tag:theconversation.com,2011:/uk/topics/poliomyelitis-75790/articlesPoliomyelitis – The Conversation2022-09-07T12:23:49Ztag:theconversation.com,2011:article/1891072022-09-07T12:23:49Z2022-09-07T12:23:49ZFears of a polio resurgence in the US have health officials on high alert – a virologist explains the history of this dreaded disease<figure><img src="https://images.theconversation.com/files/482541/original/file-20220902-13382-7ko4kb.jpg?ixlib=rb-1.1.0&rect=0%2C22%2C2995%2C2308&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Critical-care patients in the emergency polio ward at Haynes Memorial Hospital in Boston in August 1955. </span> <span class="attribution"><a class="source" href="https://newsroom.ap.org/detail/POLIOEPIDEMIC/ade0290b02e5da11af9f0014c2589dfb/photo?Query=polio%20iron%20lungs&mediaType=photo&sortBy=&dateRange=Anytime&totalCount=14&currentItemNo=6">Associated Press photo</a></span></figcaption></figure><p>Fears of polio <a href="https://history.nih.gov/display/history/Polio+Timeline">gripped the U.S. in the mid-20th century</a>. Parents were afraid to send their children to birthday parties, public pools or any place where children mingled. Children in wheelchairs served as a stark reminder of the ravages of the disease.</p>
<p>To prevent polio outbreaks, <a href="https://www.pulitzer.org/winners/david-m-oshinsky">government officials used tactics</a> now familiar in the era of COVID-19: They closed public spaces and shut down restaurants, pools and other gathering places. </p>
<p>In 1952, two years prior to the introduction of a trial polio vaccine, there were an estimated <a href="https://ourworldindata.org/polio">58,000 cases of polio and 3,145 deaths due to polio in the U.S.</a>. These cases included children who were paralyzed for life. But those numbers dropped dramatically following a widespread vaccination campaign against polio, beginning in 1955. </p>
<p>By the 1970s, there were fewer than <a href="https://www.cdc.gov/polio/what-is-polio/polio-us.html">10 cases of paralysis due to polio</a> in the U.S., and the polio virus was <a href="https://www.cdc.gov/polio/why-are-we-involved/index.htm#">considered eliminated from the U.S. by 1979</a>. Since then, collective fear of the virus has been mostly lost to history – many people alive today are lucky enough not to know someone who has experienced polio.</p>
<p>So when news broke in July 2022 that an <a href="https://www.nytimes.com/2022/07/21/nyregion/polio-case-new-york.html">unvaccinated adult man in New York had contracted polio</a> – the first case in the U.S. since 2013 – and developed paralysis from the disease, it sent a ripple of fear throughout the public health community and raised the question of whether an old foe was making a comeback. </p>
<p>I am a <a href="https://medschool.cuanschutz.edu/immunology-and-microbiology/faculty/rochford">virologist and a professor of immunology and microbiology</a> and have spent my career both teaching about and doing research on how viruses can cause disease. </p>
<p>There is no cure for polio. The only treatment is prevention. And the tool for prevention is vaccination, the same tool that <a href="https://historyofvaccines.org/history/polio/timeline">eliminated polio in the U.S. in the first place</a>. </p>
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<figcaption><span class="caption">Health experts are urging unvaccinated Americans to get vaccinated against polio.</span></figcaption>
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<h2>Life cycle of the poliovirus</h2>
<p><a href="https://www.cdc.gov/polio/what-is-polio/index.htm">Polio – or poliomyelitis – the disease</a>, is caused by the poliovirus, which is passed from person to person through the mouth. And while no one would knowingly ingest a virus, touching a contaminated object like a spoon or a glass or accidentally swallowing contaminated water can unknowingly lead to infection. </p>
<p>When someone is infected with the poliovirus, they shed the infectious virus in their feces. This is why recent reports that poliovirus has been <a href="https://www.cnbc.com/2022/08/16/polio-circulating-locally-in-nyc-area-poses-risk-to-unvaccinated-cdc-says.html">circulating in New York City wastewater for months</a> and that the virus now has been <a href="https://health.ny.gov/diseases/communicable/polio/wastewater.htm">detected in three New York counties</a> are particularly concerning. </p>
<p>In August 2022, New York State Health Commissioner Mary Basset said that the state health department is “treating the single case of polio as just the <a href="https://www.health.ny.gov/press/releases/2022/2022-08-04_polio_detected_nys.htm">tip of the iceberg of much greater potential spread</a>.” </p>
<p>“Based on earlier polio outbreaks,” she added, “New Yorkers should know that for every one case of paralytic polio observed, there may be hundreds of other people infected.” </p>
<p>A single case of polio reflects a larger potential spread of the virus because most people infected either don’t show any symptoms or have a very mild illness with <a href="https://www.who.int/news-room/fact-sheets/detail/poliomyelitis">symptoms similar to the flu</a>. But even without symptoms, an infected person is still excreting virus in their feces, which means they can be a source of infection to others. </p>
<p>The virus, which is very stable in the environment, is easily spread through surface contamination. For this reason, hand-washing is a critical prevention tool. Although many disinfecting agents, such as alcohol or diluted Lysol, fail to inactivate the virus, <a href="https://doi.org/10.1016/j.biologicals.2020.07.007">chlorine bleach does destroy it</a>. This is why public health officials started <a href="https://www.utahhumanities.org/stories/items/show/425#">chlorinating swimming pools</a> decades ago <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2038672/">in order to inactivate the polio virus</a>.<br>
Typically, the human body uses stomach acid to <a href="https://www.elsevier.com/books/mims-pathogenesis-of-infectious-disease/nash/978-0-12-397188-3">protect against ingested viruses</a>. But poliovirus can survive stomach acid to travel to your gastrointestinal tract. There, the virus reproduces itself to establish an infection. </p>
<h2>What is paralytic polio?</h2>
<p>Unfortunately, one person out of about 200 people infected with poliovirus will <a href="https://www.who.int/news-room/fact-sheets/detail/poliomyelitis">develop paralysis</a>. Scientists still don’t know why one person is susceptible to the paralytic disease while most are not. </p>
<p>In the small subset of people that get paralytic polio, <a href="https://doi.org/10.1016/j.virol.2005.09.015">the virus can attack</a> the lower motor neurons found in the brain stem and spinal cord, which are <a href="https://biologydictionary.net/motor-neuron/">important for controlling muscles</a>. Infection of those neurons leads to the muscle paralysis that is characteristic of paralytic polio. The legs are typically affected – often on only one side of the body – and paralysis can range from mild to severe. Other muscle groups can also be affected.</p>
