Third Dose of SARS-CoV-2 Vaccine: A Systematic Review of 30 Published Studies

INTRODUCTION: We analyzed published studies on the efficacy and safety of a third dose of COVID-19 vaccine in various general population settings. MATERIAL AND METHODS: We conducted systematic searches of PubMed and EMBASE for series published in the English language through November 15, 2021, using the search terms "third" or "booster" or "three" and "dose" and "COVID-19" or "SARS-CoV-2." All articles were selected according to the MOOSE guidelines. The seroconversion risk after third doses was descriptively expressed as pooled rate ratio ([seroconversion rate after 3rd dose]/[seroconversion rate after 2nd dose]). RESULTS: The search returned 30 studies that included a total of 2 734 437 vaccinated subjects. In more than 2 700 000 Israeli patients extracted from the general population, the reduction in the risk of infection ranged from 88% to 92%. Conversion rates for IgG anti-spike ranged from 95% to 100%. In cancer or immunocompromised patients, mean IgG seroconversion was 39.4% before and 66.6%. CONCLUSIONS: A third dose seems necessary to protect against all COVID-19 infection, severe disease, and death risk. This article is protected by copyright. All rights reserved.