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Vaccination is a core part of the government’s strategy against COVID-19. Andrew Parsons/EPA

Coronavirus: how to use a vaccine when it becomes available

Over 80 COVID-19 vaccine trials are currently underway. If one or more of the vaccines eventually makes it to market, how should it be used to stop or even eradicate the virus? You might think the obvious answer would be to vaccinate everyone, but that is neither practical nor feasible.

Vaccination has often been used in mass programmes to create herd immunity in the population. This works by protecting susceptible people from acquiring the disease, even if they are in contact with infectious people.

Mathematical modelling and data from vaccination programmes suggest that we don’t need to vaccinate everybody. If we protect a large enough proportion of the population, we can effectively “starve” the pathogen of a supply of new hosts.

Starving the virus

Scientists believe that for the novel coronavirus about 50% to 70% of the population needs to be resistant for the virus to stop spreading and to slowly die out. An even higher proportion is needed if we want the eradication to proceed quicker and to prevent further outbreaks.

Some people will have developed immunity to the coronavirus, but the number of people who have developed antibodies as a result of having had the disease is still far too low to reach herd immunity. The remaining protection would need to be achieved with a mass vaccination programme.

To achieve herd immunity in the UK, we would need to vaccinate millions of people. Vaccines are rarely 100% effective, so the theoretical limit of the proportion of the population that needs to be vaccinated has to be exceeded by a wide margin to account for this.

Vaccines also have side effects – some of which can be serious. For smallpox, about one in every 1,000 people vaccinated have a severe reaction, which can be life-threatening. So attempting to vaccinate 60 million people in the UK might create a significant burden on the health service, with potentially tens of thousands of deaths as a result. But many vaccines have fewer side effects, so it might be safe to apply the coronavirus vaccine widely.

Another potential difficulty is that many people might refuse to be vaccinated because of fear of the side effects or for cultural, religious or political reasons. If the government tries to impose the vaccination by force, it could lead to civil unrest, as happened in Milwaukee in 1894 or during the vaccine rebellion in Brazil in 1904.

The refusal to use the MMR vaccine is seen as the primary reason for the failure to eradicate measles and for its recent revival. Similar controversies were associated with polio vaccination, potentially affecting the virus’s eradication in parts of Africa.

Instead of trying to achieve herd immunity in the whole population, the British government could combine the vaccination programme with testing and isolation to limit the vaccine’s application to a specified location (for example, London). This strategy can be particularly useful in either early or late stages of an epidemic.

The government could combine testing with isolation to limit the number of people who need to be vaccinated. Andy Rain/EPA

Restricting movement through social distancing can be used to stop people from spreading the virus outside the containment area. Contact and social network tracing can further narrow down the susceptible people to those who might conceivably be in contact with the infected ones. This forms a ring of protection around an infected case.

Alternatively, a vaccine programme could target those at high risk because of their occupation – for example, health or care-home workers and their families.

An even more sophisticated approach uses social network analysis to establish and then target the people or groups who are potentially responsible for infecting a lot of people.

The vaccination could also be offered to those who are at risk because of their age or existing health conditions, as for seasonal flu vaccinations in many countries. The disadvantage of this approach is that many vaccines are less efficient in older people or might be dangerous for those whose immune systems are compromised.

Total eradication is almost impossible

History shows that it is incredibly difficult to eradicate a virus globally. It has only been achieved twice for major diseases of humans and animals. The smallpox vaccine was first developed and used in the 1700s, but the mass campaign to eradicate the virus stretched throughout the 20th century. As the number of cases was reduced, each outbreak was isolated, and those living close were vaccinated. Smallpox was officially eradicated in 1980.

Smallpox was finally eradicated in 1980. Everett Historical/Shutterstock

Rinderpest, a viral infection of livestock, was officially declared extinct in 2010 after programmes of mass vaccination and surveillance lasting throughout the 20th century.

Difficulties in delivering the vaccine in remote war and poverty afflicted parts of the world contribute to the long period between the campaign success in developed countries and the final complete success. Even if we manage to eradicate the novel coronavirus in countries such as the UK, there is likely to be a long period before it disappears entirely from the planet – if ever. In such a case, health workers and travellers will need to continue to be vaccinated on demand.

As we are waiting for the coronavirus vaccine, it is important that we consider now how it can be used in the most cost-effective and publicly acceptable way. We need to know how to balance the demand for herd immunity with the protection of individual rights. This requires the efforts of public health experts, doctors, psychologists and political scientists together with mathematical modellers.

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