COVID-19 vaccine effectiveness against severe omicron-related outcomes in children aged 5 to 11 years in Ontario: A Canadian immunization research network (CIRN) study

Background: Understanding how the efficacy of COVID-19 vaccines translates from clinical trials to real-world settings is critical to inform evolving vaccination policies. The objective of this study was to assess COVID-19 vaccine effectiveness (VE) against severe COVID-19-related outcomes in children aged 5-11 years, including COVID-19-related hospital admissions and multisystem inflammatory syndrome in children (MIS-C). Methods: We conducted a retrospective, population-based cohort study using linked health administrative data in the first year following the emergence of the Omicron variant (January 2 to December 31, 2022) in Ontario, Canada. Baseline differences between subgroups of interest were compared using standardized differences. We used multivariable Cox proportional hazard regression models to estimate VE by time since last vaccine dose by treating vaccination as a time-varying exposure, compared to unvaccinated children. Results: We included a total of 1,058,740 children, of which 583,867 (55.1 %) had received at least one vaccine dose by the end of the study period. In total, there were 185 COVID-19-related hospital admissions and 39 cases of MIS-C. The rate of COVID-19-related admission was substantially higher in children with an underlying comorbid condition compared to children who were previously healthy (adjusted hazard ratio [aHR] = 4.77, 95 % CI, 3.56-6.38). VE against COVID-19-related admission ranged from 93 % (95 %CI, 52-99 %) 7-29 days after a second dose to 63 % (95 %CI; 41-77 %) >= 120 days after a second dose. There was no statistically significant difference in the rate of MIS-C in children who received at least one dose of the vaccine compared to unvaccinated children (aHR = 0.71; 95 %CI, 0.38-1.34). Conclusions: We found that, for children aged 5-11 years, VE against COVID-19-related hospitalization was high in the first four months after a second dose. Children with comorbid conditions were found to be at much higher risk of COVID-19-related severe outcomes and thus may benefit most from COVID-19 vaccination.