US flu activity continues late-season decline

Robert Roos
Influenza, General

Apr 4, 2008 (CIDRAP News) – The influenza epidemic in the United States continued its late-season decline last week, with only seven states still reporting widespread flu activity, the Centers for Disease Control and Prevention (CDC) reported today.

However, the CDC received six more reports of flu-related deaths in children, raising the season's total to 59. The deaths, which occurred between Mar 8 and 23, were in Arizona, Connecticut, Minnesota (two deaths), Nevada, and Texas.

States that were still reporting widespread flu the week of Mar 22 through 29 were Connecticut, Iowa, Maine, New York, North Carolina, Pennsylvania, and Vermont. Regional flu activity was reported in 27 states and local activity in 11 more.

This year's epidemic appeared to peak in mid-February, as 49 states reported widespread flu activity for 2 weeks in a row. The number of states with widespread activity has declined each week since the week of Feb 24.

The season has been marked by a fairly poor match between two of the three viral strains in the vaccine and the dominant strains making people sick. The latest figures continue to reflect that. Only 23% of 144 influenza A/H3N2 virus isolates and 5% of 157 influenza B isolates analyzed by the CDC so far this season were matched well by the vaccine.

This season also has seen increased signs of resistance to oseltamivir, one of the two leading flu drugs, in one of the three flu subtypes, A/H1N1. Today's report says that 9.2% (73 of 797) of the H1N1 isolates tested this season had a mutation that confers resistance to oseltamivir. All tested viruses remained susceptible to the other antiviral drug, zanamivir.

As shown by lab testing, H3N2 viruses have predominated in the United States overall this season, though H1N1 strains were more common in the early weeks. Influenza B viruses have made up only 25% of specimens overall, but last week more type B than type A isolates were reported, the report says.

In other findings, the CDC says the proportion of outpatient visits to sentinel healthcare providers attributed to influenza-like illness last week dropped to the national baseline of 2.2%, down from 2.6% the previous week.

See also:

CDC's "FluView" weekly surveillance reports
http://www.cdc.gov/flu/weekly/

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