While addressing some comments from Ralph Baric on one chapter my book, I had reason to go find statistics on influenza subtype activity last year. Those interested in keeping up on recent flu activity should peruse this July, 2008, report from the CDC: Influenza Activity --- United States and Worldwide, 2007--08 Season.
Here is the breakdown on subtype activity:
During September 30, 2007--May 17, 2008, World Health Organization and National Respiratory and Enteric Virus Surveillance System collaborating laboratories in the United States tested 225,329 specimens for influenza viruses; 39,827 (18%) were positive. Of the positive specimens, 28,263 (71%) were influenza A viruses, and 11,564 (29%) were influenza B viruses. Among the influenza A viruses, 8,290 (29%) were subtyped; 2,175 (26%) were influenza A (H1N1), and 6,115 (74%) were influenza A (H3N2) viruses. The proportion of specimens testing positive for influenza first exceeded 10% during the week ending January 12, 2008 (week 2), peaked at 32% during the week ending February 9, 2008 (week 6), and declined to <10% during the week ending April 19, 2008 (week 16). The proportion positive was above 10% for 14 consecutive weeks. The peak percentage of specimens testing positive for influenza during the previous three seasons ranged from 22% to 34% and the peak occurred during mid-February to early March. During the previous three influenza seasons, the number of consecutive weeks during which more than 10% of specimens tested positive for influenza ranged from 13 to 17 weeks.
Of note, 26% of samples positive for influenza were the H1N1 subtype -- the same as the 1918 flu -- which means we all have probably been exposed to it and have some immunity. That does not mean the particular combination of genes in the 1918 flu would be harmless if it showed up again, but rather than our immune systems should be able to better recognize that bug and thus might defend against it better than the first time around.