Effectiveness of Vaccination and Previous Infection Against Omicron Infection and Severe Outcomes in Children Under 12 Years of Age

BackgroundData on the protection conferred by Covid-19 vaccination and previous SARS-CoV-2 infection against omicron infection and severe outcomes in children can inform prevention strategies. MethodsWe obtained vaccination records and clinical outcomes for 1,368,721 North Carolina residents 11 years of age or younger from October 29, 2021 to January 6, 2023. We used Cox regression to estimate the time-varying effects of primary and booster vaccination and previous infection on the risks of omicron infection, hospitalization, and death. ResultsFor children 5-11 years of age, the effectiveness of primary vaccination against infection was 59.9% (95% confidence interval [CI], 58.5 to 61.2), 33.7% (95% CI, 32.6 to 34.8), and 14.9% (95% CI, 12.3 to 17.5) at 1, 4 and 10 months after the first dose; the effectiveness of a monovalent or bivalent booster dose after 1 month was 24.4% (95% CI, 14.4 to 33.2) or 76.7% (95% CI, 45.7 to 90.0); and the effectiveness of omicron infection against reinfection was 79.9% (95% CI, 78.8 to 80.9) and 53.9% (95% CI, 52.3 to 55.5) after 3 and 6 months, respectively. For children 0-4 years of age, the effectiveness of primary vaccination against infection was 63.8% (95% CI, 57.0 to 69.5) and 58.1% (95% CI, 48.3 to 66.1) at 2 and 5 months after the first dose, and the effectiveness of omicron infection against reinfection was 77.3% (95% CI, 75.9 to 78.6) and 64.7% (95% CI, 63.3 to 66.1) after 3 and 6 months, respectively. For both age groups, vaccination and previous infection had better effectiveness against hospitalization and death than against infection. ConclusionsCovid-19 vaccination was effective against omicron infection and severe outcomes in children under the age of 12 years, although the effectiveness decreased over time. Bivalent boosters were more effective than monovalent boosters. Immunity acquired via omicron infection was very high and waned gradually over time.