A study with over half a million subjects has found the Pfizer mRNA vaccine to be 92% effective at preventing infection.
The research, published in the New England Journal of Medicine, tracked 596,618 people in Israel who had received the Pfizer-made BNT162b2 vaccine between 20 December and 1 February. The researchers matched these people up with unvaccinated participants who had the same demographic and clinical characteristics.
They found that when both doses were administered, the vaccine was 92% effective at preventing infection with COVID-19: that is, 92% of participants who tested positive to COVID-19 were in the unvaccinated population. The vaccine was also 87% effective at preventing hospitalisation, and 92% effective at preventing severe disease.
Single doses of the vaccine weren’t as good at preventing infection, but they still provided some protection against the virus. From two to three weeks after the first dose, the vaccine also appeared to lower risk of death from COVID-19. There were 41 total deaths from COVID-19 in the study, 32 (or 78%) of which were in the unvaccinated population. Statistical and demographic analyses adjusted this down to 72% effective at preventing death after one dose of vaccine.
There wasn’t sufficient data to examine whether both Pfizer vaccine doses would be more effective to lower the death rate.
Israel is on track to be one of the first countries to fully vaccinate its citizens, with over 3 million people (35% of the population) currently fully vaccinated with the Pfizer vaccine. This study is one of the earliest to look at the efficacy of the vaccine on a nation-wide scale.
The study didn’t look at specific strains of COVID-19, but the authors point out that the B1.1.7 strain was very common in Israel over the reporting period – up to 80% of new cases. This strain was first found in the UK and has been shown to be more infectious than other variants of SARS-CoV-2. This means that the vaccine is likely to be effective against this variant. Other variants of concern weren’t common enough in Israel at the time for the authors to make a judgement.
According to Johns Hopkins University, 216 million vaccine doses have been administered globally so far, and 43 million people are fully vaccinated. While Phase 3 trials of vaccines included tens of thousands of participants, nation-wide data like this study will be useful for providing extra information on their efficacy.
Ellen Phiddian is a science journalist at Cosmos. She has a BSc (Honours) in chemistry and science communication, and an MSc in science communication, both from the Australian National University.
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