As week 46 of 2009 comes to a close, the Centers for Disease Control and Prevention reports that influenza has peaked in the US. That conclusion is based on the agency’s influenza surveillance program, summarized in this figure:
Does this mean that pandemic influenza is over? Absolutely not. This is just the second wave, sparked when school began in the fall. Recall the the first wave of H1N1 infections that took place during the spring and summer:
It’s interesting to note that seasonal H1N1 and H3N2 strains are nearly gone. Of the 10,803 specimens tested by the CDC during week 45, 3,106 were confirmed as novel H1N1, one was seasonal H1N1, and no H3N2 strain was detected.
There will be more influenza to come in the winter. A catalyst might be increased travel as we come upon the holiday season. This NY Times article has a good summary of the current state of influenza and what we might expect.
Note added a few days later: Despite dire predictions to the contrary, the 2009 swine-origin H1N1 influenza virus has not mutated to increased virulence from the first to the second wave of infection. As I’ve written before, there is no evidence from any influenza pandemic that viral mutants of increased virulence in humans have emerged in successive cycles of infection.