Data from Israel, which has vaccinated the overwhelming majority of its elderly population with the Pfizer Covid-19 vaccine, suggests that mass vaccinations have prevented people from becoming severely sick, according to a new study from the Centers for Disease Control and Prevention.

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While clinical trials have found the Pfizer-BioNTech vaccine to be 95% effective in preventing Covid-19, the Israeli data offers an early glimpse into how effective the vaccine is in an uncontrolled, real-world setting.

The study, which was published Friday in the CDC’s Morbidity and Mortality Weekly Report, found that among the part of the Israeli population that has been vaccinated the most, the percentage of patients requiring ventilation drastically dropped, suggesting a reduction in severe sickness.

“Taken together, these results suggest reduced rates of severe COVID-19 following vaccination,” wrote the researchers from Ben Gurion University of the Negev, Tel-Aviv University and Maccabi Healthcare Services.

Israel launched its national vaccination campaign in December prioritizing people 60 and older, health-care workers, and people with comorbid conditions. By February, the researchers said, 84% of the population 70 and older had been fully immunized with the Pfizer-BioNTech two-shot vaccine. Only 10% of the population under 50 years old had been vaccinated by the same time, the researchers said.

The researchers compared the number of Covid-19 patients 70 and older who required a mechanical ventilator to those younger than 50 who need a ventilator. The researchers said they used the need for a ventilator, a medical instrument used to help patients breathe, to measure severe Covid-19.

Between October and February, the number of patients 70 and older who needed a ventilator fell. At the same time, the number of people under 50, a population that generally wasn’t vaccinated, who needed a ventilator rose, the study found. The country began administering shots to mostly older people Dec. 20, with a second round of shots following three weeks later.

The researchers noted a few limitations of the study. Israel implemented a strict national stay-at-home order on Jan. 8, weeks after the beginning of the vaccination campaign, that could have led to a decrease in severely sick patients who would have needed ventilators. The introduction of new variants of the coronavirus also could have affected the data, they said.

The researchers said their findings are preliminary, “important evidence of the effectiveness of vaccines in preventing severe cases of COVID-19 at the national level in Israel.”

“Receipt of COVID-19 vaccines by eligible persons can help limit spread of disease and potentially reduce the occurrence of severe disease,” they wrote.