This season’s influenza vaccine is not a great match for the viruses in circulation, data from the US Centers for Disease Control and Prevention shows.

Flu vaccines are based largely on information that lags. The flu virus is constantly changing. Each February, a panel of US experts helps the US Food and Drug Administration decide which strains to include in the upcoming flu season‘s vaccine, to be rolled out in the fall. Each shot can protect against only three or four strains of the flu, so scientists have to make their best guess about which will be circulating in the fall.

The panel bases its decisions on data about the common strains circulating around the globe throughout the year. In some years, the vaccine is a better match to strains that hit the US than others. Even in a good year, the effectiveness of the vaccine is about 60%. Other times, it can be as low as 10%.

This season, based on data from 3,636 kids and adults who got the flu between October and mid-February, the vaccine did not seem to reduce their risk of getting sick from the influenza A (H3N2) viruses that have been predominant. The overall vaccine effectiveness against infection was 16%, which is considered not statistically significant. However, there wasn’t enough enrollment in the study for a reliable understanding of how effective it was.

“I can’t remember an ineffectiveness that was much lower than that,” said Dr. William Schaffner, a professor in the Division of Infectious Disease at Vanderbilt University Medical Center, who did not work on the report. “It was definitely off-target this year.”The CDC is studying how well the current vaccine protected against severe illness. That’s largely why the agency still recommends that people get the vaccine.

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