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Swine flu: to every thing there is a season : Effect Measure
Early returns on what is happening in the southern hemisphere suggest that novel H1N1 is crowding out the expected seasonal strains, something that pandemic strains have usually done. In 1918 there was a pandemic with the H1N1 subtype that settled down as the dominant seasonal flu virus until the "Asian flu" pandemic of 1957 when it was bumped by H2N2. That subtype ruled the seasonal flu roost for only 9 years when a new subtype, H3N2 took its place in the Hong Kong flu pandemic of 1968. Both pandemics were much less severe than 1918 but still resulted in millions of excess deaths globally. In 1977 the pattern was interrupted with the return of H1N1, which didn't take the place of H3N2 but co-circulated with it. Since then we've seen mixtures of H3N2 and H1N1 each flu season, with sometimes one subtype much more prevalent, sometimes the other (a hybrid H1N2 made a sporadic appearance in 2002 but never gained a foothold and didn't represent a novel appearing virus to the population). Sometimes H1N1 and H3N2 would switch places at some point in the flu season, but those two, along with influenza B have made up seasonal influenza since 1977.
Now we have a novel H1N1 added to the mix. While novel H1N1 and seasonal H1N1 have the same subtype designation, the swine origin of the new virus means that the hemagglutinin (H) and neuriminidase (N) proteins on its surface are sufficiently different from the human adapted counterparts that it looks like a very different flu virus to most of the population. Despite its swine origin it is infecting humans and being easily transmitted from person to person. It is also continuing to circulate in the northern hemisphere at a time when most seasonal flu is at a very low level. The seasonal flu strains are essentially gone up north, almost all flu A being novel H1N1. Is this just because seasonal flu went away by itself (and we emphasize, again, the reasons for flu's seasonality remains a mystery)? Will they return, with or without novel H1N1 during next flu season up north?
We are looking to what is happening in the southern hemisphere for some clues. Australia is one of the most hard hit countries and also has a well functioning influenza surveillance system similar to one in the US. And it appears the new virus is pushing out the seasonal strains that were expected to be the main circulating flu viruses:
Tests on 138 type-A flu samples collected by a network of doctors confirmed 60 cases of the new A/H1N1 variant and only five of seasonal influenza in the eight weeks ended June 21, according to a report today by the Victorian Infectious Diseases Reference Laboratory in Melbourne.
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“We are seeing a big increase compared to the same stage of the flu season last year, and the increase is predominantly in 5- to 16-year-olds,” Kerry Chant, chief health officer for New South Wales, said in a statement today. Two children are in intensive care and two other people hospitalized, she said.
The eastern state, Australia’s most populous, has 653 confirmed cases of swine flu, the second highest number in the nation after neighboring Victoria state.
Health officials in Victoria have recorded 1,509 cases from laboratory tests and stopped analyzing specimens from patients with mild disease earlier this month as part of the state’s response to the virus.
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Victoria’s influenza sentinel surveillance network diagnosed flu-like illness in 21.9 of every 1,000 patients seen in the week ended June 21, the report showed. That’s up from 17 per 1,000 a week earlier.
The network comprises 87 general practitioners across the state who test a portion of their patients for flu to give authorities an indication of flu activity in the community.
Unlike seasonal flu, from which the elderly suffer the most death and disease, the new bug is targeting the young and causing potentially fatal complications in otherwise healthy people aged 30 to 50, pregnant women and those with asthma, diabetes and obesity, according to the WHO. (Jason Gale, Bloomberg)
Not only is the virus taking the place of the seasonal strains, it is acting like a pandemic strain usually acts, moving the age distribution to the left (younger age groups). So while the virus is not clinically very different from seasonal flu, it is epidemiologically different.
So what will happen? The most likely outcome in our view is that henceforth flu season will be dominated by a single viral lineage, the current swine origin H1N1. That may take a couple of seasons and during the transition we could have some very unpleasant flu seasons, with lots more illness than usual and increased morbidity and mortality among younger members of the population. That epidemiological difference will make this flu virus seem different and more frightening. But I'm guessing.Flu never does what we expect it to, so there are many other possibilities, some much nastier.
Over at Avian Flu Diary Mike Coston also reflects on this. He's an astute flu observer and always worth reading, no less so this time.
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Cheers,
Brad