Study Finds Flu Vaccine Over 50% Effective Against Severe Disease in Children

According to a recent study published in JAMA Network Open, the estimated effectiveness of at least one dose of the influenza vaccine against emergency department (ED) visits or hospitalization was over 50 percent across disease severity levels among nearly 16,000 U.S. children during five respiratory illness seasons.

The case-control study, conducted by New Vaccine Surveillance Network Collaborators, used a test-negative design with 15,728 children aged 6 months to 17 years who visited an ED or were hospitalized at one of eight U.S. medical centers for acute respiratory illness from November 2015 to April 2020. The team grouped patients’ demographic and clinical characteristics by flu vaccination status. Of all children, 55.4% percent were boys, 85.5 percent were aged 6 months to 8 years, 14.5 percent were 9 to 17 years, 32.9 percent were Black, 26.6 percent were Hispanic and 31.2 percent were White.

In total, 17.2 percent of participants tested positive for flu, and 82.8 percent tested negative and served as controls. Of flu patients, 61.8 percent visited an ED, 33.1 percent were hospitalized for noncritical disease and 5.1 percent were hospitalized for critical illness, including 138 children admitted to an intensive care unit (ICU), one who required extracorporeal membrane oxygenation (ECMO), 35 who were intubated and two who died.

About half (49.5 percent) of all participants were vaccinated against flu, and most of them (87.3 percent) were 6 months to 8 years old. Estimated vaccine effectiveness (VE) of one or more doses of flu vaccine was 55.7 percent for preventing flu-related ED visits or hospitalizations among children of all ages. VE was similar across disease severity levels: 52.8 percent for ED visits, 52.3 percent for noncritical hospitalization and 50.4 percent for critical hospitalization. Estimated VE was 45.8 percent against influenza A(H3N2), 64.9 percent against influenza A(H1N1)pdm09 and 56.4 percent against influenza B. VE was higher for children aged 6 months to 8 years than for children 9 to 17 years (42.6 percent, 29.2 percent to 53.5 percent).

Unvaccinated flu patients spent more days in the hospital and ICU than their vaccinated peers. Among children who visited an ED for flu, 27.5 percent had at least one underlying medical condition. Of hospitalized flu patients, 60.1 had at least one chronic condition and just 50.2 percent were given flu antiviral drugs at admission.

“Improving vaccine uptake in children may reduce influenza illness and, subsequently, ED and hospital visits in a time of increased respiratory virus co-circulation,” the researchers wrote. “More research is needed to understand how influenza vaccines protect children against a gradient of severe outcomes and whether this varies by influenza type or subtype and season.”