U.S. expresses concern about vaccine as flu cases up

By Will Dunham

WASHINGTON (Reuters) - The influenza vaccine given to Americans may not protect as well as expected, U.S. health officials said on Friday as the number of flu cases increased nationwide.

The U.S. Centers for Disease Control and Prevention said slightly more than half of the influenza virus strains reported to its surveillance system are not good matches against the strains included in this flu season's vaccine.

The number of states reporting widespread flu activity jumped to 31 this week compared with 11 a week ago, the CDC said. But Dr. Joe Bresee of the CDC's influenza division said there are no indications this flu season is worse than usual.

"Seasonal flu activity was slow to start this year but has increased sharply in recent weeks," Bresee told reporters.

One measure officials use to gauge the severity of the season is the number of flu-related child deaths. Bresee said the CDC has heard of six U.S. children who have died from the flu, a relatively low number compared with recent years.

Flu viruses mutate and change all the time, so every year a different vaccine is created as officials predict which particular strains will circulate.

The vaccine is designed to protect against three influenza strains -- two from Type A, an H1N1 and an H3N2 version, and one for Type B.

Bresee said about 30 percent of the overall strains of influenza in the United States may be a Type A strain that emerged in Australia called H3N2 A/Brisbane. It emerged too late to be included in the flu vaccine offered in the United States beginning in September and October.