Initial Effectiveness of mRNA-1273 Against SARS-CoV-2 Infection and Hospitalization in Young Children.

Background: Data on mRNA-1273 (Moderna) vaccine effectiveness (VE) in children aged 6 months to 5 years are limited. The objectives of this study were to assess mRNA-1273 vaccine effectiveness against symptomatic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and coronavirus disease 2019 (COVID-19)-related hospitalization among children aged 6 months to 5 years during the initial 5 months of the vaccination campaign rollout, as well as to determine whether VE varied by age group (6 months to <2 years vs 2 to 5 years). Methods: We used a test-negative study with linked health administrative data in Ontario, Canada, to evaluate vaccine effectiveness of mRNA-1273 against symptomatic SARS-CoV-2 infection and COVID-19-related hospitalization from July 28 to December 31, 2022. Participants included symptomatic children aged 6 months to 5 years who were tested by real-time polymerase chain reaction. The primary outcome was symptomatic infection, and the secondary outcome was COVID-19-related hospitalization. Results: We included 572 test-positive cases and 3467 test-negative controls. Receipt of mRNA-1273 was associated with reduced symptomatic SARS-CoV-2 infection (VE, 90%; 95% CI, 53%-99%) and COVID-19-related hospitalization (VE, 82%; 95% CI, 4%-99%) ≥7 days after the second dose. We were unable to detect heterogeneity in VE across age groups. Conclusions: Our findings suggest that mRNA-1273 vaccine effectiveness was initially strong against symptomatic SARS-CoV-2 infection and hospitalization in children aged 6 months to 5 years. Further research is needed to understand long-term effectiveness.