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Reported Effectiveness of COVID-19 Booster Vaccines: A Systematic Review of Early Literature and Implications for Emerging Vaccination Policy
22 Pages Posted: 7 Jun 2022More...
Background: Third 'booster' doses of COVID-19 vaccines have been shown in vaccine effectiveness (VE) studies to protect against mild, hospitalized and severe disease. These studies are of unknown comparability, particularly due to inclusion of populations with varying levels of infection and immunity. Interpretation of resulting VE estimates is complicated by study choices and uncertain applicability for public health.
Methods: We conducted a descriptive literature review on 31st March 2022 of booster COVID-19 VE studies stratified by vaccine type, comparator population, SARS-CoV-2 variant (Delta, Omicron, mixed/unspecified) and outcome (infection, hospitalization or severe/fatal disease), and discussed implications for future COVID-19 vaccination policy.
Findings: Forty-eight eligible studies were identified, many from the United States and Israel. Following exclusions, we identified 164 VE estimates; 139/160 (87%) described mRNA vaccines, mostly enrolling broad age groups. Half of estimates were against Delta (80 estimates; 50%), a third (56; 35%) against Omicron variant with the remainder including multiple variants. Eighty-eight estimates (55%) compared outcomes in boosted vs unvaccinated individuals; the others used vaccinated individuals as the comparator population. Overall, 151/160 (94%) estimates reported VE point estimates ≥50% and 111/160 (69%) reported VE ≥80%. VE appeared higher 1) when the unvaccinated rather than a fully vaccinated population was used as the comparator; 2) against the Delta variant compared with Omicron; 3) against hospitalized and fatal endpoints compared with milder endpoints; and 4) for mRNA booster vaccines compared with other booster vaccine types.
Interpretation: Booster COVID-19 vaccines appear to offer additional protection against SARS-CoV-2 infection, hospitalization and severe disease. Study design features determine the magnitude of effect and should be carefully considered when formulating booster vaccine policy.
Funding Information: This project was supported by a commissioned grant from the Health and Medical Research Fund of the Food and Health Bureau of the Hong Kong SAR Government, by the National Institute of General Medical Sciences (grant no. R01 GM139926), by the Theme-based Research Scheme (project no. T11-712/19-N) from the Research Grants Council from the University Grants Committee of Hong Kong, and an RGC Senior Research Fellowship (grant number: HKU SRFS2021-7S03).
Declaration of Interests: JN used to work for, and holds shares in, Sanofi. BJC: has received honoraria from AstraZeneca, GlaxoSmithKline, Moderna, Roche and Sanofi. The other authors declare no potential conflicts of interest.
Keywords: COVID-19 vaccine booster shot, vaccine effectiveness, methods, epidemiology
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