Background: COVID-19 vaccination during pregnancy may confer protective immunity to infants through transplacental antibody transfer. We estimated the effectiveness of maternal vaccination with a second or third mRNA COVID-19 vaccine dose during pregnancy against Delta and Omicron SARS-CoV-2 infection and hospitalization in infants.Methods: We conducted a test-negative design study in Ontario, Canada among infants younger than six months born between May 2021 and March 2022 who were tested for SARS-CoV-2 between May 2021 and September 2022. We estimated vaccine effectiveness using multivariable logistic regression.Findings: 8809 infants met eligibility criteria, including 99 Delta cases (4365 controls) and 1501 Omicron cases (4847 controls). Vaccine effectiveness for two doses was 95% (95% confidence interval (CI), 88–98%) and 97% (95%CI, 74–100%) against Delta infection and hospitalization and 45% (95%CI, 37–53%) and 58% (95%CI, 44–69%) against Omicron infection and hospitalization, respectively. Effectiveness was 73% (95%CI, 61–80%) and 81% (95%CI, 58–92%) against Omicron infection and hospitalization with three doses. Vaccine effectiveness for two doses against Omicron infection was highest with the second dose in the third (53% [95%CI, 42–62%]) compared to the first (47% [95%CI, 31–59%]) or second (37% [95%CI, 24–47%]) trimesters. Effectiveness against Omicron with two doses decreased from 57% (95%CI, 44–66%) between birth and eight weeks to 40% (95%CI, 21–54%) after 16 weeks of age.Interpretation: Maternal COVID-19 vaccination with a second dose during pregnancy was highly effective against Delta and moderately effective against Omicron infection and hospitalization in infants during the first six months of life. A third vaccine dose bolstered protection against Omicron. Vaccine effectiveness for two doses was highest with maternal vaccination in the third trimester, and effectiveness decreased in infants beyond eight weeks of age.Funding: Canadian Institutes of Health Research and Public Health Agency of Canada.Declaration of Interest: KW is CEO of CANImmunize and serves on the data safety board for the Medicago COVID-19 vaccine trial. The other authors declare no conflicts of interest.Ethical Approval: ICES is a prescribed entity under Ontario’s Personal Health Information Protection Act (PHIPA). Section 45 of PHIPA authorizes ICES to collect personal health information, without consent, for the purpose of analysis or compiling statistical information with respect to the management of, evaluation or monitoring of, the allocation of resources to or planning for all or part of the health system. Projects that use data collected by ICES under section 45 of PHIPA, and use no other data, are exempt from Research Ethics Board review. The use of the data in this project is authorized under section 45 and approved by ICES’ Privacy and Legal Office.