Moderate flu activity persists in parts of US, world

Mar 19, 2012 (CIDRAP News) – Influenza activity remained elevated in Midwestern US states but elsewhere was at low levels, while some Northern Hemisphere countries reported moderate flu activity, according to the latest US and international reports.

Although the percentage of respiratory specimens testing positive for flu rose to 23.2% in the United States—up from 21.3% the week before—the proportion of outpatient visits for influenza-like illness (ILI), at 2.2%, was still below the national baseline of 2.4%, according to a Mar 16 update from the Centers for Disease Control and Prevention (CDC).

For the week that ended Mar 10, two of the CDC's 10 regions had elevated levels of outpatient ILI: region 5, comprising Illinois, Indiana, Michigan Minnesota, Ohio, and Wisconsin, and region 7, which includes Iowa, Kansas, Missouri, and Nebraska. Those regions reported that 46.3% and 34.0%, respectively, of respiratory specimens tested positive for flu, highest in the nation.

Fifteen states reported geographically widespread influenza activity: California, Colorado, Illinois, Iowa, Kansas, Kentucky, Maryland, Michigan, Minnesota, Missouri, Nevada, New York, Oklahoma, Pennsylvania, and Virginia. Twenty-two states confirmed regional flu activity, 3 states reported local activity, and 10 states, the District of Columbia, and Puerto Rico had sporadic activity.

States reporting high ILI activity were Alabama, Arkansas, Kansas, Missouri, and Oklahoma. Illinois reported moderate activity, and 11 states reported low activity. Thirty-three states and New York City logged minimal ILI activity.

Of the 1,099 specimens that tested positive during the week, 95.5% were influenza A and 4.5% influenza B. Of the influenza A specimens that were subtyped, 394 (69.6%) were H3 (H3N2) and 172 (30.4%) were pandemic H1N1 2009 (pH1N1).

Although the CDC has antigenically characterized only a small number of influenza B viruses this flu season, 44.9% (40/89) have matched the B strain in the vaccine. The circulating A strains have matched the vaccine strains well, the agency said.

Global activity
Globally, flu activity increased in Canada, northern China, and several European countries, the World Health Organization (WHO) said in its weekly flu update on Mar 16.

In Canada, consultation rates for ILI increased from 31.3 to 38.6 per 1,000 in the last week of February, and the proportion of clinical specimens testing positive for flu increased from 10.5% to 17.9%. Institutional outbreaks also "increased markedly" in the last 2 weeks of February, the WHO said.

Canada recorded influenza B in 46.6% of positive flu specimens, up from 36.5% at the beginning of the season and much higher than in the United States.

Meanwhile, the proportion of samples testing positive for flu in Mexico fell from 30% to 25% at the beginning of March, the lowest level in 2012. Unlike most of the world, Mexico has reported a predominance of pH1N1 viruses.

In Europe, most countries are reporting level or increasing ILI activity, except for Albania, Bulgaria, Italy, and Spain, which confirmed decreasing activity, the WHO said in its Mar 16 update.

But in an update today, the European Centre for Disease Prevention and Control (ECDC) said that the continent overall reported decreasing activity. The agency said 43.2% of 1,433 specimens tested positive for flu, the second consecutive weekly drop after a peak of 46%.

Of the viruses collected at sentinel sites in Europe, 85.1% were type A and 14.9% were type B, according to the ECDC, which is the highest proportion of type B viruses yet this season.

H3N2 has been confirmed in 98.8% of all specimens positive for influenza A in Europe this flu season, according to the WHO, much higher than elsewhere in the world. The ECDC reported that circulating H3N2 strains this season are an "imperfect fit" for the H3N2 vaccine strain.

Elsewhere, northern China reported that ILI visits increased slightly, according to the WHO, to more than 3% of all doctors' visits, but remained below January's peak. ILI rates in Mongolia also increased, exceeding the national alert threshold.

Flu levels have peaked and are now declining in North Africa, the eastern Mediterranean, sub-Saharan Africa, tropical Asia, and elsewhere in the tropics, the WHO said.

In related news, the WHO reported Mar 16 in its Weekly Epidemiological Record on antigenic and genetic characteristics of animal-borne flu viruses. It recommended no new H5N1 avian flu vaccine strains.

See also:

Mar 16 CDC weekly flu activity report

Mar 16 WHO flu update

Mar 19 ECDC weekly flu update

Mar 16 Wkly Epidemiol Rec report

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