More than half of the reported influenza cases that have occurred so far this season have come from a mutated strain that the current vaccine does not appear to effectively protect against, federal health officials said Thursday.
Centers for Disease Control and Prevention Director Dr. Thomas Frieden said Thursday that 91% of the flu cases so far this season have been from the H3N2 strain, which has been associated with a high rate of hospitalizations and deaths during seasons when it is the most common.
The CDC Wednesday sent out a health advisory to health professionals stating that of the 1,200 cases of flu reported as of Nov. 22, 52% were from a certain strain of H3N2 that had since mutated from the time the vaccine for H3N2 was produced, which has reduced its effectiveness against the virus.
“We're concerned that protection from vaccination against the drifted H3N2 viruses may be lower than we usually see,” Frieden said.
The mutated strain of H3N2 only became common in September, which made it too late to create a separate vaccine for this season because the production process takes at least four months, Frieden said. “Essentially the flu change was too late for the vaccine to be changed,” he said.
Despite its reduced effectiveness, Frieden recommended people still get vaccinated because it will provide some protection against the mutated strain, as well as other forms of the virus.
Overall flu activity this season to date has been low. A total of 302 confirmed influenza-related hospitalizations have been reported as of Nov. 22. Five children already have died from flu this season, which typically runs from October through May and reaches its peak between December and February.
The number of annual deaths in the U.S. from influenza can range from 3,000 to as many as 49,000, according to the CDC.
Last season, vaccines were found to be 61% effective against the H1N1 flu strain, which was the most common form of the virus.
Follow Steven Ross Johnson on Twitter: @MHsjohnson
