The vaccine was only about 16 percent effective at reducing a person’s chance of getting a mild or moderate infection, the agency said. Experts said a good rate would be at least 50 percent.
This season’s flu vaccine has offered little to no protection against getting a mild or moderate case of influenza, the Centers for Disease Control and Prevention said this week.
In a study of more than 3,600 Americans in seven states, the C.D.C. said in a report that the vaccine was only around 16 percent effective, a rate that it said was “not statistically significant.”
“It’s not ineffective, but it’s clearly suboptimal in its efficacy,” Dr. Jesse L. Goodman, a former chief scientist at the Food and Drug Administration, said on Thursday. He reviewed the report but was not associated with it.
Still, despite the vaccine’s lackluster performance this season, which started in October and lasts through May, the C.D.C. suggested that people get inoculated, saying that it could “prevent serious outcomes.”
Scientists had warned in 2020 that the flu season, if it was severe, could possibly converge with Covid to create a dreaded “twindemic.” But coronavirus restrictions — including working from home and the use of masks — along with a high flu vaccine rate may have helped reduce caseloads the last few seasons, during which, the C.D.C. said, cases have been at a record low.
Still, even a mild flu season can be devastating. The C.D.C. estimated that during the 2019-20 flu season, around 22,000 people in the country had died and 400,000 had been hospitalized.
This season, the agency said, “influenza activity” declined in December and January, during the worst of the Omicron surge, but increased in early February.
In October and November of 2021, the agency investigated a flu outbreak at the University of Michigan, where there were 745 cases, mostly involving students who had not been vaccinated against the flu. Investigators there also found that the vaccine did not offer much protection.
Dr. Goodman said that this season’s results showed how much flu vaccines could be improved.
“The next pandemic could be an influenza pandemic,” Dr. Goodman said, “so we need better vaccines.”
Every year, scientists decide whether they need to update the flu vaccine to protect against the strains that they predict will dominate the upcoming season.
The low efficacy rate this season, Dr. Goodman said, “suggests that there was a mismatch between the strains of virus in the vaccine and what’s circulating.”
Scientists updated this season’s vaccines to offer protection against four flu viruses, including H3N2, which ended up being this season’s dominant strain, the report said. H3N2 was also dominant during the 2017-18 flu season, which experts had said was “moderately severe.”
Since the agency began calculating the vaccine’s effectiveness in 2004, the efficacy rate has been as high as 60 percent — for the 2010-11 season — and as low as 10 percent, during the first season the C.D.C. tracked it. Dr. Goodman said he would consider a rate between 50 and 80 percent to be good.
The flu is a life-threatening respiratory illness that can fill up hospital beds. It shares symptoms with Covid, including fever, coughing, a sore throat and fatigue. Adults 65 and older, those who are pregnant or immunocompromised and children under 5 are most at risk of the flu.
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