<p>In the worst cases of paralytic polio, the virus can damage the centers of the nervous system that control breathing. <a href="https://www.sciencemuseum.org.uk/objects-and-stories/medicine/iron-lung">Respirators known as “iron lungs”</a> were early medical devices that aided those with damaged respiratory muscles, helping them breathe until their muscles healed enough to work on their own. Patients could die when the paralysis was severe and sustained. </p>
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<figcaption><span class="caption">Anti-vaccination sentiments and an overall drop in routine vaccination rates during the COVID-19 pandemic have likely contributed to the resurfacing of the poliovirus in the U.S.</span></figcaption>
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<h2>Levels of severity</h2>
<p>Although polio can be devastating for those who contract the severe form of it, most people’s immune systems are well-equipped to combat it. When someone recovers from polio, researchers can detect <a href="https://doi.org/10.1016/j.immuni.2022.04.007">poliovirus-fighting antibodies in the blood</a>.</p>
<p>But even long-term survivors of paralytic polio can develop <a href="https://www.clinicalkey.com/#!/content/playContent/1-s2.0-S0003999311001353">late-onset muscle weakness and fatigue</a>, which is <a href="https://doi.org/10.1002/mus.26168">known as post-polio syndrome</a>. While the <a href="https://doi.org/10.1002/mus.20259">muscular effects of post-polio syndrome are well-recognized</a>, a number of <a href="https://doi.org/10.3389/fneur.2019.00773">other symptoms can be associated with post-polio syndrome</a>, including chronic pain, sleep disturbances, cold intolerance and difficulty swallowing.</p>
<p>Because post-polio syndrome is diagnosed only based on symptoms, there is no consensus on the <a href="https://www.clinicalkey.com/#!/content/playContent/1-s2.0-S0003999311001353">number of polio survivors who develop it</a>, but <a href="https://doi.org/10.1002/mus.26168">estimates range from 15% to upward of 80%</a>. </p>
<h2>Prevention of polio is key</h2>
<p>The decline in polio in the U.S. and globally is a direct result of the introduction of vaccines and the willingness of the public to accept them. In 1988, the World Health Organization, in partnership with Rotary International, the Centers for Disease Control and Prevention and other national governments, launched the Global Polio Eradication Initiative with the goal <a href="https://polioeradication.org/">to wipe out polio worldwide</a>, as is <a href="https://www.cdc.gov/smallpox/index.html#">the case with smallpox</a>. </p>
<p>When this initiative was launched, there were still an <a href="https://www.cdc.gov/mmwr/volumes/70/wr/mm7034a1.htm?s_cid=mm7034a1_w">estimated 350,000 children with polio in 125 countries</a>. In 2021, there were <a href="https://www.who.int/news-room/fact-sheets/detail/poliomyelitis">only six reported cases</a>. </p>
<p>Two types of polio vaccine are in use worldwide. The one used <a href="https://www.cdc.gov/media/pressrel/r990617.htm">in the U.S. since 2000</a> is an injection made from inactivated poliovirus. Inactivation kills the virus and prevents it from spreading. <a href="https://www.cdc.gov/vaccines/schedules/hcp/imz/child-adolescent.html">Children in the U.S. get this shot</a> at 2 months, 4 months and between 6 to 15 months of age, and it essentially provides lifelong protection from polio. </p>
<p>The second vaccine type, still in use in many parts of the world, is an attenuated – or weakened – form of the virus that is taken orally. In places where community transmission remains significant, <a href="https://www.npr.org/blogs/health/2012/09/21/161549217/on-the-road-to-polio-eradication-in-pakistan">like Pakistan</a>, the oral vaccine is preferred because it prevents people from getting polio and also stops person-to-person transmission. In the U.S., where person-to-person transmission of the poliovirus has been virtually nonexistent for decades, the inactivated vaccine is preferred since the focus is on preventing disease in the vaccinated person and there’s less concern about spreading the virus. </p>
<p>But in extremely rare cases, the vaccine virus mutates after it’s been excreted in feces. And if immunization levels fall below a critical threshold – as is the case in some areas of the world – <a href="https://theconversation.com/polio-in-new-york-an-infectious-disease-doctor-explains-this-exceedingly-rare-occurrence-187518">this poliovirus can cause disease</a>. The recent New York polio case has been traced back to a mutated vaccine-derived poliovirus thought to be acquired overseas.</p>
<p>Most people in the U.S. are vaccinated through routine childhood vaccinations. Because immunity to polio following vaccination is lifelong, the CDC is <a href="https://www.cdc.gov/vaccines/vpd/polio/public/index.html">not recommending booster vaccinations for the general population</a> for people who completed the full series. However, the CDC does recommend that anyone who has not been vaccinated against polio virus get vaccinated, including adults.</p>
<p>In my office, I keep a painting of <a href="https://www.salk.edu/about/history-of-salk/jonas-salk/">Dr. Jonas Salk</a>, the virologist who developed the first polio vaccine. It serves as my reminder of the importance of biomedical research to help eliminate human suffering caused by infectious diseases.</p><img src="https://counter.theconversation.com/content/189107/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Rosemary Rochford receives funding from National Institutes of Health.</span></em></p>Health officials say the new case of polio in New York state and the presence of poliovirus in the municipal wastewater suggests that hundreds more could already be infected with the disease.Rosemary Rochford, Professor of Immunology and Microbiology, University of Colorado Anschutz Medical CampusLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1889892022-08-21T20:03:12Z2022-08-21T20:03:12ZThe latest polio cases have put the world on alert. Here’s what this means for Australia and people travelling overseas<figure><img src="https://images.theconversation.com/files/479994/original/file-20220818-459-q63sig.jpg?ixlib=rb-1.1.0&rect=1%2C1%2C997%2C664&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/covid-19-measles-ebola-vaccinated-doctor-2120184491">Shutterstock</a></span></figcaption></figure><p>Until recently, polio had only been detected in a handful of countries, thanks to global eradication efforts.</p>
<p>But this year’s polio alerts in the United States, United Kingdom and Israel are a reminder that as long as poliovirus is found anywhere, it is a potential problem everywhere. </p>
<p>That could include Australia.</p>
<p>Here’s what the latest polio cases mean for Australia – including under-vaccinated communities and people travelling internationally.</p>
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<h2>The US case</h2>
<p>In July this year, a young man in Rockland County, New York, developed paralysis and was diagnosed with polio, the <a href="https://www.statnews.com/2022/07/21/n-y-state-detects-polio-case-first-in-the-u-s-since-2013/">first US case since 2013</a>.</p>
