COVID-19 vaccine effectiveness in children by age groups. A population-based study in Galicia, Spain

Background: Studies on vaccine effectiveness (VE) against COVID-19 in the pediatric population are outgoing. We aimed to quantify VE against SARS-CoV-2 in two pediatric age groups, 5-11 and 12-17-year-old, while considering vaccine type, SARS-CoV-2 variant, and duration of protection.Methods: A population-based test-negative control study was undertaken in Galicia, Spain. Children 5-11-year-old received the Comirnaty (R) (Pfizer, US) vaccine, while those aged 12-17-year-old received the Comirnaty (R) (Pfizer, US) or SpikeVax (R) (ModernaTX, Inc) vaccine. Participants were categorized into unvaccinated (0 doses or one dose with <14 days since vaccination), partially vaccinated (only one dose with >= 14 days, or two doses with <14 days after the second dose administration), and fully vaccinated (two doses with >= 14 days after the second injection). Adjusted odds ratios (OR) and their 95% confidence intervals (CI) were estimated using multiple logistic regression models. VE was calculated as (1-OR) * 100. Stratified and sensitivity analyses were performed.Results: In the fully vaccinated 5-11-year-old children, VE against the Omicron variant was 44.1% (95% CI: 38.2%-49.4%). In the fully vaccinated 12-17-year-old individuals, VE was 83.4% (95% CI: 81.2%-85.3%) against Delta and 74.8% (95% CI: 58.5%-84.9%) against Omicron. Comirnaty (R) and SpikeVax (R) vaccines showed a similar magnitude of VE against Delta [Comirnaty (R) VE: 81.9% (95% CI: 79.3%-84.1%) and SpikeVax (R) VE: 85.3% (95% CI: 81.9%-88.1%)]. Comirnaty (R) (Pfizer, US; VE: 79.7%; 95% CI: 50.7%-92.4%) showed a slightly higher magnitude of protection against Omicron than SpikeVax (R) (ModernaTX, Inc), yet with an overlapping CI (VE: 74.3%; 95% CI: 56.6%-84.9%). VE was maintained in all age subgroups in both pediatric populations, but it declined over time.Conclusions: In Galicia, mRNA VE was moderate against SARS-CoV-2 infections in the 5-11-year-old populations, but high in older children. VE declined over time, suggesting a potential need for booster dose schedules.