US flu activity continues slight drop

Influenza virus
Influenza virus

CDC influenza virus illustration

US influenza activity remained high but continued a slight decrease last week, the Centers for Disease Control and Prevention (CDC) said today in its weekly update.

For the second straight week, most key indicators are down, and the share of medical visits for influenza-like illness (ILI) and the percentage of respiratory samples that tested positive for flu were down for the third straight week.

But pediatric deaths were up a bit, and California is reporting marked increases in flu-related deaths in adults as the 2009 H1N1 strain continues to dominate nationwide. Compared with other seasonal strains, 2009 H1N1 appears to cause higher levels of severe illness in working-age adults.

Key indicators down

Today's FluView report from the CDC, which covers Jan 19 through Jan 25, said that 21.1% of 9,514 respiratory samples were positive for influenza, down slightly from 23.1% the week before. The percentage of samples positive for flu last week varied from 19.6% to 36.7% in the CDC's 10 US regions.

ILI accounted for 3.3% of visits to sentinel healthcare providers, down just a bit from 3.4% the week before and well above the seasonal baseline of 2.0%. Ten states reported high ILI activity, down from 13 the week before, while 12 states and New York City noted moderate activity, compared with 7 states a week earlier.

In terms of geographic spread, 38 states reported widespread flu, down from 41 the previous week. Ten states reported regional activity, which is the next-highest level.

The CDC reported 9 new pediatric flu deaths last week, raising the season's total to 37. The previous week saw 8 flu-associated deaths in kids. Of the 9 new deaths, 4 were attributed to 2009 H1N1, 1 to influenza B, and 4 to an "A" strain that was not subtyped. By this time last year, 45 pediatric flu deaths had been confirmed.

Also today, the California Department of Public Health (CDPH) said the state's number of confirmed flu-related deaths has increased by 52 in the past week, to a total of 147 confirmed deaths for the season. All but 4 of the 147 deaths are in adults, and state officials are investigating another 44 deaths that may also be flu-related.

The state saw an increase of 50 flu deaths the week before. California had 106 flu-associated deaths for the entire 2012-13 season, the CDPH said in a news release.

The proportion of deaths attributed to pneumonia and flu nationwide, which the CDC says generally lags behind illness indicators, rose to 8.8% from 8.1% the week before and is well above the epidemic threshold of 7.3%.

The number of lab-confirmed influenza hospitalizations climbed to 5,494, up by 879 from the week before, according to the CDC. That compares with an 870-case increase in the previous week. The numbers raise the incidence to 22.3 hospitalized cases per 100,000 population, up from 17.0 the week before.

The 2009 H1N1 strain continues to dominate. Of 2,006 strains subtyped, 1,906 (95%) were influenza A and 100 (5%) influenza B. Of the 1,147 "A" strains subtyped, 1,116 (97%) were 2009 H1N1 and the rest H3.

The circulating viruses are well matched to the vaccine strains, the CDC said.

The CDC identified one more 2009 H1N1 isolate resistant to the neuraminidase inhibitor class of antiviral drugs, which includes oseltamivir (Tamiflu) and zanamivir (Relenza). That raises the season total for resistant strains to 21, or 0.9% of the 2,254 isolates tested.

Flu increasing in Europe

Meanwhile, influenza activity continues to increase across Europe, according to an update today from the European Centre for Disease Prevention and Control (ECDC) that covers week 4 of 2014, which ended on Jan 26. Sixteen European countries reported increased flu activity, while 12 noted stable activity.

Bulgaria reported high intensity; Greece, Luxembourg, Portugal, and Spain reported medium intensity; and all other nations noted low-intensity activity. Flu is widespread in Bulgaria, Greece, Luxembourg, and Spain.

Through week 3 of this year, children under 15 are the most affected age-group in Europe, ECDC researchers noted in a separate report yesterday in Eurosurveillance.

Though percentages of circulating strains vary by country, 2009 H1N1 is the predominant strain in Europe as a whole, the ECDC said in its weekly update. In five reporting countries, 87% of hospitalized lab-confirmed cases involved the former pandemic strain.

European officials have identified two 2009 H1N1 isolates resistant to neuraminidase inhibitors so far this season.

See also:

Jan 31 CDC FluView report

Jan 31 CDPH news release

Jan 31 ECDC weekly update

Jan 30 Eurosurveillance report

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