<p>He had never been vaccinated against polio, which is not uncommon among <a href="https://forward.com/news/512089/polio-rockland-county-new-york-vaccine-orthodox-jew/">Orthodox Jewish people</a> <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8549591/">in some countries</a>. Rockland County has the highest percentage of Orthodox Jewish people in the US. Currently, only <a href="https://health.ny.gov/diseases/communicable/polio/county_vaccination_rates.htm">about 60%</a> of children in the county are vaccinated against polio, compared with <a href="https://www.cdc.gov/nchs/fastats/immunize.htm">more than 90%</a> nationally.</p>
<p>As of August 12, poliovirus was <a href="https://www1.nyc.gov/site/doh/about/press/pr2022/nysdoh-and-nycdohm-wastewater-monitoring-finds-polio-urge-to-get-vaccinated.page">still being detected</a> in sewage in New York City and other counties in New York State, indicating the virus is still circulating in the community.</p>
<p>The reason there have been no further cases of paralysis reflects the fact that only around <a href="https://www.who.int/news-room/fact-sheets/detail/poliomyelitis">one in 200 people</a> infected by the virus develops paralysis. </p>
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Read more:
<a href="https://theconversation.com/polio-in-new-york-an-infectious-disease-doctor-explains-this-exceedingly-rare-occurrence-187518">Polio in New York – an infectious disease doctor explains this exceedingly rare occurrence</a>
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<h2>A child in Israel</h2>
<p>One <a href="https://twitter.com/propublica/status/1558140096028737539">indirect link</a> to the New York man may be in Jerusalem where, in March 2022, poliovirus <a href="https://www.who.int/emergencies/disease-outbreak-news/item/2022-DON366">was found</a> in sewage and <a href="https://www.nature.com/articles/s41564-022-01201-0">one case</a> of paralysis occurred in an unvaccinated child.</p>
<p>Vaccination rates among Ultra-Orthodox Jewish people in Israel have been historically low, including <a href="https://apnews.com/article/coronavirus-pandemic-health-middle-east-religion-israel-557e9d18f3f78f4fc141eeddaaefb8eb">low uptake</a> of COVID vaccines.</p>
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<h2>UK ramps up vaccination</h2>
<p>In June this year, the UK government <a href="https://www.gov.uk/government/news/poliovirus-detected-in-sewage-from-north-and-east-london">reported</a> wastewater surveillance in north and east London between February and May had identified poliovirus on consecutive occasions. </p>
<p>This indicated a provisional “silent” outbreak and prompted health officials to instigate catch-up vaccination campaigns. No cases of paralysis have been reported.</p>
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<p>This is reminiscent of an earlier “silent” outbreak of polio in 2013-2014 when, after decades without a case, Israel <a href="https://www.pnas.org/doi/10.1073/pnas.1808798115">detected</a> poliovirus in wastewater samples in many areas, mainly in southern regions.</p>
<p>Stool surveys indicated the outbreak was restricted mainly to children under the age of ten in the Bedouin population of <a href="https://pubmed.ncbi.nlm.nih.gov/27334457/">southern Israel</a>. The virus originated in Pakistan and arrived in Israel via Cairo and then, probably, through Bedouin communities in Egypt and Israel.</p>
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Read more:
<a href="https://theconversation.com/polio-vaccine-boosters-offered-to-london-children-an-expert-explains-whats-going-on-188564">Polio vaccine boosters offered to London children – an expert explains what's going on</a>
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<h2>Hang on, hasn’t polio been eradicated?</h2>
<p>It’s tempting to think polio has been eradicated. </p>
<p>The last case of locally acquired polio in Australia <a href="https://www1.health.gov.au/internet/main/publishing.nsf/Content/cda-pubs-cdi-2002-cdi2602-cdi2602l.htm">was in 1972</a>. Australia was declared polio-free on October 29, 2000, along with the other 36 countries in the Western Pacific Region of the World Health Organization. The last case reported in Australia <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2660702/">was in 2007</a>, when a student contracted the infection in Pakistan.</p>
<p>The <a href="https://polioeradication.org">Global Polio Eradication Initiative</a>, launched in 1988, successfully eliminated wild poliovirus from all but two countries – Pakistan and Afghanistan – where in recent years there have been very few cases. </p>
<p>In <a href="https://polioeradication.org/where-we-work/afghanistan/">Afghanistan</a>, there were four cases last year and one so far this year. In <a href="https://polioeradication.org/where-we-work/pakistan/">Pakistan</a>, there was one case in 2021 and 14 so far this year.</p>
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Read more:
<a href="https://theconversation.com/polio-were-developing-a-safer-vaccine-that-uses-no-genetic-material-from-the-virus-185721">Polio: we're developing a safer vaccine that uses no genetic material from the virus</a>
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<p>The recent cases and wastewater detected polioviruses in the UK, US and Israel are not the wild variety. Instead, they are derived from the oral polio vaccine.</p>
<p>When a child receives a dose of the oral vaccine, they excrete the virus in the stool for several weeks. In very rare cases, the vaccine-derived virus mutates to a form that causes paralysis. This form is called a circulating vaccine-derived poliovirus (cVDPV). This occurs only in populations where polio vaccine coverage is low.</p>
<p>Just recently, cVDPV was reported in the Democratic Republic of the Congo, Mozambique and Yemen, as well as in wastewater in five other countries.</p>
<p>Australia, like all high-income countries, does not use the oral polio vaccine. Instead, children receive <a href="https://immunisationhandbook.health.gov.au/contents/vaccine-preventable-diseases/poliomyelitis">injectable inactivated polio vaccine</a>, which prevents paralysis but does not prevent transmission of the virus. </p>
<p>This is why so-called silent outbreaks can occur in countries that use the injectable vaccine. This is when the virus spreads from child to child but does not cause paralysis.</p>
<h2>What are the implications for Australia?</h2>
<p>Given Australia’s open international borders, there is no reason why someone who has recently received the oral polio vaccine wouldn’t enter the country and excrete the virus.</p>
<p>In Australia, at the age of five, <a href="https://www.health.gov.au/node/38782/childhood-immunisation-coverage/current-coverage-data-tables-for-all-children#five-year-olds">about 95% of children</a> are fully vaccinated against polio. </p>
<p>However, there are places with lower vaccine coverage, such as <a href="https://www.theguardian.com/australia-news/2021/aug/14/when-covid-came-to-the-anti-vax-capital-of-australia">Byron Shire</a> in northern New South Wales, with lower rates of childhood vaccination, including against polio.</p>
<p>This vaccine-hesitant community is vulnerable to the introduction of polio and has had cases of diphtheria, whooping cough, measles and tetanus in recent years.</p>
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<p>Unlike some other Orthodox Jewish communities overseas, there is no evidence this community in Australia is more vaccine hesitant than other Australians.</p>
<h2>How do we look out for cases?</h2>
