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.
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 American in an average year, and puts 200,000 into the hospital, the CDC said.
Jason - a Lumen
