BackgroundThe recommendations in several countries to stop using the ChAdOx1 vaccine has led to vaccine programs combining different vaccine types, which necessitates new knowledge on vaccine effectiveness (VE). In this study, we aimed to estimate the VE when combining the ChAdOx1 vaccine as the first dose and an mRNA vaccine as the second dose. MethodsThis nationwide population-based cohort study estimated VE against SARS-CoV-2 infection, all-cause and COVID-19 related hospitalization and death after receiving the ChAdOx1 vaccine as the first dose followed by an mRNA vaccine as the second dose. VE estimates were obtained using a Cox regression with calendar time as underlying time and adjusted for sex, age, comorbidity, heritage and hospital admission. Information on all individuals was extracted and linked from high-quality national registries. ResultsA total of 5,542,079 individuals were included in the analyses (97.6% of the total Danish population). A total of 144,360 were vaccinated with the ChAdOx1 vaccine as the first dose and of these 136,551 individuals received an mRNA vaccine as the second dose. A total of 1,691,464 person-years and 83,034 cases of SARS-CoV-2 infection were included. The VE against SARS-CoV-2 infection when combining the ChAdOx1 and an mRNA vaccine was 88% (95% confidence interval (CI): 83; 92) 14 days after the second dose and onwards. There were no COVID-19 related hospitalizations and deaths among the individuals vaccinated with the combination of the ChAdOx1 and an mRNA vaccine during the study period. ConclusionIn conclusion, this study found a reduction in the risk of SARS-CoV-2 infection when combining the ChAdOx1 and an mRNA vaccine, compared with unvaccinated individuals. This is similar to the VE of two doses of an mRNA vaccine. Longer follow-up time is needed to confirm vaccine induced protection against severe events, such as COVID-19 related hospitalization and death.
Vaccine effectiveness when combining the ChAdOx1 vaccine as the first dose with an mRNA COVID-19 vaccine as the second dose