The flu shots Americans are urged to get every winter have not been very effective this year, U.S. health officials have conceded. The number of flu cases has increased nationwide and the U.S. Centers for Disease Control and Prevention (CDC) 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 also said. “Seasonal flu activity was slow to start this year but has increased sharply in recent weeks,” Dr. Joe Bresee of the CDC’s influenza division said.
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. Flu viruses mutate and change all the time, so every year a different vaccine is created as officials predict which particular strains will circulate.
This year’s 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 U.S. may be a Type A strain that emerged in Australia called H3N2 A/Brisbane. Because it emerged late, it could not be included in the flu vaccine offered in the U.S. 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 U.S. 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 the shot is made available to the public in the fall; flu vaccines take months to create. It is sometimes difficult to determine that far in advance which strains will circulate.
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. The U.S., Canada, and parts of Europe are showing a higher resistance to the antiviral drug Tamiflu, raising questions about its potential effectiveness in the event of a human bird flu pandemic. The World Health Organization (WHO) reported elevated resistance in North America on Friday.
“One of the great conundrums of influenza is the fact that viruses appear and disappear and nobody knows really where they go from or to,” said Dr. Maria Zambon, head of the respiratory virus unit of Britain’s Health Protection Agency.
Influenza kills an estimated 36,000 Americans in an average year, and puts 200,000 into the hospital, the CDC said.