CDC Estimates This Year’s Flu Vaccine Is Only 36 Percent Effective

Early estimates suggest the flu shot only worked about a third of the time this year.

Between Nov. 2 and Feb. 3, total vaccine effectiveness was estimated at 36%, according to a report released Thursday by the CDC. When broken down by strain, however, efficacy varied widely.

H3N2, the dominant strain circulating this flu season, is notoriously more virulent and less receptive to vaccination than other types of influenza. True to form, vaccine effectiveness against this strain was low this year — approximately 25%, according to the CDC report. (That’s actually better than experts originally feared, based on trends from Canada and Australia, where the flu vaccine was only about 10% effective.) By contrast, the report says the shot worked an estimated 67% of the time against the less-severe H1N1 and 42% of the time against influenza B viruses.

The shot’s efficacy also depends on a person’s age. Among children between the ages of six months and eight years, for example, the shot reduced the risk of H3N2 illness by 59%, according to the report.

36% effectiveness may not seem very impressive, but the CDC emphasizes in the report that even small increases in immunity can have a large impact on public health. CDC data has shown that even in 2014-2015, a year when vaccine effectiveness didn’t even hit 20%, immunizations prevented as many as 144,000 flu-related hospitalizations and 4,000 deaths. Plus, people who get the shot, but who still end up getting sick, tend to have less severe illnesses than unvaccinated people.

With weeks left to go in flu season — and strains other than H3N2 starting to circulate more widely — CDC officials are still pushing vaccination, in hopes of mitigating the impact of what has been a particularly brutal epidemic.

Flu has been unusually prevalent this year, with 93% of CDC-monitored jurisdictions reporting widespread activity for three consecutive weeks in January, according to a CDC report. Hospitalizations have also broken records, and by season’s end may even eclipse the 710,000 admissions seen during the high-severity 2014-2015 season. Flu- and pneumonia-related deaths have also been common, ranging from 5.8% to 10.1% of all deaths nationwide throughout the course of the season.

During a call with reporters last week, CDC Acting Director Dr. Anne Schuchat also said that death rates are likely to rise in the coming weeks, as “the people who are likely to die may already be in the hospital.” Sixty-three pediatric deaths have been recorded so far, and only 14 of those patients had gotten the flu shot, according to the new report.

In addition to getting vaccinated, the CDC is urging people to see a doctor and stay home from school or work if they come down with the flu.