Effectiveness of mRNA Vaccines Against Omicron XBB Infection in Singaporean Children Under 5: A Population-Based Cohort Study

Limited data exists on COVID-19 mRNA vaccine effectiveness in children aged 1–4 years, particularly against Omicron subvariants. In a cohort study conducted during Omicron XBB subvariant predominant period in Singapore, completing the primary COVID-19 mRNA vaccine series significantly protected against COVID-19 infection in this age group, with 45.2% and 63.3% vaccine effectiveness in partially and fully vaccinated children, respectively. Vaccine effectiveness was also robust at 74.6% in children with prior COVID-19 infection who received at least one vaccine dose.