Vaccine effectiveness against SARS-CoV-2 infection, hospitalization, and death when combining a first dose ChAdOx1 vaccine with a subsequent mRNA vaccine in Denmark: A nationwide population-based cohort study

PLoS MedicineGram MA, Nielsen J, Schelde AB, et al. | January 10, 2022


For controlling the COVID-19 pandemic and to avoid hospitalizations and deaths from COVID-19, one of the main strategies is coronavirus disease 2019 (COVID-19) vaccination. In several countries, recommendations of terminating the use of the ChAdOx1 vaccine has resulted in vaccine programs combining different Coronavirus Disease 2019 (COVID-19) vaccine types. Researchers herein sought to attain knowledge concerning the effectiveness of the combination of the ChAdOx1 and an mRNA vaccine.

  • National registry data on all Danish residents during the study period (February 9 to June 23, 2021) were used to estimate the vaccine effectiveness (VE).

  • Data from a total of 5,542,079 individuals (97.6% of the total Danish population) were analyzed.

  • Vaccination was done with the ChAdOx1 vaccine as the first dose in 144,360 individuals, and an mRNA vaccine was administered in 136,551 of these individuals as the second dose.

  • Included were a total of 1,691,464 person-years and 83,034 SARS-CoV-2 infections.

  • An estimated protection of 88% was recorded against Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection in correlation with receiving vaccination with the combination of ChAdOx1 and an mRNA vaccine.

  • Overall findings indicate that combining the ChAdOx1 and an mRNA vaccine results in large reduction in the risk of SARS-CoV-2 infection when compared with unvaccinated individuals.

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