Flu season continues its spring-like thaw

Melting icicle
Melting icicle

BeyondMyLens / iStockphoto

Flu's winter grip on the nation continued to thaw last week, with a key marker—clinic visits for flulike illness—falling below the national baseline for the first time in 14 weeks, the US Centers for Disease Control and Prevention (CDC) said today.

The clinic visit marker had hugged the 2% baseline level for the past few weeks, but it fell to 1.7% last week, the CDC said. However, the situation varied in different parts of the country, with 4 of the CDC's 10 regions still reporting clinic visit activity above baseline for flu.

Another marker that declined last week was the percentage of respiratory specimens that tested positive for flu, which fell to 8.4% from 8.8% the previous week. The 2009 H1N1 virus is still the dominant strain among subtyped influenza viruses, and the percentage of influenza B viruses detected was 31.3%, down slightly from the week before.

Seven more pediatric flu deaths were reported, bringing the season's total to 75 this season. One was related to the 2009 H1N1 virus, five were linked to unsubtyped influenza A viruses, and one was associated with influenza B. Some of the deaths from unsubtyped flu viruses occurred earlier in the flu season.

The percentage of overall deaths from pneumonia and flu popped above the seasonal baseline last week to 7.1, but it remained well below the epidemic threshold.

This flu season's cumulative rate for hospitalizations was 30.4 per 100,000 people, which is lower than the 41.2 per 100,000 people rate reported at the same point during last flu season, when the H3N2 virus was the dominant strain. However, about 60% of hospitalization this season have been in people ages 18 to 64 years old, an increased level in this age-group that health officials saw the last time that the 2009 H1N1 was the dominant strain—during the pandemic of 2009-10.

No states reported high flu activity, one of the measures the CDC uses to gauge clinic visits. That number was down from two states the previous week. Texas was the only state to report moderate flulike illness activity.

On the geographic spread scale, only four states were listed as widespread: Connecticut, Massachusetts, New Jersey, and New York. That number was the same as the previous week.

Antiviral resistance testing programs found three more 2009 H1N1 specimens that were resistant to oseltamivir (Tamiflu), raising the season's total so far to 43.

Low to moderate levels in Europe

In European countries, flu activity remained at moderate- to low-intensity levels, with stable or decreasing patterns in most countries, according to the latest update today from the European Centre for Disease Prevention and Control (ECDC).

Increasing trends were reported by six countries: Austria, Croatia, the Netherlands, Poland, Romania, and the United Kingdom (Northern Ireland).

The region is still seeing co-circulation of H3N2 and the 2009 H1N1 virus, a different pattern than seen in North America, where the 2009 H1N1 virus has been the dominant strain all season.

The percentage of respiratory specimens that tested positive for flu was 36%, up from 32% the previous week.

See also:

Mar 21 CDC weekly FluView report

Mar 21 CDC situation update

Mar 21 ECDC weekly influenza update

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