Flu Season Just Beginning, but Could Ramp Up Quickly

The geographic spread of actual flu cases presents a slightly different picture, according to the CDC. From that perspective, Colorado and California had widespread flu activity, 13 other states had regional activity, and 20 states had local activity. He called this season's pattern "unusual but not unprecedented." In the past 35 years, the flu season has peaked in March four times and in April twice, "so we can't predict the timing of activity nor when the season will end," he said. He added that those who haven't received a flu shot should still get one.

Most of the viruses characterized in the United States this season have been well matched to the vaccine, he said. "Slightly below 50% of the B viruses identified in the United States this season have been similar to the component in the vaccine," but very few B viruses have circulated, he reported. He also noted that the WHO mentioned the option of including two B strains in next season's vaccine, given the perennial difficulty of predicting which of the two B lineagesVictoria and Yamagatawill be more common. Today's CDC weekly flu update shows that U.S. flu activity is still low as measured by the number of outpatient medical visits for influenza-like illness (ILI), and only one state reported high ILI activity last week. He said all three flu subtypes are circulating in the United States, but H3N2 viruses have made up more than 70% of the isolates identified, and type B viruses have been very few.

"The timing of influenza and how severe it is each year is highly variable and hard to predict," he said. "One factor is that the viruses circulating this year are quite similar to the viruses last year, so possibly there is high immunity to the viruses in the population." He said. In response to a question, he said the unusually mild winter may also be a factor: "We do know that in cold, less humid times, flu viruses remain more viable for longer, and in cold weather people tend to cluster inside more," where they can share infections.

Influenza activity has remained low for the 2011-2012 influenza season, with a slight increase in the first week of February, according to Lisa Grohskopf, MD, MPH, who presented data during this week’s Advisory Committee on Immunization Practices meeting.

Grohskopf, associate chief for policy and liaison activities for the CDC’s Epidemiology and Prevention Branch, Influenza Division, told committee members that influenza A has predominated this season, accounting for 90% of the more than 78,000 respiratory specimens tested. Of the influenza A viruses that were subtyped, 84% were influenza A (H3N2), but influenza A (H1N1)pdm09 and influenza B viruses have been detected.

Just why the US influenza season has been so tame thus far is uncertain, but the possible reasons include mild weather, vaccination, and high population immunity owing to a fairly stable set of circulating viruses, a Centers for Disease Control and Prevention (CDC) official said today.