US flu season nears its end; global levels low

Apr 22, 2011 (CIDRAP News) – Influenza activity in the United States continued to fade last week, but the lingering signs included reports of four more flu-related deaths in children, the Centers for Disease Control and Prevention (CDC) said today.

Meanwhile, the World Health Organization's latest global update said flu was generally low and decreasing worldwide, but there were a few patches of activity in the tropics.

Most of the US flu indicators were down, according to the CDC. The proportion of outpatient medical visits attributed to flu-like illness (ILI) was 1.3%, down from 1.4% last week and well below the national baseline of 2.5%. On the basis of medical visits, ILI activity was low in all 50 states. Only 6.8% of respiratory specimens tested for flu were positive, compared with 9.1% a week earlier.

No states reported geographically widespread flu activity, whereas two states still had widespread cases the week before. Another 20 states had some local flu activity, while the rest reported sporadic cases or none.

The four flu-related pediatric deaths reported last week raise the season's total to 95, the CDC reported. Three of the deaths were linked to influenza B infections and one to a 2009 H1N1 infection.

The proportion of deaths ascribed to pneumonia and flu last week was at the epidemic threshold of 7.8%, slightly below the 8.0% of the week before. Last week was the 12th consecutive week that this indicator was at or above the epidemic threshold.

Eight more 2009 H1N1 virus isolates that were resistant to oseltamviir were reported last week, bringing the season total to 26. Only two oseltamivir-resistant H3N2 isolates have been reported this season.

Of the 201 flu isolates that were identified last week, 63% were influenza A and 37% were type B, similar to the previous week's pattern, the CDC said. Among the type A isolates, 62% were H3N2 (41% a week earlier), 13% were 2009 H1N1 (28% a week earlier), and the rest were not subtyped.

WHO says season ending
The WHO update, covering the week of Apr 3-9, said global activity was generally low and declining, with cases in temperate parts of the Northern Hemisphere back to baseline levels in most areas, indicating the season is ending.

In Canada the ILI consultation rate dropped and was slightly below the expected rate for the time of year, the WHO said. Mexico had few virus detections but an increasing proportion of 2009 H1N1 isolates. The outbreak reported in March in the northern Mexican state of Chihuahua included six 2009 H1N1-related deaths, all in adults who were previously healthy, the report noted.

In Europe, flu cases have returned to baseline in 36 of 50 countries, the WHO said. The proportion of influenza B detections has increased as overall cases have dropped, and type B made up 70% of isolates for the week. But for the whole season, 71% of isolates were type A, and 96% of those were 2009 H1N1. The latter virus accounted for 90% of 3,072 hospital cases in which viruses were subtyped.

The report notes localized flu outbreaks in a few places in the tropics. Venezuela had a wave of 2009 H1N1 cases that apparently peaked in early April, leading to at least 12 deaths, all in people with preexisting conditions.

In addition, H3N2 flu was circulating in the central part of sub-Saharan Africa, with cases in the Democratic Republic of Congo and Rwanda. Also, Kenya has had some influenza B cases, while previously reported H3N2 and type B activity has decreased in Madagascar.

The WHO said the flu season has not started in temperate regions of the Southern Hemisphere. Australia continues to report "low but unusual levels" of H3N2 viruses, most of them in the Northern Territory and Queensland.

See also:

Apr 21 WHO flu update

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