Selection Effects and COVID-19 Mortality Risk after Pfizer vs. Moderna Vaccination: Evidence from Linked Mortality and Vaccination Records

Abstract

The Pfizer and Moderna COVID-19 vaccines have often been described as providing similar protection against mortality. However, most comparisons do not take into account the potential selection effects for who gets vaccinated, with how many doses, when, and with which vaccine. We report evidence on large selection effects and use a novel method to control for these effects. Instead of studying COVID-19 mortality, we study the COVID-19 Excess Mortality Percentage (CEMP), defined as COVID-19 deaths divided by non-COVID natural deaths for the same population. The CEMP measure uses non-COVID-19 natural deaths as a proxy for population health and thus controls, albeit imperfectly, for selection effects. We study adults vaccinated with the Pfizer-BioNTech (BNT162b2) and Moderna (mRNA1273) vaccines, and report mortality risk (RMR) for each vaccine relative to the unvaccinated and to the other vaccine, using linked mortality and vaccination records for Milwaukee County, Wisconsin, through June 30, 2022. The vaccines provided similar protection for persons aged 18-59, but for persons aged 60+, RMRs for two-dose Pfizer vaccinees were over twice those for Moderna vaccinees. Pfizer two-dose RMR for ages 60+ over April 2021-June 2022 was 248% of Moderna [95% CI=175%,353%]; in the Omicron period, Pfizer RMR was 57% versus 23% for Moderna. Both vaccines demonstrated waning two-dose effectiveness over time, especially for ages 60+. For booster recipients, the Pfizer-Moderna gap narrowed and became statistically insignificant. Younger persons (aged 18-59) were well-protected against death by two doses of either vaccine and highly protected by three doses (no deaths in our sample among over 100,000 vaccinees). These results support booster importance for ages 60+, especially for Pfizer recipients.

The Online Appendix for this paper is available at http://ssrn.com/abstract=4321773

Note:
Funding Information: This project was funded by the National Institutes of Health, award 3 UL1 TR001436-06S1.

Declaration of Interests: The authors have no competing interests.

Ethics Approval Statement: Approved by the Medical College of Wisconsin Human Research Review Board.

Keywords: COVID-19 mortality rates; COVID-19 Excess Mortality Percentage; vaccine effectiveness, Moderna vaccine, mRNA1273, Pfizer vaccine, Pfizer-BioNTech vaccine, BNT162b2.