Most people infected with influenza show no symptoms and only a small proportion become unwell enough to seek medical help, a new study shows.
But since influenza can severely affect the elderly, pregnant women, Aboriginal and Torres Straight Islanders and those with chronic diseases, experts advise those who routinely come into contact with high-risk people to get vaccinated.
The Flu Watch study, published in The Lancet Respiratory Medicine today, sought to establish the extent and severity of flu infection in the community.
This is vital for ensuring that government measures to stop its spread – such as stockpiling antiviral medicine – are useful and cost-effective, the researchers said.
Current estimates of the flu’s impact on the community are based on surveillance of people who seek medical attention for flu-like symptoms. This approach underestimates the number of people who have been infected with the virus, the researchers said, because not every infected person will go to a doctor.
It also over-estimates the severity of the illness as only really unwell people seek medical attention, end up in hospital or die from flu-related complications.
The researchers tracked five sets of households across England over six flu seasons between 2006 and 2011, including the swine flu pandemic (2009).
They found fewer than one in five (18%) unvaccinated people were infected by the flu each winter; 23% of these infections had symptoms and only 17% had severe enough symptoms to see a doctor. The study did not include children under the age of five and people over the age of 65.
The research suggests many people who think they have a cold during winter might actually have the flu.
Head of the epidemiology unit at the Victorian Infectious Diseases Reference Laboratory (VIDRL) and adjunct professor in infectious diseases epidemiology, Heath Kelly, said the study illustrated how the messaging about flu did not reflect the science.
“Flu is always portrayed as a disease of sudden onset and severe symptoms that sends you to bed, with the chance of ending up in hospital or ICU. All of that’s true but it’s a tiny, tiny proportion of influenza cases,” he said.
“What a lot of people might be having is what I call immunising infections – you get an infection, it’s not severe enough to go and seek medical care but it gives you protection. It’s exactly what you want from vaccination.”
But Dr Alan Hampson, adjunct senior research fellow at Federation University and chairman of the Influenza Specialist Group, a pharmaceutical industry-funded body that works closely with government to raise awareness of the flu, interpreted the study differently.
He recommended vaccinating people who might suffer only a mild infection, to control the spread of influenza and prevent severe cases.
Dr Kelly said according to research undertaken at the Victorian Infectious Diseases Reference Laboratory over the last seven years, the flu vaccine is between 50% and 60% effective, so a shot may not prevent infection.
Hampson and Kelly agreed that those in high-risk groups, which according to the Australia’s National Centre for Immunisation Research and Surveillance includes Aboriginal and Torres Strait Islander people over the age of 15, pregnant women, those over 65 and people with medical conditions that puts them at risk of complications, should get a flu shot.
Vaccines for people in these groups are provided free under the National Immunisation Program.
Kelly and Hampson also advised people who come in contact with high-risk groups be vaccinated.