Flu activity declining across Northern Hemisphere

Mar 25, 2011 (CIDRAP News) – Most measures of influenza activity in the United States continued their gradual decline last week, according to the Centers for Disease Control and Prevention (CDC), and similar trends were reported in Europe and other parts of the Northern Hemisphere.

Eighteen US states—all in the North except California, Nevada, and Virginia—reported widespread flu activity last week, down from 31 states the week before, the CDC said in its weekly flu update. Another 22 states had regional activity.

The share of medical visits to sentinel providers for influenza-like illness (ILI) dropped to the national baseline of 2.5%, compared with 3.0% a week earlier.

Six flu-related deaths in children were reported, bringing the seasonal total to 77, the CDC said. The previous week brought 11 reports of pediatric flu deaths. Of the latest deaths, two were linked to influenza A/H3N2 viruses, two to influenza B viruses, one to a 2009 H1N1 virus, and one to a type A virus that was not subtyped.

The proportion of all deaths attributed to pneumonia and flu—an indicator that tends to lag other flu measures—stayed at 8.6% last week, the same as the week before. That marked the eighth consecutive week the figure was at or above the epidemic threshold of 8.0%.

Of 6,144 respiratory specimens tested by US flu-surveillance labs last week, 18.9% were positive, compared with 21.1% a week earlier, the CDC reported. About 28% of the positive samples were influenza B and 72% were type A. Of the type A isolates, 37.8% were H3N2, 28.4% were 2009 HN1, and 33.8% were not subtyped.

Antigenic characterization of isolates continued to show the vast majority were well matched to the flu vaccine. Two more 2009 H1N1 isolates were found to be resistant to neuraminidase inhibitors, bringing the seasonal total to 5 of 670 isolates tested, the report says. No resistance to neuraminidase inhibitors has been found in H3N2 or type B viruses.

Europe also saw a continuing decline in flu activity last week, according to the European regional office of the World Health Organization (WHO).

Of 40 countries reporting, 23 had low-intensity flu activity and 17 had medium-intensity activity, the WHO said. In geographic terms, most countries reported only sporadic cases or local outbreaks, though 10 had regional or widespread activity.

The WHO also said that "sentinel hospitalizations" for severe acute respiratory infections generally declined but remained above pre-season levels in some countries.

Testing of sentinel specimens from patients with ILI or acute respiratory infections showed that 33% were positive for flu, down from a peak of 46% in the fourth and fifth weeks of the season, the report says.

Influenza B accounted for 70% of the 298 sentinel specimens that tested positive in Europe last week. Of the type A viruses that were subtyped, 83% were 2009 H1N1 and 17% were H3N2.

In a separate global report covering the week of Mar 6 to 12, the WHO said flu activity appears to be declining in most areas of the Northern Hemisphere.

The agency reported generally moderate flu activity in North Africa and the Middle East, citing Algeria, Tunisia, Iran, and Oman as countries continuing to report cases.

Flu cases are generally fading across northern Asia, the WHO reported. ILI activity in northern China has stayed below the level of the past three seasons, but Japan saw a slight increase after several weeks of declines.

Flu activity has remained low in the tropics, the WHO said. However, some signs of increased activity were seen in Guatemala, Ecuador, and Kenya.

See also:

Mar 25 CDC flu update

Mar 25 WHO EuroFlu bulletin

Mar 25 WHO global flu update

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