Flu activity is continuing to pick up in the U.S., settling into a more typical pattern following the emergence of the H1N1 influenza virus in 2009.
Unlike the unusual 2009-2010 flu season, which saw extensive disease activity from August to October almost entirely from the pandemic H1N1 strain with little through the winter, this season appears to be following the pattern of most previous years, according to Dr. Dan Jernigan, deputy director of the CDC's influenza division.
Flu activity, which was high in New York City and three states in the southeast as of December 25 , is expected to increase and peak in late January following gatherings over the holidays and children's return to school, Jernigan told MedPage Today.
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"Flu is now with us," he said. "All the indexes that we follow indicate that the season has started."
Most of the circulating viruses (70 percent) are influenza A strains, and the vast majority of those are H3N2, with very little 2009 H1N1, Jernigan said. The other 30 percent are influenza B strains.
All testing so far has indicated that the three strains included in this year's trivalent vaccine are well matched to those that are circulating, Jernigan said.
It is not too late to get the vaccine, and getting vaccinated is particularly important, he said, because the predominant strain, H3N2, has a propensity to cause more severe outcomes, and thus, more deaths in the very young and people older than 65. He pointed to the 2007-2008 flu season, another in which H3N2 was dominant, when there was a spike in pneumonia and influenza mortality.
"That's why we really wanted to push the vaccine earlier this year and still are pushing," he said.
For the week ending December 25, representing the most recent data, New York City and the states of Georgia, Alabama, and Mississippi already had high levels of influenza activity.
Jernigan said all of the major indicators CDC follows show that activity is increasing -- 21 percent of specimens tested were positive for influenza, the proportion of outpatient visits for flu-like illness is above the national baseline (2.7 percent versus 2.5 percent), and the percentage of deaths attributed to pneumonia and influenza is at the epidemic threshold for this time of year (7.5 percent).
The epidemic threshold is "the point at which the observed proportion of deaths attributed to pneumonia or influenza was significantly higher than would be expected at that time of the year in the absence of substantial influenza-related mortality," according to the CDC website.
With H3N2 causing most of the illnesses, the age groups typically suffering more severe outcomes from flu -- the very young and the very old -- are expected to bear the brunt of the effects this season as well, Jernigan said.
The H1N1 pandemic, in contrast, was relatively mild in the older population, possibly because of previous exposure to similar viruses.
Jernigan said it is unlikely that the 2009 H1N1 strain will cause widespread illness again unless it changes substantially, because there is immunity built up through natural infection and vaccination once a new virus has moved through a population.
It is estimated that at least half of the U.S. population has been exposed to the pandemic strain one of those two ways.
The pandemic strain has not changed significantly, and Jernigan said that will likely not occur until other parts of the world build up the same immunity to the virus as the U.S.
He noted that the U.K. and other parts of the world are still having widespread H1N1 activity this season because they did not have as much disease -- or vaccination rates as high -- as in the U.S. during the pandemic.
"Once it's burned through that population, then the virus will have to do some changing in order for it to come back. And that's what influenza viruses do -- they constantly change," Jernigan said.
"So we would anticipate that 2009 H1N1 will be with us for a while and each year it will have some slight changes to it that will make us have to change our vaccine, just like we do for all past influenza viruses each year."