Flu Activity Peaked in February, with Influenza A Viruses Predominating

In the most recent Morbidity and Mortality Weekly Report, the Centers for Disease Control and Prevention (CDC) provides a summary of influenza activity in the U.S. for the 2010-2011 season.

According to the CDC, influenza activity peaked in early February, and influenza A (H3N2) viruses predominated. The proportion of influenza B viruses reported was highest early in the season, with the majority of these viruses reported from the southeastern states, and 2009 influenza A (H1N1) viruses became the most common in several regions in the later part of the season. Almost all influenza viruses sent to CDC for further characterization were antigenically similar to one of the components of the 2010-2011 Northern Hemisphere vaccine.

In comparison with the past three seasons, the 2010-2011 influenza season was less severe than the pandemic year (2009-2010) and the 2007-2008 season, but more severe than the 2008-2009 influenza season, as determined by the percentage of deaths resulting from pneumonia or influenza, the number of influenza-associated pediatric deaths reported, and the percentage of visits to outpatient clinics for ILI. However, all age groups were affected substantially during the 2010-2011 season because of widespread cocirculation of influenza A (H3N2), 2009 influenza A (H1N1), and influenza B viruses.

Hospitalization rates overall were similar to rates reported during the 2007-2008 influenza season, when influenza A (H3N2) was the predominant strain. Cumulative hospitalization rates reported for the 2010-2011 season for persons aged <65 years were lower than rates reported during the pandemic, but the rates reported in these age groups were higher than those reported during the 2007-2008 season. However, the cumulative hospitalization rate for persons aged 65 years was lower than during the 2007-2008 season, but higher than the rate reported during the pandemic.