US flu level continues drop amid low global activity

Apr 8, 2011 (CIDRAP News) – Flu indicators in the United States and other Northern Hemisphere locations continue to plummet, with global surveillance showing only a few active areas in tropical and Southern Hemisphere locations that will soon be approaching their flu season.

The US Centers for Disease Control and Prevention said in its flu surveillance update today that the number of doctor's visits for flulike illness continued a 7-week descent, falling to 1.6% of visits, down from 2.0% the previous week. For the first time since Dec 4, 2010, all 10 of the CDC's regions are below their regional baselines for flu visits.

The number of respiratory specimens that tested positive for flu last week also continued a steady decline, falling from about 14% the week before to 11%. The profile of the circulating strains didn't change much over the past week. About two-thirds were influenza A, with H3N2 making up 40% of that group. And about a third of the viruses were influenza B.

Sampling of viral isolates found more antiviral resistant viruses, including one H3N2 that was resistant to oseltamivir (Tamiflu) and 10 more 2009 H1N1 isolates that showed resistance to the drug.

Only three states reported widespread geographic spread last week—Connecticut, Idaho, and York. That number is seven fewer than last week's report. Ten states reported regional spread.

The CDC said it received reports of two pediatric flu deaths last week, raising the season's total to 91. One of the deaths was linked to the 2009 H1N1 virus and the other involved unsubtyped influenza A. Overall, the percentage of deaths from pneumonia and flu was lower but still remained above the epidemic threshold for the 10th week in a row.

At the international level, the World Health Organization (WHO) said today that flu levels are declining or have shrunk to baseline levels in many Northern Hemisphere regions, and that the Southern Hemisphere's flu season hasn't started yet. Globally, the circulating flu strains continue to match those included in seasonal flu vaccines, except for a small number of influenza B viruses from the Yamagata lineage.

In connection with a recent 2009 H1N1 outbreak in parts of Mexico's Chihuahua state, which borders the United States, genetic sequencing of isolates from three cases at Mexico's Institute of Diagnosis and Epidemiological Reference found that they are homologous to currently circulating viruses, the WHO reported. The outbreak has not excessively burdened the area's health system, and no other outbreaks have been reported in the rest of Mexico, it added.

In tropical regions, flu continues to circulate in sub-Saharan Africa, with Kenya and Uganda reporting cocirculation of 2009 H1N1 and influenza B viruses, according to the WHO. Over the past few weeks Madagascar has reported increasing flu activity, most of it H3N2 and influenza B.

Australia continues to report low levels of H3N2 activity, a trend seen during many of its summer months.

See also:

Apr 8 CDC flu update

Apr 8 WHO global flu update

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