US flu activity ebbs further as influenza B surges

Mar 1, 2013 (CIDRAP News) – Most US influenza activity indicators showed a continued decrease last week as influenza B for the first time this season became the predominant strain, according to new data presented today by the Centers for Disease Control and Prevention (CDC).

Of 7,609 respiratory specimens tested by reporting labs, 16.9% were positive for flu, about the same as the previous week, when 16.8% tested positive.

The mid-Atlantic states of Delaware, Maryland, Pennsylvania, Virginia, and West Virginia, plus Washington, DC, were responsible for that measure staying level. That area saw 28.6% of samples testing positive for flu, up from 25.9% the week before. It contributed by a good measure the most samples to the CDC database, 6,630, and was the only one of 10 US regions reporting an increase.

Flu was reported as widespread in only 12 states, down from 22 the week before. The proportion of doctors' visits for influenza-like illness was 2.7%, above the national threshold of 2.2% but down slightly from the previous week's level of 2.8%.

The proportion of deaths attributed to pneumonia and flu was 8.4%, down from 8.6% the week before but above the epidemic threshold of 7.5%. Three new pediatric deaths were reported to the CDC, raising the total for the season to 81.

The cumulative rate of influenza-associated hospitalized nudged up slightly in today's report, from 34.2 to 36.7 per 100,000 population. The percentage of flu-related hospitalizations in the elderly also edged up, from more than 50% to more than 51%.

For the first time this year influenza B outpaced influenza A among US specimens tested. Of the 1,288 specimens that were positive for flu, 683 (53.0%) were influenza B and 605 (47.0%) were influenza A. In the previous week influenza B accounted for 45.1% of specimens testing positive for flu.

H3N2 continues to dominate 2009 H1N1 (pH1N1) among the influenza A strains, accounting for 86% of A strains that were subtyped.

Flu also continued to slow in Canada and Mexico, according to a Feb 26 report from the Pan American Health Organization (PAHO). The agency also reported declining respiratory disease in Central American and the Caribbean, while South America was experiencing activity "within the expected range for this time of year."

In Europe, 19 of 28 nations reported high- to medium-intensity flu transmission and wide geographic spread, but only 6 countries reported increasing trends last week compared with 11 in the previous week, the European Centre for Disease Prevention and Control (ECDC) reported today.

The overall proportion of influenza-positive specimens remained high at 50%, but it decreased from last month's peak, the agency said. The continent is experiencing a 50-50 split between influenza A and B, and pH1N1 accounted for 60% of influenza A strains that were subtyped.

See also:

Mar 1 CDC weekly FluView report

Feb 26 PAHO update

Mar 1 ECDC update

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