CDC sees US, global H3N2 flu activity

Oct 8, 2010 (CIDRAP News) – In its review of summer flu activity in the United States and across the globe today, the US Centers for Disease Control and Prevention (CDC) said that patterns, which included a few clusters and sporadic illness, were typical.

The CDC's report, however, which appears in today's issue of Morbidity and Mortality Weekly Report (MMWR), describes H3N2 influenza activity both globally and domestically, a strain often linked to more severe flu.

The report comes during the first official week of the US flu season.

Of respiratory samples from the United States that were analyzed over the summer, 326 (1.3%) were positive for influenza, of which 261 were influenza A and 65% were influenza B. Of the 185 influenza A viruses that were subtyped, most (70%) were H3N2, with rest the 2009 H1N1 subtype.

The number of positive samples grew slowly through late summer, with the largest portion coming from the southeastern states. Two unrelated H3N2 clusters were reported in Iowa in early July, and in August Maryland's health department reported an influenza B outbreak in children visiting the United States as part of an international exchange program. About 35 of the 400 children who were sick were treated at a local hospital, and influenza B was confirmed in 8 cases.

Outpatient visits for flu-like illnesses stayed below the national baseline, and mortality stayed below epidemic thresholds over the summer except for three nonconsecutive weeks. No pediatric flu deaths were reported.

Globally, H3N2, influenza B, and 2009 H1N1 viruses circulated over the summer, with influenza B predominant during the first part of the summer, the 2009 H1N1 becoming more common after early July, and H3N2 becoming the most commonly identified subtype since late August.

Yesterday at a press briefing on the flu vaccine, CDC officials said they anticipated more H3N2 activity this year, which is often linked to more severe flu infections. They listed the likelihood of H3N2 circulation as another good reason for Americans to get their flu immunizations.

The CDC said in an editorial note that accompanied the flu report that the disease clusters it received, along with sporadic reports of 2009 H1N1 and influenza B infections, are typical for summer months.

The agency said today and yesterday at the media briefing that antigenic analysis suggests the circulating strains are similar to the ones in the vaccine strains, which suggests the vaccines will be a good match.

The World Health Organization (WHO) today in a report on recommendations for the Southern Hemisphere's 2011 flu vaccine said the same thing and added that the vast majority of 2009 H1N1 viruses were sensitive to oseltamivir (Tamiflu), and the few resistant viruses detected were mostly linked to prophylaxis or treatment. Most 2009 H1N1 viruses and all H3N2 viruses that were tested were resistant to the adamantane antiviral drugs amantadine and rimantadine, the WHO said.

The WHO noted that a small number of seasonal H1N1 viruses it saw came from China and were closely related to the Brisbane seasonal H1N1 strain. In its analysis of influenza B strains, the WHO said the Victoria lineage, included in this season's vaccines for the Northern Hemisphere, continues to predominate, though the Yamagata lineage had recently become predominate in China at low levels.

In a separate report on Oct 5 on international flu activity in over the past 2 weeks, the CDC said the number of respiratory samples in southern China that have tested positive for influenza has risen about 8% in September, with H3N2 accounting for most of the activity.

In Chile, flu activity dipped in September, with H3N2 accounting for about half of the samples.

Though flu activity dropped sharply in New Zealand, Australian officials reported a late-season rise in 2009 H1N1 activity.  Elsewhere, two Thai provinces reported a rise in flu activity in September, with 2009 H1N1 as the primary subtype.

See also:

Oct 8 MMWR report

Oct 5 CDC international flu situation update

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