US flu activity remains high

Feb 18, 2011 (CIDRAP News) – Flu activity in the United States remains at high levels, with all areas of the country now reporting that doctor's visits for flu-like illnesses are above regional baselines, the US Centers for Disease Control and Prevention (CDC) said today.

Thirty-seven states reported widespread flu activity last week, the same as the previous week's total. Ten states reported regional activity, up one from the previous week.

The proportion of respiratory samples that tested positive for flu rose last week to 35% from 31.7% the week before. Overall the H3N2 influenza virus reclaimed its place as the dominant flu strain, a week after the 2009 H1N1 strain edged it out by a slim margin.

Of 3,306 positive flu specimens, 78% were influenza A, of which 39% were H3N2, 28.5% were 2009 H1N1, and 30.9% weren't subtyped. The proportion of influenza B viruses was 21.4%, down slightly from the week before. The CDC said the predominant virus has varied by week, region, and even between states in the same region. However, over the last few weeks it has seen an increasing proportion of 2009 H1N1 viruses.

In an update on the flu season today in Morbidity and Mortality Weekly Report (MMWR), the CDC said the largest numbers of influenza B specimens in November and December came from the southeastern region.

So far all of the influenza viruses that the CDC has tested for antiviral resistance have been sensitive to oseltamivir (Tamiflu) and zanamivir (Relenza).

Hospitalizations related to lab-confirmed flu that the CDC tracks through its FluSurv-NET system continued to rise, with the highest rates still occurring in patients younger than 5 and those age 65 and older.

The share of deaths attributed to flu and pneumonia rose sharply last week to 8.9%, from 8% the previous week. The epidemic threshold is 8%.

Of five pediatric flu deaths that were reported, three were linked to the 2009 H1N1 virus, one was from influenza B, and the other was from an undetermined subtype. In the MMWR report, the CDC said more than half of the pediatric flu deaths reported this season occurred after Jan 16.

Overall, the percentage of doctor's visits for flu-like illness last week dropped slightly to 4.5% from 4.6% the week before. The level is still well above the national baseline of 2.5%.

In the MMWR article, the CDC said flu activity can be unpredictable but often peaks in February. The agency said substantial activity can occur as late as May, and it encouraged healthcare providers to continue offering the vaccine to people age 6 months and older who haven't been immunized yet.

See also:

Feb 18 CDC weekly flu surveillance report

Feb 18 MMWR report

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