US flu activity continues to decline

Apr 5, 2013 (CIDRAP News) – As attention shifts to China and a novel H7N9 strain of avian flu, seasonal influenza in the United States continued to ebb, the US Centers for Disease Control and Prevention (CDC) reported today.

The proportion of outpatient visits for influenza-like illness (ILI) held steady at 1.8% last week, the CDC said in its weekly update, a level below the national baseline of 2.2%. But the percentage of respiratory specimens that tested positive for influenza dropped to 11.3% from 13.2% the week before and 16.3% the week before that.

The proportion of deaths attributed to pneumonia and influenza fell to 7.4%, below the epidemic threshold of 7.5%, which is where the rate stood the week before.

Three of the country's 10 regions were above baseline levels for outpatient ILI, the same as the previous week. Only 4 states reported widespread flu activity, while 7 had regional activity; 26 states, the District of Columbia, and Puerto Rico experienced local activity; and 12 states saw sporadic activity.

One pediatric flu death was reported to the CDC, raising the season's total to 111. The death was caused by an influenza A virus that was not subtyped.

Of 555 specimens that tested positive for flu last week, 75% were influenza B and 25% influenza A. The B-strain prevalence is a common occurrence late in the flu season. Of the A strains that were subtyped, H3N2 outpaced pandemic 2009 H1N1 (pH1N1) viruses more than two to one.

Over the course of the season, however, H3N2 has dominated. Of 1,970 viruses antigenically characterized since last October, 1,158 (59%) have been H3N2, 201 (10%) pH1N1, and 609 (31%) influenza B.

Flu activity elsewhere
Influenza activity has also tapered in Canada and Mexico, according to an Apr 3 update from the Pan American Health Organization (PAHO). No region in Canada reported widespread flu activity, where influenza B detections have increased. Mexico is reporting that more than 80% of isolates in recent weeks are H3N2, the agency said.

South America is reporting increased respiratory infections but the numbers are within limits for this time of year, PAHO said.

In Europe, all countries are reporting declining flu levels or a return to baseline, the European Centre for Disease Prevention and Control (ECDC) said today in its weekly update.

"After more than 3 months of active transmission, a long period compared to other years, the 2012/13 influenza season is waning and slowly moving towards its close," the agency said.

Of 26 countries reporting, 19 had medium-intensity and 7 low-intensity flu activity. Bulgaria was the only nation to report increased activity, and no country reported high-intensity activity.

The proportion of influenza-positive specimens in Europe remained high at 40% but continued to decline, the ECDC said. For the season, 47% of specimens from sentinel European labs have been influenza A and 53% influenza B. Of the A strains, 63% have been pH1N1, in contrast to the situation in North America.

See also:

Apr 5 CDC update

Apr 3 PAHO update

Apr 5 ECDC update

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