<p>For years, wastewater monitoring has been routinely implemented in many countries. This acts as an early warning system to identify and rapidly mitigate the spread of many pathogens, <a href="https://theconversation.com/sewage-surveillance-is-the-next-frontier-in-the-fight-against-polio-105012">including poliovirus</a>, hepatitis viruses and, recently, SARS-CoV-2 (the virus that causes COVID).</p>
<p>At wastewater treatment facilities, sewage from an entire region is combined. This allows scientists to <a href="https://www.nature.com/articles/s41564-022-01201-0">detect pathogens</a> at the population level and before anyone presents with symptoms.</p>
<p>In December 2017, Victoria’s environmental testing program <a href="https://www.health.vic.gov.au/media-releases/health-surveillance-system-detects-poliovirus">detected</a> a rare type of poliovirus in pre-treated sewage from the Western Treatment Plant in Melbourne. </p>
<p>No cases of paralytic polio were detected but all Victorians up to the age of 19 were offered three doses of vaccine, free of charge, as part of catch-up arrangements.</p>
<p>Australia’s poliovirus infection outbreak response plan <a href="https://www.health.gov.au/sites/default/files/documents/2022/05/poliovirus-infection-outbreak-response-plan-for-australia.pdf">focuses on</a> clinical surveillance (where health workers report suspected cases to health authorities) and laboratory investigations of people who present with acute paralysis. </p>
<p>While the plan refers to examples of wastewater surveillance overseas, it does not propose a specific strategy in Australia. </p>
<p>Other than Victoria, it is not clear where wastewater polio surveillance is being conducted in Australia.</p>
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Read more:
<a href="https://theconversation.com/sewage-surveillance-is-the-next-frontier-in-the-fight-against-polio-105012">Sewage surveillance is the next frontier in the fight against polio</a>
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<h2>What happens next?</h2>
<p>Australia is just as vulnerable to importations of poliovirus – both wild and vaccine-derived – as any other country.</p>
<p>Australia should ensure routine wastewater surveillance for poliovirus is conducted, at least in metropolitan areas.</p>
<p>Community-based vaccination campaigns should be sensitively conducted in vaccine-hesitant communities, such as in Byron Shire, to achieve high coverage.</p>
<p>Education should also be provided through GPs to parents planning to travel to Jerusalem, New York City and Rockland County. They should ensure all travelling family members are fully vaccinated against polio. Visitors to Israel may be able to access a dose of oral polio vaccine in that country for their children (which will prevent them being infected) but this is not available in the US.</p>
<p>Poliovirus enters the body through the mouth, usually from hands contaminated with the stool of an infected person. So parents should also pay special attention to their children’s hand hygiene, particularly if travelling overseas to any of the locations mentioned.</p><img src="https://counter.theconversation.com/content/188989/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michael Toole receives funding from the National Health and Medical Research council.</span></em></p>Polio cases in the US, UK and Israel remind us that this could also happen in Australia. Here’s what we should watch out for.Michael Toole, Associate Principal Research Fellow, Burnet InstituteLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1885642022-08-15T10:12:44Z2022-08-15T10:12:44ZPolio vaccine boosters offered to London children – an expert explains what’s going on<figure><img src="https://images.theconversation.com/files/478904/original/file-20220812-3904-gx33u5.jpg?ixlib=rb-1.1.0&rect=19%2C0%2C6507%2C4354&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/vaccination-concept-little-kid-medical-face-1914690256">Studio Romantic/Shutterstock</a></span></figcaption></figure><p>In June, the <a href="https://www.gov.uk/government/news/poliovirus-detected-in-sewage-from-north-and-east-london">UK Health Security Agency</a> reported that poliovirus had been detected in sewage in north and east London between February and May 2022.</p>
<p>Following this, people were advised to ensure their children were <a href="https://www.england.nhs.uk/london/2022/07/20/get-your-child-vaccinated-against-poliovirus-urges-londons-nhs/">up to date</a> with their polio vaccinations.</p>
<p>On August 10, the <a href="https://www.gov.uk/government/publications/vaccination-strategy-for-ongoing-polio-incident-jcvi-statement/joint-committee-on-vaccination-and-immunisation-statement-on-vaccination-strategy-for-the-ongoing-polio-incident">UK Joint Committee on Vaccination and Immunisation</a> (JCVI) recommended that a further booster dose of polio vaccine be offered to all children in London aged between one and nine.</p>
<p>So what’s happened between June and now, and why this change in vaccination policy?</p>
<p>First, a bit of background. Poliomyelitis, or <a href="https://www.nhs.uk/conditions/polio/">polio</a>, is a devastating disease that has historically seen paralysis and death around the world, mainly in children. It’s caused by an RNA virus (poliovirus) that spreads easily from person to person, usually through virus shed in faeces. </p>
<p>The vast majority of infections with poliovirus actually go unnoticed, but a small proportion of those infected will develop paralysis (or paralytic poliomyelitis), which can lead to respiratory failure or long-term deformities.</p>
<p>In the 1950s, <a href="https://www.sciencemuseum.org.uk/objects-and-stories/medicine/polio-20th-century-epidemic">two polio vaccines</a> were developed: a live attenuated vaccine administered orally (the Sabin vaccine), and an inactivated vaccine given by injection (the Salk vaccine). A live attenuated vaccine is based on a virus that’s still able to reproduce, but is weakened so it doesn’t cause disease. An inactivated vaccine, on the other hand, cannot reproduce.</p>
<p>Both vaccines are highly effective at preventing paralytic poliomyelitis. The oral vaccine in particular can induce strong immunity <a href="https://www.nature.com/articles/d42859-020-00015-6">in the gut</a> and so is better at reducing faecal shedding of the virus, and therefore <a href="https://www.futuremedicine.com/doi/10.2217/fmb.15.19">reducing transmission</a>. </p>
<p>However, the oral vaccine can very occasionally <a href="https://www.ncbi.nlm.nih.gov/books/NBK236293/">cause paralysis</a> (about two to three cases per million doses). For this reason, most countries, including the UK, now prefer to use the inactivated vaccine. The oral vaccine is still used in a small number of countries though.</p>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/poliovirus-in-london-sewage-what-you-need-to-know-185744">Poliovirus in London sewage – what you need to know</a>
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<h2>Wastewater monitoring</h2>
<p>Children who receive the live vaccine will shed it for a short time in their faeces, which is why we might detect “<a href="https://www.gov.uk/government/publications/polio-detection-of-vdpv2-in-london-sewage-samples/immediate-actions-in-response-to-detection-of-vaccine-derived-polio-virus-type-2-vdpv2-in-london-sewage-samples">vaccine-like</a>” polioviruses in wastewater. This normally happens two or three times a year in the UK, where this weakened version of the virus is introduced to the sewage by a child who received the oral vaccine overseas.</p>
