Every winter I can count on doing at least one story about the extent of seasonal influenza. This year? Nothing, nada, zilch. I can’t remember the last time this happened.
So what gives? It seems that while the H1N1 novel influenza virus was capturing all the attention last spring and fall, the seasonal flu viruses decided to take a break.
It’s not that seasonal flu has been entirely absent. Although the incidence of the usual influenza viruses that circulate each winter has been noticeably lower, cases are still occurring. The virus also still has the ability to make some people very sick. Minnesota has had two confirmed deaths this past winter from seasonal flu; the most recent death was last month. So just because we haven’t seen much seasonal influenza doesn’t mean we can stop taking it seriously.
The behavior of flu viruses, the CDC reminds us, is notoriously difficult to predict:
Although influenza activity has declined recently, additional waves of influenza activity due to 2009 H1N1, seasonal influenza viruses, or both, may occur later this influenza season. Flu season can last as late as May. Even if the U.S. doesn’t experience a sharp increase in influenza activity during the remaining winter or spring (another "wave" of influenza), continued low level circulation of influenza viruses may continue during this time.
In fact, if you’ve been following the news, you may have recently seen rumblings from the state of Georgia about a possible new spike in the number of H1N1 cases.
It’s interesting to note how the emergence of the 2009 N1N1 novel influenza virus may have altered the pattern we usually see with seasonal flu. From the science blog community comes this intriguing post, written way back last September, that explores the theoretical workings of these dynamics, based on a small study of the transmission of flu viruses among ferrets. Researchers at the University of Maryland found that even when the ferrets were infected with both H1N1 and seasonal viruses, it was the H1N1 virus that was passed on to other animals:
So if co-infection is possible, how is it that swine seems to be crowding out the seasonal viruses? In this set-up, the evidence suggests it is happening at the level of transmission. If the animal is infected with swine and seasonal virus, it can infect other animals but in this case only with the swine flu virus. There was transmission from inoculated animals to contact and respiratory droplet animals caged with them (as described), but the transmitted cases were only swine virus. In other words, although an inoculated animal was infected with two viruses, only the swine virus is passed on. How this works we don’t know yet.
It’s a pretty big leap, of course, from influenza among ferrets to influenza among humans. There’s a lot we don’t understand yet about how various flu viruses interact with each other and how factors such as virulence and immunity can influence how easily a particular virus spreads. It just goes to show, however, how tricky and complicated these microbes are – and why I’m not sorry I took the time last fall to get vaccinated against both seasonal flu and the H1N1 virus.