Pfizer Vaccine is 94% Effective, says Study

Pfizer vaccine 94% effective
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The largest real-world study has found that the Pfizer vaccine is 94% effective at preventing COVID-19.

Researchers at The Clalit Research Institute and Harvard University have conducted the first real-world analysis of the efficacy of the Pfizer vaccine. This large-scale study took place in a nationwide mass-vaccination setting in Israel. At a time when Israel’s cases were at their peak and authorities were reporting almost 12,000 new cases per day. Researchers analyzed data from over 1 million people who had received the Pfizer vaccine between 20th December 2020 and 1st February 2021. They then matched them with unvaccinated controls according to their clinical characteristics and demographics.

The swift nationwide rollout of Israel’s COVID-19 vaccination campaign provided the Clalit Research Institute with a unique opportunity to assess, through its rich digital datasets, the effects of the vaccine in a real-world setting in all population sub-groups

Prof. Ran Balicer, senior author of the study

Although randomized clinical trials are the ‘gold standard’ for evaluating vaccine efficacy, their highly controlled setting does not assess for all factors. Nor can one estimate vaccine efficacy in different subpopulations. The experimental design used for the real-world study allowed for a more detailed analysis of the vaccine’s effectiveness. Moreover, researchers could assess the vaccine’s efficacy across a range of subpopulations, age-groups, and outcomes. The study outcomes included documented infection, symptomatic COVID-19, severe disease, hospitalization, and death.

This combination of evidence from randomized trials and observational studies is a model for efficient medical research, something which is especially important in COVID times.

Prof. Miguel Hernán of the Harvard T.H. Chan School of Public Health

However, the study’s design did not allow for the assessment of asymptomatic infections or viral transmission in a mass-vaccination setting.

94% Effective at Preventing Infection

The results, published in the New England Journal of Medicine, further confirm the findings of Pfizer’s phase 3 clinical trials. The authors of the study report that the Pfizer vaccine is 94% effective against symptomatic infection, 92% against severe disease, and 97% against hospitalizations.

The results also correlate well with recent population-level trends in Israel, which have seen a sustained decline in hospitalization and severe disease in the mostly-vaccinated older age groups, alongside a delayed decline among younger age groups for whom vaccination began several weeks later.

Prof. Ran Balicer, senior author of the study

At the time of the study, the UK variant was prevalent in Israel and the results further prove the Pfizer vaccine’s effectiveness against the new variants. Therefore, the results further provide hope that COVID-19 vaccines can likely put an end to the pandemic.


Dagan, N., Barda, N., Kepten, E., Miron, O., Perchik, S., Katz, M. A., . . . Balicer, R. D. (2021). BNT162b2 mRNA Covid-19 vaccine in a nationwide mass Vaccination Setting. New England Journal of Medicine. doi:10.1056/nejmoa2101765


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