US flu activity rises, shifts toward 2009 H1N1 strain

Feb 11, 2011 (CIDRAP News) – The nation's flu activity increased again for the fourth week in a row, and for the first time the number of specimens testing positive for the 2009 H1N1 virus topped the H3N2 strain, which for the past several weeks had dominated, the Centers for Disease Control and Prevention (CDC) said today.

Most indicators have increased, including doctor's visits for flu-like illness. That marker rose to 4.6%, up from 4.0% the week before, which puts the level well above the national baseline of 2.5%. The only one of the CDC's 10 regions in which doctor's visits for flu were still below the regional baseline is the one that includes Arizona, California, Guam, Hawaii, and Nevada.

Flu also expanded its reach and intensity last week, with 37states now reporting widespread activity, 7 more than the previous week. Nine states reported regional activity, and the District of Columbia was the only location reporting local activity. Four states plus Guam, Puerto Rico, and the US Virgin Islands reported sporadic activity.

Hospitalizations related to lab-confirmed flu that the CDC tracks through its FluSurv-NET system reflected the increasing levels of flu across the country, with the highest rates in two groups: patients younger than5 and those ages 65 and older.

Eleven pediatric flu deaths were reported last week, raising the total for the season to 30 from 18 states. Of the newly reported fatalities,4 were linked to influenza B, 2 to 2009 H1N1, 1 to H3N2, and 4 to undetermined influenza A subtypes.

Overall rates of deaths from flu and pneumonia dropped slightly, from 8.5% to 8%, but the percentage was still above the level expected for this time of year, the CDC said.

The percentage of respiratory specimens testing positive for influenza dipped slightly last week to 31.7%, down from 32.9% the previous week. Of the 2,377 positive specimens, 77.6% were influenza A and 22.4% were influenza B, which showed a small shift toward influenza B. Of the type A isolates, 28.3% were H3 viruses and 28.5% were 2009 H1N1 strains, and the rest were not subtyped. The percentages for H3 and 2009 H1N1 were 43 and 25.3,respectively, the week before.

In its flu update summary, the CDC said flu typically peaks in January or later during most years. It said the circulating strains are still a good match to the viruses included in this season's flu vaccines and that antiviral resistance testing since Oct 1, 2010, has found no instances of flu virus resistance to oseltamivir (Tamiflu) or zanamivir (Relenza).

Elsewhere in North America, flu activity is declining across most parts of Canada except for the Atlantic provinces as well as Mexico, the World Health Organization (WHO) said in its global update today. In Canada, as in the United States, influenza B detections slightly increased. The dominant influenza strain in Canada is still H3N2.

See also:

Feb 11 CDC flu situation summary

Feb 11 CDC weekly flu surveillance report

Feb 11 WHO global flu update

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