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COVID‐19 Vaccine Efficacy and the Evidence on Boosters

23 Pages Posted: 30 Dec 2021

See all articles by Bernard S. Black

Bernard S. Black

Northwestern University - Pritzker School of Law

David Thaw

University of Pittsburgh - School of Law; University of Pittsburgh - School of Information Sciences; Yale University - Information Society Project; University of Pittsburgh - Graduate School of Public & International Affairs; National Defense University - College of Information and Cyberspace



Background: The need for COVID‐19 vaccine booster shots is controversial. Krause et al. and others have argued that need for a COVID‐19 booster for all adults has not been sufficiently established. The EU, UK, Canada, and Israel approved boosters for all adults, but U.S. regulators initially limited booster eligibility, waited nearly two months before allowing, and even longer before recommending boosters for all adults, with public health officials sending mixed messages on booster value.

Methods: We summarize vaccine efficacy against four endpoints: any infection, symptomatic infection, hospitalization, and death for the four principal vaccines used in developed Western countries (BNT162b2, mRNA1273, Ad26.CoV2.S, and ChAdOxS‐1), and evidence for waning efficacy over time, based on review of regulatory submissions and studies which met defined inclusion criteria.

Findings: Evidence on vaccine efficacy across multiple studies supports the conclusions that: (i) the mRNA vaccines experience significant declining efficacy after approximately six months, especially against infection but also against severe disease, with Pfizer declining faster than mRNA1273, but (ii) both mRNA vaccines outperform the Ad26.CoV2.S and ChAdOx1‐S viral vector vaccines. Booster doses greatly strengthen antibody levels and reduce both symptomatic infection and severe disease.

Interpretation: Strong epidemiological evidence supports the value of a booster dose for COVID vaccines, roughly 6 months after initial vaccination. Boosters both protect those who receive them and provide large spillover benefits to others, both vaccinated and unvaccinated, by preventing downstream infections, some of which will lead to hospitalization and death; reducing shortage risk for monoclonal antibodies, antiviral medications and other currently scarce COVID treatments; and reducing hospital overload (and thus improving survival rates. The emergence of the Omicron variant strengthens their value. Comprehensive evaluation of vaccination dosage and timing, including boosters, are part of proactive public health response to COVID risk.

Funding Information: No outside funding.

Declaration of Interests: None of the authors have any conflicts to disclose.

Ethics Approval Statement: Our study does not involve human subjects research and is exempt from review by an institutional review board.

Keywords: COVID-19, SARS-CoV-2, vaccine, vaccine efficacy, vaccine booster, BNT162b2, mRNA1273, Ad26.COV2.S, ChAdOxS-1, SARS-CoV-2 variants

Suggested Citation

Black, Bernard S. and Thaw, David, COVID‐19 Vaccine Efficacy and the Evidence on Boosters. Available at SSRN: or

Bernard S. Black (Contact Author)

Northwestern University - Pritzker School of Law ( email )

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Chicago, IL 60611
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312-503-2784 (Phone)

David Thaw

University of Pittsburgh - School of Law ( email )

3900 Forbes Ave.
Pittsburgh, PA 15260
United States


University of Pittsburgh - School of Information Sciences ( email )

Pittsburgh, PA 15260
United States

Yale University - Information Society Project ( email )

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New Haven, CT 06520-8215
United States

University of Pittsburgh - Graduate School of Public & International Affairs ( email )

Pittsburgh, PA 15260-0001
United States

National Defense University - College of Information and Cyberspace ( email )

300 5th Ave
Ft McNair
Washington, DC 20319
United States

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