Preprint Study Co-Authored by CDC Network Finds ‘Reduced Effectiveness’ of Repeat Flu Vaccine

A preprint study co-authored by the CDC’s U.S. Flu Vaccine Effectiveness Network Investigators found that repeat flu inoculations may be “higher” than non-repeat vaccines.

The U.S. Flu Vaccine Effectiveness Network Investigators consists of study sites at Arizona State University Tempe; University of Michigan; Washington University at St. Louis; Duke University; University Hospitals Cleveland; University of Pittsburgh; Baylor Scott & White Health; Kaiser Permanente Washington.

The study, titled, “Reduced effectiveness of repeat influenza vaccination: distinguishing among within-season waning, recent clinical infection, and subclinical infection,” found that “[i]f repeat vaccinees tend to vaccinate substantially earlier in a season, waning protection could make the risk of infection among repeat vaccinees appear higher than in non-repeat vaccinees.”

“Throughout our analysis, we assumed that influenza vaccination with any type of influenza vaccine confers complete protection in a subset of vaccinees. We did not consider ‘leaky’ vaccine effects, where vaccines are partially protective in all recipients, and which can lead to an observed decline in [vaccine effectiveness] without the need for waning vaccine protection,” the study added.

A 2022 study from the World Health Organization (WHO) and the National Institutes of Health (NIH) similarly found estimates that suggest that “vaccination in the previous year attenuates vaccine effectiveness,” meaning that previous flu inoculation provides less efficacy than anticipated.

“The estimated effects of vaccination in the previous year are concerning and warrant additional investigation, but are not consistent or severe enough to support an alternative vaccination regimen at this time,” the study concluded, although it still advocated for flu vaccines.

American Faith reported that the 2022-2023 influenza vaccine was only 48% effective in reducing flu risk and only 40% effective at preventing hospitalization.

As of December 2023, 22% of adults received the flu vaccine, compared to 25.2% the previous year.

As of November 4, 32.6% of children received the most recent flu vaccine, down from 36.5%.

“About one-third of Americans report having gotten a flu vaccine so far this season,” the CDC stated. “Adult flu vaccine coverage in the United States has hovered at around 50 percent of the population for years. Historically, most flu vaccination has taken place during the month of October.”