Flu Shot Effective at Keeping Kids Out of Hospital

Influenza vaccines were effective at preventing hospitalizations and outpatient visits among U.S. kids in recent years, but uptake remained low and fell further in the years since the COVID-19 pandemic, researchers found.

Vaccine effectiveness against hospitalization ranged from 28% during the 2021-2022 flu season to 67% in the 2023-2024 season and against outpatient visits ranged from 28% in 2021-2022 to 56% in 2023-2024, reported Samantha Olson, MPH, of the CDC, and colleagues in Pediatrics.

A separate study from Europe, also published in Pediatrics, found vaccine effectiveness against primary care visits was 68% during the 2022-2023 season and 71% in the 2023-2024 season.

The two studies “add to a large body of research consistently demonstrating the effectiveness of influenza vaccination in preventing severe outcomes,” Eric Chow, MD, MS, MPH, of Public Health Seattle & King County and the University of Washington in Seattle, and colleagues wrote in an accompanying commentary.

However, Chow and colleagues noted that the “real-world value of these findings depends on how they are communicated.”

Olson and colleagues wrote that the U.S. vaccine effectiveness estimates were “similar to estimates from seasons before the COVID-19 pandemic.” However, vaccine uptake has been lower in recent seasons, ranging from 44% in the 2022-2023 season to 51% in the 2021-2022 season.

“Sharing data that measure how effective flu vaccines are in recent years might help improve the uptake of flu vaccines and prevent additional flu morbidity and mortality in children,” Olson told MedPage Today in an email.

The findings come as childhood influenza vaccination in the U.S. was recently moved from a universal recommendation to one based on shared clinical decision-making, endangering a group facing a high risk of flu complications, experts say.

Earlier this year, the CDC, under HHS Secretary Robert F. Kennedy Jr., drastically reduced the number of universally recommended vaccines in childhood from 17 to 11, contrary to the advice of public health experts. Though a judge temporarily blocked those and other changes to vaccine recommendations, there already has been damage from confusion surrounding U.S. vaccine policy, experts have said.

Among nearly 20,000 kids included in the study by Olson and colleagues, 14% of hospitalizations or outpatient visits for acute respiratory illness were positive for flu.

The 2021-2022 season had the lowest estimates of influenza vaccine effectiveness, but noted that during this time, “there was almost no influenza circulation which impeded sufficient data to make an informed vaccine virus selection.”

Overall, vaccine effectiveness skewed higher for younger kids — those 6 months to 8 years of age — at 33% in the 2021-2022 season, 55% in the 2022-2023 season, and 63% in the 2023-2024 season. (Among kids ages 9 to 17 years, vaccine effectiveness was 35%, 37%, and 48%, respectively.)

Vaccine effectiveness was 53% against influenza A/H1N1, 43% against A/H3N2, and 69% against influenza B/Victoria, they reported.

Olson and colleagues used data from 7 U.S. pediatric medical centers within the New Vaccine Surveillance Network. Kids included in the study were age 6 months to 17 years who were hospitalized or received outpatient care for acute respiratory illness.

Limitations included that sample sizes were small during the 2021-2022 flu season, with enrollment possibly affected by the pandemic, and that the study was dependent on accurate classification of vaccination status and influenza virus infection. There also may have been residual confounding the research team was not able to address, such as socioeconomic factors that may have affected the likelihood of vaccination or influenza infection.