By Angel Adegbesan and Madison Muller | Bloomberg

Flu shots aren’t preventing this season’s dominant strain of the virus from causing mild or moderate illness that requires medical treatment, according to an interim analysis by U.S. government-led researchers.

The formulation’s efficacy at preventing such cases was estimated at 16%, which was statistically indistinguishable from being unvaccinated, according to the study led by the U.S. Centers for Disease Prevention and Control. The findings were reported Thursday in the agency’s Morbidity and Mortality Weekly Report.

Vaccination is still recommended for people older than 6 months as long as the virus is circulating, the CDC said. The shots can still prevent serious cases, hospitalizations, intensive-care admissions and death, and may work against other strains arising later in the season, the authors said.

Health officials have long advocated for flu shots to protect against the cold-weather viral illness that usually kills about 36,000 American annually, most of them older or in poor health. However, flu mutates constantly and the vaccine, which is reformulated each year, never gives perfect protection, leading some people to doubt its overall benefits.

Widespread focus on the pandemic and public concerns about shots to prevent Covid may have also had an impact on vaccination rates. Doses distributed this season as of the third week in February fell 10% compared with a year ago, according to the CDC’s website.

The analysis was based on six months of data from more than 3,600 people with acute respiratory illnesses in California, Michigan, Pennsylvania, Tennessee, Texas, Washington and Wisconsin. Low numbers of cases kept the researchers from estimating efficacy in different age groups, the study said.

Estimates of vaccine efficacy could still change by the end of the season as more people get infected or if other strains of the virus predominate later in the season, the authors said. U.S. flu cases typically peak between December and February, but can last into May.

“Influenza activity is difficult to predict, and strategies to prevent influenza illness remain important to reduce strain on health care services,” the authors concluded.

The CDC also suggested using antiviral medications, which include Tamiflu, in addition to vaccination. Such medications can also offer protection against flu strains that might circulate later in the year.

With the success of messenger RNA vaccinations against Covid-19, vaccine makers Moderna Inc. and the Pfizer Inc.-BioNTech SE partnership are looking into using the same approach for flu vaccines. While researchers anticipate the technology will one day allow scientists to improve upon the efficacy of existing flu shots, it may be a long, complicated task.

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