Although the new influenza H1N1 strain has been receiving a great deal of attention, apparently last season’s H1N1 and H3N2 strains continue to circulate, at least in the United States. FluView, the weekly surveillance report from the CDC, indicates that during weeks 17 and 18, seasonal influenza A (H1), A (H3), and B viruses are co-circulating with novel influenza A (H1N1). Half of all influenza viruses detected are the swine-like H1N1 viruses. The statistics are shown in this bar graph:
Seasonal influenza in the US peaked in week 6 (February) and declined by week 16. With the emergence of the new H1N1 strains, there came an upsurge in influenza activity during weeks 17 and 18.
Two aspects of these data are noteworthy. First, it is very interesting that influenza viruses of three different subtypes – H3N2, and two H1N1 – are circulating at the same time. On at least two previous occasions, the seasonal influenza strain was replaced by the new pandemic virus. In 1957, when the H2N2 strain emerged, it replaced the previously circulating H1N1 strain. Then in 1968, the H2N2 strain was replaced by the H3N2 strain. The reasons for the replacement of influenza virus strains during these two episodes of antigenic shift remain obscure. Should we expect the new H1N1 strain, if it goes on to be the pandemic strain, to replace previously circulating human influenza A viruses? When the H1N1 influenza virus re-emerged in 1977, it did not replace the H3N2 viruses – although it did not go on to be the next pandemic strain. Whether or not the new H1N1 viruses will dominate is anyone’s guess. Clues might come from which strains are isolated in the southern hemisphere in the coming weeks.
I am also intrigued by the observation that circulation of last season’s H3N2 and H1N1 strains, nearly over by week 16, has suddenly been revived along with introduction of the new H1N1 strain. Put another way, why is there a general upsurge in influenza cause by all three A subtypes? Perhaps it’s just an unusual outbreak late in the season, which has certainly happened in the past, only this time, there is a new virus thrown into the mix.