How effective is a COVID-19 booster vaccine?

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The largest real-world observational study of the third ‘booster’ dose of the mRNA COVID-19 vaccine demonstrates its effectiveness against severe SARS-CoV-2-related illness, hospitalization, and death.

A collaboration of researchers from Clalit Research Institute (Ramat Gan, Israel) and Harvard Medical School (MA, USA) conducted the largest real-world study assessing the effectiveness of a third ‘booster’ dose of mRNA vaccine against severe SARS-CoV-2 Delta variant infection. Analyzing data from Israel’s health record databases, the findings demonstrated that receiving the third dose of Pfizer/BioNTech BNT162b2 vaccine is indeed effective in protecting against severe COVID-19 infection outcomes, compared to receiving only two doses.

The study, led by co-first authors Noam Barda and Noa Dagan (both affiliated with Clalit Research Institute and Harvard Medical School), took place from July 30 2021 until September 23 2021, at the same time as Israel’s fourth wave of coronavirus. During this period, the Delta (B.1.617.2) variant was the dominant strain for new infections in the country. As many countries are also experiencing a resurgence of SARS-CoV-2 infections, thought to be due to waning immunity of vaccines previously administered and the heightened infectivity of the Delta variant, the findings of this study are important now more than ever.

Data from 728,321 individuals ages 12 or above who had received their third dose of the BNT162b2 vaccine were carefully matched and compared with a control group of another 728,321 individuals who had only received two doses of the same vaccine, at least five months prior to recruitment for the study. The methods used to select and match the groups considered a range of demographic, geographic and health-related characteristics associated with risk of infection and severe disease as well as maintaining health and health-seeking behavior. The study also excluded immunocompromised patients and healthcare workers due to concerns of residual confounding.

After measuring the primary outcomes of the individuals (hospital admission for COVID-19, severe COVID-10 disease, and COVID-19-related death) and conducting multiple analyses, the results revealed that compared to the control group, individuals with the third dose had a 93% lower risk of hospitalization, 92% lower risk of severe COVID-19 disease, and 81% lower risk of COVID-19-related death. Interestingly, the vaccine effectiveness was also found to be similar for different age groups (40-69 and 70+), sexes and comorbidities.

“To date, one of the main drivers of vaccine hesitancy has been a lack of information regarding the effectiveness of the vaccine. This careful epidemiological study provides reliable information on third-dose vaccine effectiveness, which we hope will be helpful to those who have not yet decided about vaccination with a third dose,” commented co-author Ben Reis (Harvard Medical School).

Not only are the findings of this study critical in the context of COVID-19 vaccination program roll-out and the wider pandemic, but the study also highlights the potential for other extraordinary epidemiological discoveries made possible by comprehensive datasets.

Source: Barda N, Dagan N, Cohan C et al. Effectiveness of a third dose of the BNT162b2 mRNA COVID-19 vaccine for preventing severe outcomes in Israel: an observational study. The Lancet doi:10.1016/S0140-6736(21)02249-2 (2021) (Epub ahead of print).

Click here to view the press release.

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