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Selection Effects and COVID-19 Mortality Risk After Pfizer vs. Moderna Vaccination: Evidence from Linked Mortality and Vaccination Records (WP-23-02)

Vladimir Atanasov, Natalia Barreto Cham, Jeffrey Whittle, John Meurer, Benjamin Weston, Qian (Eric) Luo, Andy Yuan, Lorenzo Franchi, Ruohao Zhang, and Bernard Black

Many studies report that the Pfizer-BioNTech (BNT162b2) and Moderna (mRNA1273) COVID-19 vaccines provide similar protection against mortality, with a modest edge for Moderna due to slower waning. However, most comparisons of Pfizer to Moderna, or of either vaccine to the unvaccinated, do not address selection effects for who gets vaccinated, with how many doses, when, and with which vaccine. The researchers report evidence on large selection effects and use a novel method to control for these effects. Instead of studying COVID-19 mortality, they 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. The authors report mortality risk (RMR) for each vaccine relative to the unvaccinated and to the other vaccine, using linked mortality and vaccination records for all adults in Milwaukee County, Wisconsin, through June 30, 2022. Both vaccines provided similar protection for persons aged 18-59. However, for persons aged 60+, RMRs for two-dose Pfizer vaccinees were consistently over twice those for Moderna. 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+. A possible explanation for the Moderna advantage is a higher dose (100 μg versus 30 μg for Pfizer). For booster recipients, the Pfizer-Moderna gap is smaller and 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 among over 100,000 vaccinees). These results support booster importance for ages 60+, especially for Pfizer recipients. They suggest, but do not prove, that older persons may benefit from a larger vaccine dose.

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Vladimir Atanasov, Brinkley-Mason Term Professor of Business, William & Mary

Natalia Barreto Cham, Department of Economics, University of Illinois, Urbana-Champaign

Jeffrey Whittle, Professor of Medicine, Medical College of Wisconsin

John Meurer, Professor of Pediatrics and Community Health, Medical College of Wisconsin

Benjamin Weston, Associate Professor, Department of Emergency Medicine, Medical College of Wisconsin

Qian (Eric) Luo, Assistant Research Professor, Milken Institute School of Public Health, George Washington University

Andy Yuan, Postdoctoral Scholar, Pritzker School of Law, Northwestern University

Lorenzo Franchi, Post-Baccalaureate Research Fellow, Pritzker School of Law, Northwestern University

Ruohao Zhang, Assistant Professor of Data Science, Economics and Environmental Studies, Centre College

Bernard Black, Nicholas J. Chabraja Professor and IPR Associate, Northwestern University

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