The Clalit Research Institute, part of a large Israeli health system, analyzed data on 1.2 million people, of whom approx. half had received the Pfizer BioNTech vaccine. Researchers compared patients who received the vaccine with similar individuals who had not.
The rate of symptomatic Covid-19 – meaning people who were infected with coronavirus and felt ill – dropped 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 placebo. Then they looked to see how many people got sick in each group, and found that the vaccine greatly reduced the disease.
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 did not randomly select who received the vaccine and who did not.
It can create problems; people who choose to be vaccinated, for example, may also be more likely to take other steps to protect themselves.
However, researchers said they were trying to account for these types of effects. And observational studies are important because they can provide some insight into how well vaccines protect humans under real conditions.
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