Flu activity continues to slow, CDC says

Mar 7, 2008 (CIDRAP News) Seasonal influenza activity in the United States slowed last week for the second week in a row, the Centers for Disease Control and Prevention (CDC) reported in a surveillance update today.

Widespread activity was reported in 47 statestwo fewer than the previous week. Regional activity was reported in Massachusetts, Mississippi, and New Mexico. The District of Columbia reported only local activity.

For the week that ended Mar 1, 4.8% of the visits to sentinel healthcare providers in the CDC's network were for influenza-like illnesses (ILI), a decrease from the 5.7% reported the previous week, the CDC said. The percentage of visits for ILI decreased in seven of nine regions across the country, the same as for the previous week.

Nine flu-related pediatric deaths were reported last week, though one of them occurred during the 2006-07 flu season. The number was up from three reported the previous week, and it pushes the total for this season to 32.

Laboratories in the CDC's surveillance network tested 8,814 specimens for influenza last week, of which 2,401 (27.2.%) were positive, according to the report. In the previous week, 30% of 7,726 specimens tested positive.

The CDC's update says 6.1% (45 of 743) of flu viruses tested this season had a mutation that confers resistance to oseltamivir; all were the H1N1 subtype. The percentage is higher than last season, but last week's number was down slightly from the 6.8% reported the previous week.

CDC data continue to show that the H3N2 and B components of this year's vaccine don't match up well with most of the H3N2 and B viruses in circulation.

Fourteen percent (12 of 86) of the H3N2 viruses antigenically characterized by the CDC so far this season matched the A/Wisconsin/67/2005-like strain used in the vaccine, but 78% (67 of 86) were the Brisbane/10/2007-like strain, the CDC said. The Brisbane strain evolved from the Wisconsin strain but is distinct from it.

For influenza B, 7% (6 of 89) of the viruses belonged to the Victoria lineage represented in the vaccine, but 90% (83 of 89) belonged to the Yamagata lineage, the report said.

The Solomon Islands strain of H1N1 used in the vaccine is a good match for 77% (141 of 191) of the H1N1 viruses analyzed so far, according to the CDC. Thirteen percent (25 of 191) of the viruses were another strainA/Brisbane/59/2007-like.