Influenza Vaccine Effectiveness Among Children: 2011–2020

To determine the effectiveness of the inactivated influenza vaccine against influenza illness in the outpatient pediatric population from 2011 to 2020.

Children and adolescents (≥6 months to 17 years) from the US Influenza Vaccine Effectiveness Network from 5 sites (Michigan, Pennsylvania, Texas, Washington, and Wisconsin) were included in the study. The study took place over 9 influenza seasons from 2011 to 2012 through 2019 to 2020. Patients with acute respiratory illness were enrolled after 2 consecutive weeks of increasing local influenza activity at the onset of the influenza season.

Influenza testing was performed using real-time polymerase chain reaction assays developed by the US Centers for Disease Control and Prevention. Vaccination status was obtained from electronic health records or immunization information systems. A patient ≤9 years was considered fully vaccinated if they had received 2 doses of influenza vaccine in the current season or 1 vaccine dose in the current season with 2 doses in 1 or more previous influenza seasons. Children ≥9 years were considered fully vaccinated if they had received 1 dose of current season influenza vaccine. Patients who received the influenza vaccine within 13 days before the onset of illness or those who received the live attenuated influenza vaccine were excluded from the study.

This study included 24 148 children. Of patients, 6767 (28%) tested positive for influenza with the most common being influenza A(H3N2), A(H1N1)pdm09, and influenza B, respectively. Forty one patients had both influenza A and B. Of patients, 9472 (39%) were fully vaccinated with the majority having received a quadrivalent vaccine. Twenty nine percent of cases compared with 43% of controls were fully vaccinated. Vaccination coverage in influenza-negative patients was highest among those aged 6 to 59 months (52%). Pooled vaccine effectiveness (VE) against any influenza illness was 46% (95% confidence interval [CI], 43–50]. VE varied by virus type/subtype with similar estimates for influenza A(H1N1)pdm09 (57% [95% CI, 51–62]) and influenza B (54% [95% CI, 49–59]) and lowest for influenza A(H3N2) (33% [95% CI, 27–39]).

There was not a statistical difference in VEs between participants 5 to 8 years and 9 to 17 years. In children 6 to 59 months, VE was substantially higher compared with VE in children 5 to 8 and 9 to 17 years. Children 6 to 59 months had substantially higher VEs for influenza A subtypes (47% [95% CI, 37–55] for A[H3N2] and 63% [95% CI, 54–70] for A(H1N1)pdm09) and influenza B [95% CI, 53–71] compared with the other age groups.

Pooled analysis over 9 influenza seasons demonstrated vaccine effectiveness in 46% of pediatric patients with the highest rates among those aged 6 to 59 months.

There continues to be parental concern regarding the effectiveness of the seasonal influenza vaccine.¹ It is known that children younger than 5 years old, especially those younger than 2, are at higher risk of developing serious influenza-related complications. This study provides further evidence that vaccination against influenza is effective at decreasing rates of infection in the outpatient pediatric patient population, especially in this vulnerable group of children <5 years of age.