Influence of age on the effectiveness and duration of protection in Vaxzevria and CoronaVac vaccines

Background High rates of virus transmission and the presence of variants of concern can affect vaccine effectiveness (VE). Both conditions occur in low-income countries, which primarily use viral vector or inactivated virus vaccine technologies. Such countries conducted few VE analyses, and most lack the power to evaluate effectiveness in subgroups. Methods The present retrospective cohort study evaluated the effectiveness of Vaxzevria and CoronaVac vaccines for COVID-19-related infection in 75,919,840 Brazilian vaccinees from January 18 to July 24, 2021. Study outcomes included documented infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), COVID-19–related hospitalization, ICU admission, and death. We estimated VE using Cox regression adjusted for individual demographic characteristics. Results Vaccination with Vaxzevria or CoronaVac was effective against SARS-CoV-2 infection and highly effective against hospitalization, ICU admission, and death in individuals up to 79 years. From 80-89 years of age, Vaxzevria led to 89.9%(95CI:87.7-91.7) VE against death versus 67.2%(95CI:63.6-70.5) for CoronaVac. Above 90 years, 65.4%(95CI:46.1-77.8) protection was conferred to Vaxzevria-vaccinated individuals versus 33.6%(95CI:21.9-43.5) in CoronaVac-vaccinated individuals. Furthermore, the post-vaccination daily incidence rate shows a stepwise increase from younger to elder decades of life. Conclusions Vaxzevria demonstrated overall effectiveness against severe COVID-19 up to 89 years and CoronaVac up to 79 years of age. There is a stepwise effectiveness reduction for both vaccines for each decade of life. Our results suggest that individuals aged 80 years or older may benefit from an expedited booster dose. Ongoing evaluations, including any additional vaccines authorized, are crucial to monitoring long-term vaccine effectiveness.