<p>This isn’t dangerous in itself, but it’s possible that if these viruses continue to circulate in a population, over time they can mutate, and possibly revert to a version that causes paralysis. These then become classified as <a href="https://polioeradication.org/polio-today/polio-prevention/the-virus/vaccine-derived-polio-viruses/">vaccine-derived polioviruses</a>. Circulation of vaccine-like and vaccine-derived polioviruses is more likely when fewer children are up to date with their vaccinations.</p>
<figure class="align-center ">
<img alt="A mother holds her son on her lap while they are seen by a doctor." src="https://images.theconversation.com/files/478924/original/file-20220812-15-w384kk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/478924/original/file-20220812-15-w384kk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/478924/original/file-20220812-15-w384kk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/478924/original/file-20220812-15-w384kk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/478924/original/file-20220812-15-w384kk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/478924/original/file-20220812-15-w384kk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/478924/original/file-20220812-15-w384kk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Polio is included in routine childhood vaccinations.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/family-getting-covid-vaccine-home-doctor-1994923175">Jacob Lund/Shutterstock</a></span>
</figcaption>
</figure>
<p>This is the process which appears to be playing out in London now. From the JCVI’s statement, it’s clear that since the virus was first detected in February, it has evolved and acquired mutations that <a href="https://www.gov.uk/government/publications/vaccination-strategy-for-ongoing-polio-incident-jcvi-statement/joint-committee-on-vaccination-and-immunisation-statement-on-vaccination-strategy-for-the-ongoing-polio-incident">increase the risk</a> of paralytic disease. </p>
<p>These same genetic analyses show a high diversity of viruses being detected, which suggests the virus is circulating in separate networks of people across affected areas of north London.</p>
<p>The JCVI also stated that occasional positive results are being recorded in areas beyond where the virus was originally detected, including Enfield, Barnet and some areas south of the Thames. So it does appear that the virus is continuing to circulate, and may be circulating more widely than before. If this continues, it would only be a matter of time before we’d start to see cases of paralysis among children who are not vaccinated.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/polio-was-recently-detected-in-sewage-in-the-uk-heres-what-else-scientists-look-for-in-our-wastewater-185799">Polio was recently detected in sewage in the UK – here's what else scientists look for in our wastewater</a>
</strong>
</em>
</p>
<hr>
<h2>Vaccination is key</h2>
<p>Monitoring of wastewater for polioviruses is carried out by many countries primarily to identify when such viruses are <a href="https://www.cambridge.org/core/journals/epidemiology-and-infection/article/role-of-environmental-poliovirus-surveillance-in-global-polio-eradication-and-beyond/DBB1EC7A25FBB252D7EDF9F2F7939FE3#">circulating in the community</a>, before these viruses start to cause paralysis. The fact the issue was detected in London earlier this year and we are yet to see any cases of paralysis shows that such monitoring is achieving its objectives.</p>
<p>The only effective way to control spread of the virus, and importantly, to prevent the emergence of paralytic disease, is to ensure vaccine coverage is as high as possible. Fortunately we have an ample supply of effective and safe vaccines.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1557363689367502850"}"></div></p>
<p>In England the policy has been to give three doses of polio vaccine within a baby’s first 16 weeks, a further booster shortly after three years, and <a href="https://www.nhs.uk/conditions/polio/">again at age 14</a>. The polio vaccinations are usually given in combination with vaccinations against other conditions.</p>
<p>Part of the logic behind the advice for <a href="https://www.gov.uk/government/publications/vaccination-strategy-for-ongoing-polio-incident-jcvi-statement/joint-committee-on-vaccination-and-immunisation-statement-on-vaccination-strategy-for-the-ongoing-polio-incident">an additional booster dose</a> is that <a href="https://academic.oup.com/cid/article/71/8/1984/5754490?login=false">since 2012</a> the UK has been offering a whooping cough vaccine to pregnant women to protect their babies in the early weeks after birth. This is the same vaccine used in children at age three which also contains a dose of the inactivated polio vaccine. </p>
<p>There is some evidence that high levels of maternal antibodies to polio may <a href="https://www.gov.uk/government/publications/vaccination-strategy-for-ongoing-polio-incident-jcvi-statement/joint-committee-on-vaccination-and-immunisation-statement-on-vaccination-strategy-for-the-ongoing-polio-incident">reduce the effectiveness</a> of the first three vaccine doses, meaning children who miss their booster at age three could still be susceptible.</p>
<p>In the affected areas of London, uptake of the polio booster at age three is <a href="https://www.gov.uk/government/publications/vaccination-strategy-for-ongoing-polio-incident-jcvi-statement/joint-committee-on-vaccination-and-immunisation-statement-on-vaccination-strategy-for-the-ongoing-polio-incident">particularly low</a>, so there is concern that many children in these areas may have inadequate protection. The <a href="https://www.gov.uk/government/news/all-children-aged-1-to-9-in-london-to-be-offered-a-dose-of-polio-vaccine">programme</a> will begin in these areas.</p>
<p>While children who have missed their booster will be most vulnerable, according to the JCVI, offering an additional dose to those who are vaccinated will boost antibody levels and may help to reduce asymptomatic shedding of the virus.</p><img src="https://counter.theconversation.com/content/188564/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Paul Hunter consults for the World Health Organization. He receives funding from National Institute for Health Research, the World Health Organization and the European Regional Development Fund.</span></em></p>The move follows recent detections of poliovirus in London’s wastewater.Paul Hunter, Professor of Medicine, University of East AngliaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1875182022-07-22T21:53:50Z2022-07-22T21:53:50ZPolio in New York – an infectious disease doctor explains this exceedingly rare occurrence<figure><img src="https://images.theconversation.com/files/475697/original/file-20220722-18-x3p6wo.jpg?ixlib=rb-1.1.0&rect=2%2C0%2C1794%2C1196&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Polio is endemic only in Afghanistan and Pakistan in 2022.</span> <span class="attribution"><a class="source" href="https://newsroom.ap.org/detail/Polio/221757d74c22419095b557a6d3e4ab75/photo?Query=polio&mediaType=photo&sortBy=arrivaldatetime:desc&dateRange=Anytime&totalCount=1705&currentItemNo=2">Sarah Poser, Meredith Boyter Newlove/CDC via AP</a></span></figcaption></figure><p><em>The first case of polio in the U.S. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5779436/">since 2013</a> was announced by New York state <a href="https://health.ny.gov/press/releases/2022/2022-07-21_polio_rockland_county.htm">health officials on July 21, 2022</a>. The U.S. resident had not been vaccinated.</em></p>
