The strongest evidence to date demonstrating the effectiveness of a third dose of mRNA Covid-19 vaccine comes from an observational study from Israel which reported 93% effectiveness for admission to hospital, 92% for severe disease and 81% for Covid-19 related deaths when compared to those who had received 2 doses of the vaccine (Pfizer, BNT162b2 mRNA) at least 5 months before.1
This was a large population-based study involving over a million people 16 years or older (one-half in each group) who were eligible for the third dose (median age 52 y); those living in long-term facilities, healthcare workers and those medically confined to their homes were excluded. Vaccine effectiveness was evaluated at least 7 days after receipt of the third dose. Median follow-up period was 13 days for both groups.
Overall effectiveness of the third dose vs 2 vaccine doses was 93% (88-97) for admission to hospital, 92% (82-97) for severe disease and 81% for death (59-97). Effectiveness of the third dose was similar between males and females and between individuals 40-60 years and those at least 70 years of age; effectiveness could not be determined in the younger age group due to small number of adverse outcomes.
What makes this study stand out among the previous works2,3 is that it controlled for important possible confounders, including sociodemographic factors, clinical factors, and behavioral factors related to Covid-19. Limitations include its observational nature and exclusion of certain at risk groups, such as nursing home residents and healthcare workers.
Given the increasing number of Covid-19 cases in many communities at this writing, the news that a booster shot of an mRNA vaccine provides further protection in preventing Covid-19 is very welcome!
Bonus Pearl: Did you know that in a study measuring the immune response after the third dose of an mRNA vaccine (Moderna) in those 60 years of age or older, the median antibody titer rose 50-fold!4
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- Barda N, Dagan N, Cohen C, et al. Effectiveness of a third dose of the BNT162b2 mRNA Covid-19 vaccine for preventing severe outcomes in Israel: an observational study. Lancet, published online October 29, 2021. https://www.thelancet.com/action/showPdf?pii=S0140-6736%2821%2902249-2
- Bar-On YM, Goldberg Y, Mandel M, et al. Protection of BNT162b2 vaccine.N Engl J Med 2021; published online Sept 15, https://doi.org/10.1056/nejmoa21114255.
- Patalon T, Gazit S, Pitzer VE, et al. Short term reduction in the odds of testing positive for SARS-CoV-2; a comparison between two doses and three doses of the BNT162b2 vaccine. medRxive 2021;published online Aug 31. https://doi.org/10.1101/2021.008.29.21262792 (preprint).
- Eliakim-Raz N, Liebovici-Weisman Y, Stemmer A, et al. Antibody titers before and aftera third dose of SARS-CoV-2 BNT162b2 vaccine in adults ages ≥60 years. JAMA. Published online November 5, 2021. doi:10.1001/jama.2021.19885 https://jamanetwork.com/journals/jama/fullarticle/2786096
Disclosures: The listed questions and answers are solely the responsibility of the author and do not necessarily represent the official views of Mercy Hospital-St. Louis, Massachusetts General Hospital, Harvard Catalyst, Harvard University, their affiliate academic healthcare centers, or its contributors. Although every effort has been made to provide accurate information, the author is far from being perfect. The reader is urged to verify the content of the material with other sources as deemed appropriate and exercise clinical judgment in the interpretation and application of the information provided herein. No responsibility for an adverse outcome or guarantees for a favorable clinical result is assumed by the author. Thank you!