US flu markers show steady rise

Mar 2, 2012 (CIDRAP News) – Flu activity in the United States showed a moderate spike last week, with a jump in states reporting widespread activity, the US Centers for Disease Control and Prevention (CDC) reported today.

The number of respiratory specimens testing positive for flu, another marker of flu activity, also showed a steady rise last week, increasing to 18.4%, compared with 14.4% the week before.

Increasing flu activity comes amid one of the latest starts to flu season in nearly three decades, and the CDC has warned that the season is not over and said it expects further increases in the weeks ahead.

Though the level of influenza-like illnesses (ILI) is still relatively low overall, three regions of the country reported levels at or above baseline: the northeast, the northern Midwest, and the central Midwest.

Nationally, the percentage of doctors visits for ILI held steady at 1.9%, putting it below the national baseline of 2.4%.

So far H3N2 is the most commonly detected strain, though the CDC has noted a growing proportion of pandemic 2009 H1N1 (pH1N1) viruses, especially in western and southwestern states.

The CDC also reported the first detection this season of an oseltamivir-resistant pH1N1 virus.

One more pediatric flu death was reported, raising the total so far this season to four. The death occurred during the week that ended Feb 4 and involved an unsubtyped influenza virus. Overall the percentage of deaths from pneumonia and flu rose to 7.3% but stayed below the epidemic threshold.

Six states reported widespread flu activity, an increase from two the week before. They are California, Colorado, Illinois, Nevada, Oklahoma, and Virginia. Eighteen states reported regional geographic spread.

Globally, flu activity in Northern Hemisphere countries is low but increasing in parts of North America and most of Europe, the World Health Organization (WHO) said today. It added that some areas appear to have peaked, including a few countries in Europe, northern Africa, and the Middle East.

The dominant strain in most areas is H3N2, though China and surrounding countries are reporting mostly influenza B, and Mexico is reporting predominantly pH1N1.

Though most viruses that WHO laboratories have characterized this season are closely related to the ones included in the seasonal vaccine, the committee that met recently to recommend the strains to include in the 2012-13 Northern Hemisphere vaccine noted increasing antigenic drift in H3N2 and a rise in the number of influenza B strains from the Yamagata lineage not included in this year's vaccine. The findings led to the group's Feb 23 recommendation that next season's vaccine contain two strain changes.

The European Centre for Disease Prevention and Control (ECDC) said in its weekly update today that Europe's flu season has also shown a later start than past seasons, with the number of countries reporting high- or medium-intensity activity last week holding steady compared with the previous week.

Bulgaria and Italy have reported decreasing flu indicators for the past 3 weeks in a row, which suggests they are the first two European countries where flu has peaked.

The number of respiratory samples that tested positive for flu was about the same as the week before, but the number of influenza B specimens rose to 11.1%, the highest so far this season, the ECDC said.

See also:

Mar 2 CDC weekly flu update

Mar 2 ECDC influenza surveillance report

Mar 2 WHO influenza update

Feb 23 CIDRAP News story "WHO picks two new strains for next season's flu vaccine"

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