<p><em>Polio was a common cause of paralysis in children before <a href="https://theconversation.com/lessons-from-how-the-polio-vaccine-went-from-the-lab-to-the-public-that-americans-can-learn-from-today-145604">safe and effective vaccines were invented</a> in the mid-20th century. Thanks to global vaccination campaigns, polio is now almost eradicated, with <a href="https://polioeradication.org/polio-today/polio-now/this-week/">only 13 cases of endemic wild poliovirus reported</a> in 2022 to date worldwide.</em></p>
<p><em>The New York patient reportedly contracted a form of polio that can be traced back to the live, but weakened, poliovirus used in the <a href="https://www.cdc.gov/vaccines/vpd/polio/public/index.html">oral polio vaccine</a>. This version of the vaccine has <a href="https://www.cdc.gov/media/pressrel/r990617.htm">not been used in the U.S. since 2000</a>. Health officials said the virus affecting the male patient, who has muscle weakness and paralysis, <a href="https://health.ny.gov/press/releases/2022/2022-07-21_polio_rockland_county.htm">likely originated somewhere overseas</a>, where oral vaccines are still administered.</em></p>
<p><em><a href="https://scholar.google.com/citations?user=6yMIM1MAAAAJ&hl=en&oi=ao">William Petri</a> is an infectious disease specialist and chair of the World Health Organization’s <a href="https://polioeradication.org/tools-and-library/current-research-areas/polio-research-committee/">Polio Research Committee</a>. Here he explains what <a href="https://www.cdc.gov/vaccines/vpd/polio/hcp/vaccine-derived-poliovirus-faq.html">vaccine-derived poliovirus</a> is and why the inactivated polio vaccine administered in the U.S. today can’t cause it.</em></p>
<h2>What are the two kinds of polio vaccine?</h2>
<p>Vaccines introduce a harmless version of a pathogen to your body. The idea is that <a href="https://www.cdc.gov/vaccines/hcp/conversations/understanding-vacc-work.html">they train your immune system</a> to fight off the real germ if you ever encounter it.</p>
<p>The oral polio vaccine, <a href="https://doi.org/10.1126/science.288.5471.1593">originally developed by Albert Sabin</a>, uses a live but weakened poliovirus that one swallows in a sugar cube or droplet. Scientists weaken – or attenuate – the virus so it can no longer cause disease. </p>
<p>The other kind of polio vaccine was <a href="https://doi.org/10.1126/science.288.5471.1593">originally developed by Jonas Salk</a>. It contains inactivated, dead virus. It is administered by an injection.</p>
<p>In the U.S., children receive the inactivated polio vaccine <a href="https://www.cdc.gov/vaccines/schedules/hcp/imz/child-adolescent.html">at 2, 4 and 6 months of age</a>. It provides nearly complete protection from paralytic polio.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/475699/original/file-20220722-12-3gho47.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="adult hands administer a drop of medicine to a boy with open mouth" src="https://images.theconversation.com/files/475699/original/file-20220722-12-3gho47.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/475699/original/file-20220722-12-3gho47.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/475699/original/file-20220722-12-3gho47.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/475699/original/file-20220722-12-3gho47.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/475699/original/file-20220722-12-3gho47.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/475699/original/file-20220722-12-3gho47.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/475699/original/file-20220722-12-3gho47.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">A boy in Pakistan receives a dose of the oral polio vaccine.</span>
<span class="attribution"><a class="source" href="https://newsroom.ap.org/detail/PakistanPolio/23a6599533e34577b64f4932393fa80a/photo?Query=polio&mediaType=photo&sortBy=arrivaldatetime:desc&dateRange=Anytime&totalCount=1705&currentItemNo=17">AP Photo/Muhammad Sajjad</a></span>
</figcaption>
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<h2>How can the live vaccine lead to a case of polio?</h2>
<p>The weakened form of the live virus in the oral vaccine cannot cause disease. However, because the vaccine is given orally, the weakened virus is excreted in the feces and can <a href="https://www.who.int/news-room/questions-and-answers/item/poliomyelitis-vaccine-derived-polio">spread from someone who is vaccinated to their close contacts</a>. If the weakened virus circulates person to person for long enough, it can mutate and regain its ability to cause paralysis.</p>
<p>The <a href="https://www.who.int/news-room/questions-and-answers/item/poliomyelitis-vaccine-derived-polio">mutated virus can then infect people</a> in communities with poor sanitation and low vaccination rates, causing disease and even paralysis. </p>
<p>This is an exceedingly rare occurrence. With more than 10 billion doses of the oral polio vaccine administered since 2000, there have been <a href="https://polioeradication.org/polio-today/polio-now/this-week/circulating-vaccine-derived-poliovirus/">fewer than 800 cases of vaccine-derived polio</a> reported.</p>
<p>Apparently, the current patient in New York was somehow exposed to a mutated poliovirus that had been transmitted after vaccination overseas. Earlier this summer, routine surveillance spotted <a href="https://www.who.int/news/item/22-06-2022-vaccine-derived-poliovirus-type-2-(vdpv2)-detected-in-environmental-samples-in-london--uk">vaccine-derived poliovirus in London’s sewage system</a>, but no cases have been reported there. </p>
<h2>Why use the oral vaccine anywhere if it comes with this risk?</h2>
<p>There’s a positive aspect to the fact that the weakened live virus can circulate in the community once oral vaccine recipients shed it in their feces. Traveling a feces-to-oral route, it can <a href="https://polioeradication.org/polio-today/polio-prevention/the-vaccines/opv/">help induce immunity</a> even in people who weren’t directly vaccinated. The oral polio vaccine is also <a href="https://doi.org/10.1093/infdis/jiu128">cheaper and easier to administer</a> than inactivated polio vaccines.</p>
<p>Most importantly, the live-virus vaccine stops transmission of wild poliovirus in a way that the inactivated-virus vaccine does not. The <a href="https://doi.org/10.1126/science.abb8588">eradication of polio</a> in the Americas, Europe and Africa has been accomplished solely through the use of the live oral vaccine. Once polio has been wiped from a continent, then it is safe to stop using the oral live vaccine and use only the inactivated vaccine, <a href="https://www.cdc.gov/cpr/polioviruscontainment/diseaseandvirus.htm">which does prevent disease in recipients</a> and does not pose the rare risk of vaccine-derived paralytic polio.</p>
