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

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.

The Type B strain chosen for this year's vaccine also was not a good match for most of the B virus strains seen in the United States this flu season, Bresee said.

"While a less-than-ideal virus match between the viruses in the vaccine and those circulating viruses can reduce vaccine effectiveness, we know from past influenza studies that the vaccine can still protect enough to make illness milder or prevent flu-related complications," Bresee said.

Bresee noted that decisions on the composition of the annual vaccine are made about nine months before it is made available to the public in the fall, and it is sometimes hard to know that far in advance which strains will circulate.

Flu vaccines take months to make.

Bresee also said some resistance is being reported to the antiviral drug Tamiflu, made by Switzerland's Roche Holding AG and Gilead Sciences Inc of the United States. Of the viruses tested in CDC flu labs, 4.5 percent are resistant to the drug, Bresee said.

Influenza kills an estimated 36,000 Americans in an average year, and puts 200,000 into the hospital, the CDC said.

(Editing by Maggie Fox and Xavier Briand)