Pfizer's COVID-19 vaccine performs just as well in the real world as it did in trials, a landmark Israeli study of more than 1 million people suggests. It was 94% effective.

• Pfizer-BioNTech's two-dose coronavirus vaccine was 94% effective at protecting against COVID-19 in a real-world study.
• In late-stage clinical trials before it was authorized, it was 95% effective.
• The new Israeli study of the Pfizer vaccine involved more than 1 million people.

Pfizer and BioNTech's vaccine is just as effective at protecting against symptomatic COVID-19 in the real world as it was in clinical trials, an Israeli study published in the New England Journal of Medicine Wednesday suggests.

The study found that the vaccine was 94% effective at protecting against coronavirus with symptoms, seven days after the second dose, when two shots were given 21 days apart. This is comparable to the 95% effectiveness reported in late-stage clinical trials in November.

The Israeli study also showed that Pfizer's vaccine was 90% effective at preventing people from passing on the virus, after two doses.

The shot worked just as well across all age groups, with slightly lower efficacy for people with co-existing medical conditions, the researchers said. About one-quarter of the participants were over 60 years old.

"These results strengthen the expectation that newly approved vaccines can help to mitigate the profound global effects of the COVID-19 pandemic," the authors said.

This was the largest study so far examining how well the shot works outside of clinical trials and against the coronavirus variant first found in the UK, called B.1.1.7. Israel has immunized about 35% of its population already, according to Johns Hopkins University, the most of any country worldwide.

About 80% of Israelis with COVID-19 are infected with the highly contagious B.1.1.7 variant: The authors said the results showed that the vaccine worked against it. The variant first detected in the UK is estimated to be 30-50% more infectious, and has spread across the world, including to the US, where it is expected to become the most common coronavirus in the country by March, according to Centers for Disease Control and Prevention predictions in January.

"This is the kind of vaccine that gives us hope that herd immunity may be possible," Raina MacIntyre, professor of biosecurity at the University of New South Wales in Sydney, told Bloomberg.

MacIntyre, who was not involved with the study, said that at the levels of efficacy seen in Israel, vaccinating about 60% to 70% of the population should be enough to prevent widespread infections, illness, and death and "have the best chance of resuming normal life and opening up society."

There is ongoing debate worldwide about whether to delay the second dose of Pfizer's vaccine beyond the three weeks that was studied in trials. In the UK, the second dose can be delayed up to 12 weeks.

The study authors reported that fourteen to twenty days after just one dose, Pfizer's shot was 72% effective at protecting against hospitalization, 74% effective at preventing COVID-19 deaths, and 57% effective at protecting against milder COVID-19 illness with symptoms.

The true figure for protecting against symptomatic COVID-19 after one dose was actually lower, at 29%. The authors said the 57% was an "estimate" that took into account high levels of coronavirus in Israel at the time, but it is not clear exactly how the numbers were crunched. The shot did not work so well at stopping people from passing on the virus after just one dose, with 29% effectiveness 14-20 days after the first dose.

The trial did not include anyone who had previously tested positive for coronavirus, health workers, or nursing home residents. There was some early evidence from the US that people who have been infected before with coronavirus produced a higher immune response after a single dose of vaccine than those who hadn't, so the efficacy number for a general population could be higher.

The study authors did not report on the safety of the vaccine.

The study was conducted between December 20 and February 1 by researchers from the Clalit Research Institute, one of the top four largest Israeli healthcare providers, Ben-Gurion University of the Negev in Israel, and Harvard University in the US.

The researchers compared more than half a million people over 16 years old who had had Pfizer's shot, with the same number who were not immunized. The participants in each group were closely matched in terms of medical conditions, ethnicity, and age - a process that could get harder as the Israeli vaccination efforts continue and there are fewer people who have not had a shot.