H3N2 Viruses – Responsible for the High Rates?

This is an excerpt from the article, “Flu Activity Picks up Nationwide.”  For more information please visit cdc.gov.

CDC tracks influenza activity year-round and publishes a report weekly on Fridays. According to this surveillance, the proportion of people seeing their health care provider for ILI in the United States has been elevated for four consecutive weeks, climbing sharply from 2.8% to 5.6% during that time. Last season, which was relatively mild, ILI peaked at 2.2 percent. Comparatively, during 1998-1999 and 2003-2004, which were moderately severe seasons, ILI peaked at 7.6%. During 2007-2008, another moderately severe season, ILI peaked at 6.0%. During the 2009 H1N1 pandemic, ILI peaked at 7.7%.

h2n2

While the timing of influenza seasons also is impossible to predict, based on past experience it’s likely that flu activity will continue for some time. During the past 10 influenza seasons, ILI remained at or above baseline for an average of 12 consecutive weeks, with a range of 1 week (2011-2012 season) to 16 weeks (2005-2006 season). During the pandemic, the proportion
Twenty-nine states and New York City are now reporting high levels of influenza-like-illness and another 9 states are reporting moderate levels of ILI. Ten states are still reporting low or minimal ILI. (These are California, Connecticut, Hawaii, Kentucky, Maine, Montana, Nevada, New Hampshire, South Dakota and Wisconsin). The District of Columbia and 2 states did not have enough information to calculate an activity level. of visits to doctors for ILI remained above the national baseline for 19 consecutive weeks.

Information about flu-related hospitalizations is collected from 15 states to calculate a rate of laboratory-confirmed influenza-associated hospitalizations. Right now, cumulative influenza hospitalization rates are 8.1 per 100,000 people. According to Bresee, “This is high for this time of year.”

Influenza-associated pediatric deaths have been reportable to CDC since the 2004-2005 season. To date, CDC has received reports of 18 pediatric deaths this season. More information about reported pediatric deaths is available at the Influenza-Associated Pediatric Mortality web application.

One factor that may indicate increased severity this season is that the predominant circulating type of influenza virus is influenza A (H3N2) viruses, which account for about 76 percent of the viruses reported. Bresee explains “typically ‘H3N2 seasons’ have been more severe, with higher numbers of hospitalizations and deaths, but we will have to see how the season plays out.”

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