Early-season rise in US flu activity continues

Dec 7, 2012 (CIDRAP News) – Influenza is widespread in eight US states, and some other indicators of an early flu season continue to rise, the Centers for Disease Control and Prevention (CDC) said today.

The percentage of patients' respiratory samples testing positive for flu grew to 20.7% in the week ending Dec 1, up from 15.2% the week before. And, as before, the H3N2 strain continues to dominate, which can be a sign of a more severe flu season.

The percentage of flu-positive specimens varied widely by region, from 6.2% in the Southwest to 33.5% in Midwestern states. Southern states also reported percentages above the national average.

The CDC reported three new flu-related deaths in children, bringing the season's total to five. One of those deaths was attributed to influenza B and the other two to influenza A viruses that were not subtyped.

The agency reports geographic spread of flu as assessed by state epidemiologists. This week Alabama, Alaska, Mississippi, New York, North Carolina, Ohio, Rhode Island, and South Carolina reported widespread activity, 15 states reported regional activity, 15 experienced local activity, and 12 had sporadic activity

In its state-by-state summary of influenza-like illness (ILI) activity, the CDC classified four states as having high levels: Alabama, Louisiana, Mississippi, and Texas. Three states are experiencing moderate activity, 6 states low activity, and 37 states minimal activity.

Two other flu indicators, however, are below epidemic thresholds: the proportion of outpatient medical visits for ILI (at 1.9%, below the 2.2% threshold) and pneumonia and influenza mortality (6.5% of all deaths, below the 6.8% threshold).

Of the 1,139 respiratory specimens that tested positive for flu during the week, three fourths were influenza A and one fourth influenza B. Of the 410 flu A viruses that were subtyped, 406 (99%) were H3 and only 4 (1%) were pandemic 2009 H1N1 (pH1N1).

The CDC reported no antiviral-resistant flu isolates, nor were any infections with novel H3N2 viruses found.

In contrast to the US situation, flu activity in the rest of the Northern Hemisphere has picked up only slowly, the World Health Organization (WHO) said in an update.

H3N2 activity in Canada has increased slightly, the agency said, while in Europe flu activity remains low, with only sporadic detections.

Tropical countries seeing slightly increased activity include Honduras, Cambodia, and Sri Lanka, the WHO said. The Southern Hemisphere is reporting only sporadic activity.

Globally, the H3N2 strain also dominates, according to WHO data. Flu A is outpacing flu B 70% to 30%, while H3N2 has accounted for 82% of type A viruses so far this season, compared with 18% for pH1N1.

See also:

Dec 7 CDC weekly flu update

Dec 7 WHO flu update

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