A real-world study of Pfizer's COVID-19 vaccine involving more than half a million inoculated people has demonstrated its overwhelming effectiveness after two doses were given, marking a major milestone for the shot. Even after one dose, the vaccine proved effective at preventing serious illness and death.
According to the independently reviewed study out of Israel's Clalit Research Institute, published Wednesday in the New England Journal of Medicine, two doses of the Pfizer/BioNTech shot were 94% effective at cutting symptomatic COVID-19 cases across all age groups.
"The study provides the first large-scale peer-reviewed evaluation of the effectiveness of a COVID-19 vaccine in a nationwide mass-vaccination setting," the institute said in a statement Thursday.
The vast majority of data on coronavirus vaccine efficacy has been the result of controlled lab conditions in clinical trials, but Israel's speedy vaccine rollouts have provided researchers with the first large-scale real-world data. In many ways, the data showed that benefits seen in smaller, controlled trials translate to the general public.
In collaboration with experts from Harvard University, researchers compared about 600,000 vaccinated people over the age of 16 to the same sized control group of unvaccinated people, with similar age, sex and health history. None of the participants previously tested positive for COVID-19.
The study, involving about 1.2 million people total, showed just one shot of the vaccine is 57% effective in protecting against symptomatic infections after two weeks. The vaccine also proved to be 62% effective at preventing severe disease after one shot, and 92% effective after two.
In terms of preventing hospitalization, the shot was 74% effective after one dose and 87% effective after two.
"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," said lead author Ran Balicer. "These results show convincingly that this vaccine is highly effective against symptomatic COVID-19, one week after the second dose."
The study indicates the vaccine may be effective against the coronavirus variant identified in the U.K. — the dominant strain of the virus in Israel during the time of the study, according to researchers, marking "Israel's third and largest wave of coronavirus infection and illness."
"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," said Miguel Hernán of the Harvard T.H. Chan School of Public Health.
The results reflect those of earlier studies in Israel that focused on symptomatic infections. Infections have fallen in Israel as nearly half of the population has been vaccinated, and the country has eased its national lockdown, reopening stores, schools and many other workplaces in recent weeks.
While the study provides compelling evidence that getting even one shot to the majority of people will dramatically reduce infection rates, several companies, including Pfizer, are now running trials to determine whether a third shot would increase effectiveness against new strains, including those first found in Brazil and South Africa.