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Why does flu keep coming back?
Flu is unique among human diseases. It circulates constantly in cool, dry areas of east Asia, conditions the virus prefers, but when temperatures drop during the northern and southern winters, it breaks out and begins a tour of the relevant hemisphere. Because it spreads from person to person efficiently in exhaled droplets, and can be picked up from contaminated surfaces, nearly everyone is exposed.
Winter flu: All the essential facts you need to know
And unlike, say, measles, having flu once doesn’t make you immune to catching it. The virus is uniquely talented at dodging our immune systems. The big haemagglutinin protein on its surface gets most of your immune system’s attention, and this protein constantly mutates at seven hotspots. Every few years it racks up such a number of mutations that many antibodies you made to your last infection don’t recognise the virus, and you get sick again.
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You still have some immunity to kinds of flu that are only a little different from viruses you have seen before, which is why much winter flu isn’t as severe as flu can be. The strains best able to evade this kind of prior immunity dominate the annual epidemic in each hemisphere, so we only need one vaccine per season – but a new one each year.
Why are people worried this year?
A record number of flu strains are currently circulating, two in the influenza B group and two influenza A strains, H1N1 and H3N2. H3N2 is the real problem. Our strongest immunity is to the first kind of flu we caught. Between 1918 and 1968, no H3N2 viruses circulated as winter flu, so .
That includes elderly people, whose faltering immune systems make them more vulnerable. There are , amounting to This year’s H3N2 seems to be especially severe: in Australia in the winter just past, it caused more than three-quarters of all flu cases (see diagram), and more than . The likelihood of dying was relatively high, with most deaths – although not all.
“We don’t really know what makes some winter flu viruses more severe than others,” says Colin Russell at the University of Amsterdam. It’s a mix of the virus’s inbuilt weaponry to defeat our immune system and our system’s ability to recognise and respond to it fast enough. With this year’s H3N2, it could be either or both, says Derek Smith at the University of Cambridge – virologists can’t yet tease all the variables apart.
What is flu?
Flu is a small virus with just 11 genes made of RNA. One type, influenza B, infects only humans, and two strains of it are circulating this year: Yamagata and Victoria. The other common type of flu is influenza A. Its many strains mostly, and in large part harmlessly, infect waterfowl, but three varieties have adapted to humans. Those three plus a few more infect other mammals, notably pigs.
Influenza A strains are named after their two main surface proteins, haemagglutinin (H) and neuraminidase (N). There are 18 types of H and 11 of N in bird viruses, and immune-system antibodies that attack one type don’t recognise another. Only viruses H1N1, H2N2 and H3N2 have fully adapted to humans, and only H1N1 and H3N2 currently circulate in us. People sometimes catch other strains of bird flu, for example H5N6, but they cannot spread.
Dominant strains of both A and B circulate together in the northern and southern hemisphere winters, infecting up to half of all people and causing disease in between 10 and 15 per cent. Influenza A is of extra concern because new viruses, or just viral genes, sometimes jump from birds to humans and the novel virus can cause an especially severe pandemic flu.
This article appeared in print under the headline “Winter flu: Everything you need to know”
