The Clalit Research Institute, part of a large Israeli health system, analyzed data on 1.2 million people, about half of whom had received the Pfizer-BioNTech vaccine. Researchers compared patients who received the vaccine with similar individuals who hadn’t.
The rate of symptomatic Covid-19 — meaning people who were infected with the coronavirus and felt sick — decreased by 94% among people who received two doses of the vaccine, according to a press release from Clalit. The rate of serious illness decreased by 92%.
In the Pfizer trial, researchers randomly assigned patients to receive either the vaccine or a placebo. Then they looked to see how many people got sick in each group, and found that the vaccine sharply reduced illness.
Pfizer’s study was a randomized controlled trial, the gold standard in clinical research. The Israeli study, on the other hand, was observational, meaning researchers didn’t randomly pick who got the vaccine and who didn’t.
That can pose problems; people who choose to get vaccinated, for example, may also be more likely to take other steps to protect themselves.
Researchers said they tried to account for those types of effects, however. And observational studies are important because they can offer certain insight into how well vaccines protect people under real-world conditions.