US flu markers show further signs of decline

Mar 15, 2013 (CIDRAP News) – Flu activity in the United States last week continued its slow retreat, with more areas of the country returning to regional baselines and fewer respiratory samples testing positive for the virus.

In its weekly update, the US Centers for Disease Control and Prevention (CDC) said, however, that one key marker increased slightly—the percentage of outpatient visits for flulike illness, which rose from 2.3% to 2.6% last week, keeping it above the national baseline.

The percentage of respiratory swabs that were positive for flu dropped from 17.2% to 14.3% last week. In the past few weeks, influenza B edged out H3N2 as the dominant strain, which isn't unusual for late in the season. The CDC said influenza B was more common than influenza A in all 10 of its regions.

Antiviral resistance testing detected an H3N2 sample that was resistant to oseltamivir (Tamiflu). Earlier testing found two 2009 H1N1 samples that were resistant to the drug.

The level of flu-related hospitalizations rose slightly again last week, from 38.5 per 100,000 population to 39.6 per 100,000. Seniors are still the hardest-hit group.

Twelve more pediatric flu deaths were reported to the CDC last week, pushing the season's total so far to 99. Nine, reported between Nov 24 and Mar 9, were linked to influenza B.

The marker for overall deaths from pneumonia and flu, which has been running well above the epidemic threshold for several weeks but falling recently, declined slightly to 7.6%, nearly reaching the threshold of 7.5%.

Only eight states still reported wide geographic spread of flu, down from nine the week before. States still reporting wide spread are located in the northeastern and western parts of the country.

In other parts of North America, flu activity in Canada continued to drop below expected levels, according to a Mar 13 update from the Pan American Health Organization (PAHO). As in the United States, influenza B detections are increasing and flu hospitalization rates are highest in people aged 65 and older.

In Mexico, H3N2 is still the dominant strain, according to the PAHO report.

Meanwhile, flu is still causing substantial illness in Europe, but markers continue to decline, according to a weekly surveillance update from the European Centre for Disease Prevention and Control (ECDC). The percentage of respiratory specimens that tested positive for flu last week was 54%—still high, but down from the peak of 61% earlier this year.

Finland and Germany were the only countries that reported high-intensity activity, and Portugal and Wales reported increasing trends.

Of European flu virus detections last week, 52% were influenza B and 48% were influenza A. Testing during the previous week showed an even split between the two virus types.

At the global level, the 2009 H1N1 virus is the dominant strain, followed by H3N2 and influenza B, the World Health Organization (WHO) said today in a virologic update.

WHO said southern China is seeing increased activity from the 2009 H1N1 flu strain. Sporadic flu detections continue to be reported from Africa, with influenza B as the dominant strain.

Flu activity in the Southern Hemisphere remains low, according to the WHO. The flu season there generally runs from May through October.

See also:

Mar 15 CDC weekly flu update

Mar 13 PAHO influenza situation report

Mar 15 ECDC flu surveillance report

Mar 15 WHO virological update

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