Effectiveness of first, second, and third COVID-19 vaccine doses in solid organ transplant recipients: A population-based cohort study from Canada.

Limited data exists on the effectiveness of a third COVID-19 vaccine dose in solid organ transplant recipients. We conducted a population-based cohort study using linked healthcare databases from Ontario, Canada to answer this question. We included solid organ transplant recipients (n = 12,842) as of December 14, 2020, with follow-up until November 28, 2021. We used an extended Cox proportional hazards model with vaccination status, including BNT162b2, mRNA-1273, and ChAdOx1 vaccines, modeled as a time-dependent exposure. Individuals started in the unvaccinated category (reference) and could contribute person-time to first, second, and third doses. Over a median follow-up of 349 days, 12.7% (n = 1632) remained unvaccinated, 54.1% (n = 6953) received 3 doses, and 488 (3.8%) tested positive for SARS-CoV-2 (of which 260 [53.3%] had a clinically important outcome [i.e., hospitalization or death]). Adjusted vaccine effectiveness against infection was 31% (95% CI: 2, 51%), 46% (95% CI: 21, 63%), and 72% (95% CI: 43, 86%) for one, two, and three doses. Vaccine effectiveness against clinically important outcomes was 38% (95% CI: 4, 61%), 54% (95% CI: 23, 73%), and 67% (95% CI: 11, 87%). Vaccine effectiveness in solid organ transplant recipients is lower than the general population, however, vaccine effectiveness improved following a third dose.