Effectiveness of mRNA and ChAdOx1 COVID-19 vaccines against symptomatic SARS-CoV-2 infection and severe outcomes with variants of concern in Ontario

Abstract

Background SARS-CoV-2 variants of concern (VOC) are more transmissible and have the potential for increased disease severity and decreased vaccine effectiveness. We sought to estimate the effectiveness of BNT162b2 (Pfizer-BioNTech), mRNA-1273 (Moderna), and ChAdOx1 (AstraZeneca) vaccines against symptomatic SARS-CoV-2 infection and severe outcomes (COVID-19 hospitalization or death) caused by the Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), and Delta (B.1.617.2) VOCs during December 2020 to May 2021. Methods We conducted a test-negative design study using linked population-wide vaccination, laboratory testing, and health administrative databases in Ontario, Canada. Findings Against symptomatic infection caused by Alpha, vaccine effectiveness with partial vaccination (≥14 days after dose 1) was higher for mRNA-1273 (83%) than BNT162b2 (66%) and ChAdOx1 (64%), and full vaccination (≥7 days after dose 2) increased vaccine effectiveness for BNT162b2 (89%) and mRNA-1273 (92%). Protection against symptomatic infection caused by Beta/Gamma was also higher with partial vaccination for mRNA-1273 (77%) than BNT162b2 (60%) and ChAdOx1 (48%), and full vaccination increased effectiveness for BNT162b2 (84%). Against Delta, vaccine effectiveness after partial vaccination tended to be lower compared to Alpha for mRNA-1273 (72% vs. 83%) and BNT162b2 (56% vs. 66%), but was similar to Alpha for ChAdOx1 (67% vs. 64%). Full vaccination with BNT162b2 increased protection against Delta (87%) to levels comparable to Alpha (89%) and Beta/Gamma (84%). Vaccine effectiveness against hospitalization or death caused by all VOCs was generally higher than for symptomatic infection after partial vaccination for all three vaccines. Interpretation Our findings suggest that even a single dose of these 3 vaccines provide substantial protection against these 4 VOCs, and 2 doses likely provide higher protection. Jurisdictions facing vaccine supply constraints might consider delaying second doses to more rapidly achieve greater overall population protection.