<p><a href="https://polioeradication.org/nopv2/">A new and safer oral polio vaccine</a> that has been engineered not to mutate is now <a href="https://doi.org/10.1126/science.abb8588">replacing the earlier live-virus vaccine</a>. Thus, even this extremely rare complication of polio vaccination should soon become a thing of the past.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/C0KmNNP_Wrg?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">WHO: Polio Eradication – Reaching Every Last Child.</span></figcaption>
</figure>
<h2>How close is the world to eradicating polio?</h2>
<p>Thanks to <a href="https://polioeradication.org">tremendous global effort</a>, two of the three viruses that cause polio have been eradicated. The world is now on the verge of eradicating the final one, wild poliovirus 1 (WPV1).</p>
<p><a href="https://polioeradication.org/polio-today/polio-now/this-week/">Today endemic polio is found</a> only in Pakistan, with 12 cases of paralytic polio so far in 2022, and Afghanistan, with just one case this year. Africa has two cases, imported from overseas, which are being contained by additional vaccination campaigns.</p>
<p>Once wild poliovirus has been eradicated from the planet, vaccination efforts may be able to switch to the inactivated polio vaccine, eliminating the risk of any future vaccine-derived cases.</p><img src="https://counter.theconversation.com/content/187518/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>William Petri receives research funding from the NIH and the Gates Foundation.</span></em></p>The oral polio vaccine – which is no longer given in the US – relies on a live but weakened virus that can actually be passed from person to person.William Petri, Professor of Medicine, University of VirginiaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1215982019-09-13T11:37:14Z2019-09-13T11:37:14ZA newly designed vaccine may help stamp out remaining polio cases worldwide<figure><img src="https://images.theconversation.com/files/292096/original/file-20190911-190007-1t409cv.jpg?ixlib=rb-1.1.0&rect=272%2C13%2C1870%2C1387&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The oral polio vaccine is most commonly used in the developing world, despite one big problem.</span> <span class="attribution"><a class="source" href="https://phil.cdc.gov/Details.aspx?pid=18249">CDC/Alan Janssen, MSPH</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Public health organizations around the world have been fighting for global eradication of polio since 1988. Through massive vaccination efforts, the <a href="http://polioeradication.org/polio-today/history-of-polio/">incidence of polio has gone down 99%</a> since then, with the virus eradicated from most of the countries on Earth.</p>
<p>But there have been many setbacks.</p>
<p>One particularly serious threat has surfaced over the last 15 years. Among poorly vaccinated populations, an increasing number of polio cases are due to strains of the virus that originate from one version of the vaccine itself. The Sabin vaccine, which is taken orally, is <a href="https://www.who.int/biologicals/areas/vaccines/poliomyelitis/en/">composed of live but weakened viruses</a> that won’t sicken recipients but will still create lasting immunity against polio.</p>
<p>However, through genetic changes, the weakened vaccine virus can reacquire the ability to cause paralytic polio. How this happens and how to prevent it are under active research. A new vaccine deliberately constructed to prevent the poliovirus from regaining virulence may be the answer.</p>
<h2>Virus in vaccines, attenuated or killed</h2>
<p>The <a href="https://www.who.int/en/news-room/fact-sheets/detail/poliomyelitis">virus that causes polio</a> infects the cells of the throat and intestine. People usually catch it by ingesting food or water contaminated with fecal matter from an infected person.</p>
<p>Most people infected with the polio virus have no symptoms at all; about a quarter of infections result in flu-like symptoms. However, in about 1 out of every 200 cases, the virus <a href="https://doi.org/10.1016/j.virol.2005.09.015">invades the cells of the central nervous system</a>, causing paralysis.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/292094/original/file-20190911-190050-1xdqj0r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/292094/original/file-20190911-190050-1xdqj0r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/292094/original/file-20190911-190050-1xdqj0r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=594&fit=crop&dpr=1 600w, https://images.theconversation.com/files/292094/original/file-20190911-190050-1xdqj0r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=594&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/292094/original/file-20190911-190050-1xdqj0r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=594&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/292094/original/file-20190911-190050-1xdqj0r.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=746&fit=crop&dpr=1 754w, https://images.theconversation.com/files/292094/original/file-20190911-190050-1xdqj0r.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=746&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/292094/original/file-20190911-190050-1xdqj0r.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=746&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Illustration of a poliovirus particle.</span>
<span class="attribution"><a class="source" href="https://phil.cdc.gov/Details.aspx?pid=22498">CDC/ Sarah Poser</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Poliovirus is a very simple virus. It is composed of a shell, or capsule, made of protein. Inside the capsule is a single strand of ribonucleic acid, or RNA, that encodes the genetic information to make new virus particles. There are three poliovirus strains – PV1, PV2 and PV3. Immunity to one strain does not confer immunity to the other two, so both the original Salk and Sabin vaccines included all three.</p>
<p>The Salk vaccine, which is injected, is composed of killed viruses; the Sabin vaccine, taken orally, contains live but weakened viruses. Such weakened vaccine viruses are known as “attenuated.” The oral vaccine is both more effective and easier to administer than the injected vaccine, and so it has been the primary weapon for worldwide eradication of polio.</p>
<p>Once a child is immunized with the oral polio vaccine, the weakened virus lives in their intestine for several weeks, allowing the child to develop immunity by building up antibodies. During this time, recipients shed live viruses in their feces. The weakened viruses in the vaccine itself do not cause polio, and this shedding can help to <a href="http://polioeradication.org/polio-today/polio-prevention/the-vaccines/opv/">immunize unvaccinated people</a> that come into close contact with recently vaccinated individuals. </p>
<p>But shortly after the oral vaccine became widely used in the early 1960s, researchers discovered that some shed viruses had <a href="http://www.virology.ws/2015/09/10/why-do-we-still-use-sabin-poliovirus-vaccine/">reacquired the ability to cause paralysis</a>.</p>
<p>As the viruses multiply in the intestine, they undergo genetic changes, some of which can reverse or circumvent the original genetic changes that had made them less virulent. These vaccine-derived virulent viruses very rarely cause their vaccinated host to get polio, but, when they circulate in the population, they put unvaccinated individuals at grave risk.</p>
