US flu continues slow rise with plenty of flu B activity

Flu activity increased again last week, and though illnesses are at relatively low levels, all three strains are circulating, including plenty of influenza B, especially in the southern states, the US Centers for Disease Control and Prevention (CDC) said today in its weekly update.

Typically, most influenza B activity occurs toward the end of the flu season, but last season saw very little influenza B activity and was marked by two waves of influenza A, the first from 2009 H1N1 and the second from H3N2.

Maps show a few hot spots

Nationally, flu markers that the CDC uses to track activity are still below their baselines, but for two regions, the percentage of clinic visits for flulike illness have reached their baselines. One is in the southwest in a region that includes Arkansas, Louisiana, New Mexico, Oklahoma, and Texas. The other is in the central part of the country in a region that includes Iowa, Kansas, Missouri, and Nebraska.

The flulike illness activity map, which also tracks outpatient visits, shows high activity levels for Louisiana and Puerto Rico, low activity for nine states—most of them in the southeast—and minimal activity elsewhere.

On the geographic spread map, two states report widespread activity: Louisiana and Maryland. Alabama is the only state that reported regional spread.

Flu strains vary by region

At clinical labs last week, 3.2% of tests on respiratory specimens were positive for flu, up from 2.4% the previous week.

Of flu positives at clinical labs, 75.4% were influenza B and 24.6% were influenza A. So far, 63% of the flu viruses and 75% of the influenza B positives this season have been from the south and southeast regions.

At public health labs, testing has found slightly more influenza B than influenza A. Of subtyped influenza A viruses last week, 58.5% were H3N2 and 41.5% were 2009 H1N1. "The predominant virus varies by region," the CDC said, noting circulation of all three strains.

No new pediatric flu deaths were reported last week, keeping the season's total at two.

The CDC said it's too early to characterize the timing of the flu season or predict what strains will dominate and how severe illnesses will be. It urged people to get vaccinated.

See also:

Nov 8 CDC FluView report

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