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A third COVID-19 vaccination shot appeared to significantly curb a delta-led surge in cases and prevent severe illness, according to a study in Israel, the first country to offer boosters to seniors.

Twelve days or more after a booster dose of the vaccine made by Pfizer Inc. and its partner BioNTech SE, the risk of a confirmed infection decreased more than elevenfold relative to people given only two shots, researchers from Israel’s Ministry of Health and key scientific institutes found. A third dose was associated with at least a tenfold reduction in the risk of falling seriously ill, according to the research released Friday.

The more-infectious delta variant has fueled a resurgence of COVID-19 cases and hospitalizations in many countries, including Israel, prompting plans to administer extra doses to those already immunized. Israeli authorities approved a Pfizer-BioNTech booster shot for vulnerable citizens on July 12 and began offering it on July 30 to all people 60 or older who had been immunized for at least five months. On Sunday, the government expanded the booster drive to all residents age 12 and above.

“The results of such a policy are of importance for countries that seek strategies to mitigate the pandemic,” said Yinon M. Bar-On from the Weizmann Institute of Science and colleagues in the paper. “Our findings give clear indications of the effectiveness of a booster dose even against the currently dominant delta variant.”

The analysis is based on medical findings from the health ministry database extracted on Aug. 24. The researchers chose 12 days as the time it took the booster to curb confirmed infections.

The data may be subject to biases and confounding, the scientists said. Still, a more conservative sensitivity analysis suggested a third dose yields about a fivefold improvement in protection against confirmed infection.

The case for administering additional vaccine doses is controversial, since it’s seen reducing supplies needed to immunize people in low- and middle-income countries, where vaccination rates remain stubbornly low.

U.S. public health advisers were set to meet Monday to discuss boosters, with the White House eager to start a nationwide rollout by Sept. 20. The rush is in response to waning antibody levels in the face of delta, the most-infectious coronavirus variant to emerge so far.

Israel’s swift vaccination campaign led to more than half of its people being fully vaccinated against COVID-19 by the end of March, driving the incidence of cases to less than 2 cases per million per day by June from about 900 cases in mid-January, the study showed.

Cases in Israel have climbed in recent months as the delta variant became the dominant cause of COVID-19 there.

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