<p>Because of this risk, most developed countries, including the United States, have <a href="https://www.cdc.gov/vaccines/vpd/polio/public/index.html">stopped using the oral Sabin vaccine</a>, relying instead on several injections of the Salk vaccine. However, oral polio vaccine is still the most available, and reliable, vaccine in developing countries.</p>
<h2>Fine-tuning the vaccination effort</h2>
<p>One of the polio strains – type 2 polio, or PV2 – was <a href="http://polioeradication.org/news-post/global-eradication-of-wild-poliovirus-type-2-declared/">declared eradicated</a> worldwide in 2015. The challenge then became preventing new cases of type 2 polio that could stem from virus shed by newly vaccinated people. </p>
<p>To prevent these new cases of vaccine-derived type 2 polio, the World Health Organization campaigned to replace the standard oral polio vaccine with one consisting of only PV1 and PV3. This <a href="https://www.who.int/immunization/diseases/poliomyelitis/en/">switch was completed in 2016</a>. </p>
<p>Unfortunately, the vaccine-derived type 2 poliovirus continues to circulate and cause paralytic polio. Globally, <a href="http://polioeradication.org/polio-today/polio-now/this-week/">104 polio cases due to this virus occurred in 2018</a>, which was three times the number of cases due to wild viruses not derived from the vaccine. As the number of children who have no immunity to type 2 poliovirus increases, this number may rise.</p>
<p>Health officials are campaigning to eliminate all oral vaccines and rely on injected vaccines containing killed virus. However, this plan imposes logistic and financial burdens on developing countries. Meanwhile, oral type 2 vaccines are needed to stop any outbreaks of type 2 polio. And health officials have not yet figured out how to make sure unvaccinated children don’t get polio from mutated viruses shed by vaccinated people.</p>
<h2>Ensuring virulence can’t be restored</h2>
<p>But what if researchers created a vaccine from a weakened live virus that’s unlikely revert to virulence? That’s one of the strategies that <a href="https://doi.org/10.2217/fmb-2019-0196">several research groups</a> are working on now.</p>
<p>As part of this effort, scientists have now mapped out in detail the <a href="https://doi.org/10.1016/j.cell.2017.03.013">steps that allow the PV2 vaccine strain to regain virulence</a>. </p>
<p>Just three simple genetic mutations, each of which alone has a small effect, when combined, dramatically increase the virulence of the PV2 strain in mice in the lab. And, all three are found in the viruses shed by <a href="https://doi.org/10.1016/j.cell.2017.03.013">recent vaccine recipients</a>.</p>
<p>Virulence can also be reestablished in the intestine if the genetic material of the PV2 strain recombines with that of another virus. The second virus can be the weakened PV1 or PV3 from the vaccine, <a href="https://doi.org/10.1016/j.cell.2017.03.013">or a related virus</a> such as coxsackie, a common virus in children.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/292101/original/file-20190911-190050-1or0jb1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/292101/original/file-20190911-190050-1or0jb1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/292101/original/file-20190911-190050-1or0jb1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=479&fit=crop&dpr=1 600w, https://images.theconversation.com/files/292101/original/file-20190911-190050-1or0jb1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=479&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/292101/original/file-20190911-190050-1or0jb1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=479&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/292101/original/file-20190911-190050-1or0jb1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=601&fit=crop&dpr=1 754w, https://images.theconversation.com/files/292101/original/file-20190911-190050-1or0jb1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=601&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/292101/original/file-20190911-190050-1or0jb1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=601&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Rather than making the vaccine from purified virus as Albert Sabin did, today’s researchers are tweaking the virus they will use to create the vaccine.</span>
<span class="attribution"><a class="source" href="http://www.apimages.com/metadata/Index/Watchf-Associated-Press-Domestic-News-Ohio-Unit-/486be8a50c8f406a9ece810ead1dee5d/156/0">AP Photo/Gene Smith</a></span>
</figcaption>
</figure>
<h2>Designing the virus for the vaccine</h2>
<p>Having discovered exactly how the type 2 vaccine virus regains virulence, scientists figured out ways that these specific genetic changes could be prevented. Using the tools of molecular biology, <a href="http://dx.doi.org/10.2139/ssrn.3400856">they made four important changes to the PV2 genome</a> that should stop it from regaining virulence.</p>
<p>Part of the virus’s RNA genome has to fold up into a lollipop-like shape in order for proteins to be made. This structure is disrupted in the weakened vaccine strain, but a single mutation in the virulent strain allows it to reassemble. So that can’t happen, the scientists changed the genetic sequence of the RNA in a way that no single nucleotide change would let the RNA fold into the stable lollipop structure again.</p>
<p>Second, they changed the genetic sequence of the enzyme that copies the RNA to make it more accurate. That way fewer genetic mutations would occur in the vaccine recipient’s intestine. </p>
<p>Third, another change to the same enzyme reduced the chance that the virus could pick up genes by recombining with other viruses in the recipient.</p>
<p>And, fourth, they rearranged the virus’s genes so that replacing certain regions of its own RNA with genetic information from a wild virus, such as coxsackie, would be lethal for it.</p>
<p>Researchers have produced two candidate PV2 viruses that grew well in experimental cells, were not virulent in a mouse model and were genetically stable. <a href="https://doi.org/10.1016/S0140-6736(19)31279-6">A phase 1 clinical trial of vaccines made from these viruses</a> showed that they were well tolerated, produced an immune response and had reduced (but not zero) reversion to virulence compared to the original oral vaccine for PV2.</p>
<p>These and other new, <a href="https://doi.org/10.1371/journal.ppat.1003001">rationally designed viruses</a> have the potential to provide good protection from polio with a reduced risk of creating new outbreaks due to circulating vaccine-derived polio virus.</p>
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<p class="fine-print"><em><span>Patricia L. Foster receives funding from the US Army Research Office. She is a member of the Union of Concerned Scientists and Concerned Scientists at Indiana University.</span></em></p>A challenge in eradicating polio comes from a version of the vaccine itself, which relies on live but attenuated virus. Rationally designing a new vaccine could help get rid of polio once and for all.Patricia L. Foster, Professor Emerita of Biology, Indiana UniversityLicensed as Creative Commons – attribution, no derivatives.