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Most people who got a booster shot of the Pfizer Inc. vaccine had similar or fewer side effects than they did after the second dose, according to a preliminary study conducted by Israel’s largest health maintenance organization.

Of the 4,500 respondents to the survey, 88% reported “a similar or better feeling” than their reaction to the previous dose, with 31% saying they had localized effects like pain or swelling in the area of injection, according to a statement from Clalit Health Services late Sunday.

About 15% of people had other symptoms like tiredness, muscle aches or fever. Less than 1% reported difficulty breathing or chest pains.

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Israel became the first country in the world to disseminate booster shots widely in a bid to quell a surge in new cases. The U.S. hasn’t yet sanctioned the use of third doses, although Pfizer said it plans to meet with U.S. regulators for authorization. About 500,000 Israelis have received the third dose so far.

Clalit said the results of the study, involving people vaccinated between July 30 and Aug. 1, still needed to be analyzed “carefully” given the short testing period. Some people who received boosters may have been suffering from significant side effects that precluded their participation in the survey, it said.