Flu season just got worse

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CDC finds vaccine less effective against most common strain; current virus has mutated

CDC announced on December 4 that the current influenza vaccine may not be the best protection against H3N2—the most common seasonal influenza strain that is currently circulating in the United States.

Roughly half of the H3N2 viruses that have been analyzed are drift variants, meaning these viruses have mutated and are different from this season’s vaccine virus. In addition, CDC said it was too late to make new influenza vaccines that could better protect against H3N2.

The result could mean a more severe flu season, according to CDC. Not only is protection decreased with a less effective vaccine, but H3N2 strains have historically caused more severe influenza illness, hospitalizations, and deaths. So far this year there have been five pediatric deaths as well as reports of nursing home and school outbreaks, according to CDC officials.

However, CDC is still recommending that Americans get vaccinated with the available vaccine.

“Vaccination has been found to provide some protection against drifted viruses in past seasons. Also, vaccination will offer protection against other flu viruses that may become more common later in the season,” said Joseph Bresee, MD, Chief of the Influenza Epidemiology and Prevention Branch at CDC.

Vaccinated individuals may also have a milder illness if they do become infected.

According to CDC, during the 2007–08 flu season, the predominant H3N2 virus was a drift variant; yet the vaccine had an overall efficacy of 37% and 42% against H3N2 viruses.

Tom Frieden, MD, MPH, CDC Director, urged a three-pronged approach: vaccination, prompt treatment for people at high risk of complications, and preventive health measures, such as staying home if one is sick in order to reduce the spread of influenza.

If a high-risk individual shows symptoms of influenza, it’s recommended that the person contact their health care provider about getting antiviral medications promptly. Studies have found that antiviral medication is most effective when taken in the first 48 hours after symptoms appear. 

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