I’ve been wondering for some time why seasonal flu is apparently being suppressed by H1N1. Much of the reporting I’ve seen notes the fact but doesn’t seem to get to the bottom of why it’s happening. Is that because doctors and public-health officials themselves aren’t sure? Are there at least plausible working theories?
This Washington Post article hints at one possible factor:
When a person is infected with one respiratory virus (such as rhinovirus, which causes colds), the chance of catching a different virus (such as flu) declines greatly. Part of the reason is that the first infection provokes what’s called “innate immunity” — a flood of interferon and other cellular hormones that defend the body in a general way without specifically targeting the invader. That protection can last weeks, breaking chains of transmission and slowing a flu epidemic.
A similar form of interference occurs between strains of flu, which is one of the reasons there’s been almost no “seasonal flu” in recent months. The strains circulating last season and still occasionally found this season — H3N2, other forms of H1N1 and influenza B — have all been outcompeted by the upstart H1N1.
In fact, even if there isn’t a third wave, the new H1N1 may well spell the end of one or more of the families of flu virus that have been circulating for decades. That’s what’s happened in previous pandemics, at least.
The H1N1 family arrived in 1918 with the Spanish flu. In the 1957 pandemic, the new virus was in the HN2 family; it drove all H1N1 strains to extinction. In 1968, the new virus was an H3N2. It spelled the end of the H2N2 family, which disappeared. H1N1 returned in 1977, apparently the result of an accidental release from a laboratory in Russia or China.
Have you seen/heard/read any good pieces about intra-flu-strain competition? It seems central to H1N1’s story arc at the moment. The questions about H1N1’s dominance — and the possible answers — could be interesting to keep in mind as you continue reporting on swine flu and seasonal flu.
