US flu activity may still be increasing

Mar 3, 2005 (CIDRAP News) Influenza activity in the United States has been increasing since late December and may not have peaked for the season yet, the Centers for Disease Control and Prevention (CDC) said today.

In the week that ended Feb 19, 33 states had widespread flu activity and 15 states had regional activity, the CDC said in an update in Morbidity and Mortality Weekly Report. Thirty-eight states and New York City have had widespread flu cases for at least one week this season.

This year's pattern of flu differs sharply from 2003-04, when significant outbreaks began in October and activity peaked in December. In 16 of the past 27 seasons, the CDC said, cases have peaked in February or later, suggesting that this year is fairly typical.

So far this season, nine children have died of causes related to flu, as compared with 153 children last season, the CDC said. But the number of deaths is expected to rise before the end of the season because many flu outbreaks among schoolchildren have been reported.

The agency also said two different surveillance systems covering 14 urban areas have reported fairly low rates of flu-related hospitalization among children so far this winter, in comparison with past years.

One way the CDC measures flu activity is to use about 1,500 "sentinel" healthcare providers who report the percentage of their patient visits that are due to flu-like illness. In the week that ended Feb 19, 5.7% of visits were for flu-like illnesses, which was above the national baseline level of 2.5%. The percentage has been above the baseline for 6 weeks in a row, the agency said.

More than 84% of virus samples analyzed this season have been influenza A, and more than 99% of those have been the H3N2 strain, the CDC said. The agency has antigenically characterized 228 influenza A(H3N2) isolates. About 45% (103) of those resembled the recently identified A/California strain; the rest were A/Fujian.

The California strain has been growing more common since mid-January, the CDC said. The Fujian strain was included in this year's flu vaccine, but the California strain was not. Though the two strains are related, the vaccine may be less effective against the California strain, the agency said.

CDC. Update: Influenza activityUnited States, 2004-05 season. MMWR 2005 Mar 3;54(8):193-6 [Full text]