An elderly woman receives a booster shot of her vaccination against the coronavirus disease (COVID-19) at an assisted living facility, in Netanya, Israel January 19, 2021. REUTERS/Ronen Zvulun

Most people who received a third dose of Pfizer Inc’s COVID-19 vaccine felt similar or fewer side effects than they did after receiving the second shot, according to an initial survey in Israel.

Israel began offering the booster shots about 10 days ago to people over age 60 as part of efforts to slow the spread of the highly contagious Delta variant. That effectively turned Israel into a testing ground for a third dose before approval by the US Food and Drug Administration.

Israel’s largest healthcare provider, Clalit, said on Sunday it had administered a third dose of the Pfizer/BioNTech vaccine to more than 240,000 people.

About 4,500 people, all of whom received the booster shot from July 30 to Aug. 1, responded to questions and were included in the survey.

Eighty-eight percent of participants in the survey said that in the days after receiving the third shot, they felt “similar or better” to how they felt after the second shot.

Thirty-one percent reported some side effect, the most common being soreness at the injection site.

About 0.4% said they suffered from difficulty breathing, and 1% said they sought medical treatment due to one or more side effect.

Ran Balicer, Clalit’s chief innovation officer, said that even though the results are “initial and self-reported,” they allow a comparison of side effects with the second dose, and “it turns out that in most cases they are similar or less in the booster.”

“Although we do not yet have long-term research on the efficacy and safety of the third booster dose, for the personal risk management of any person aged 60 plus, these findings continue to point to the benefit of immunization now, along with careful behavior among adults and avoiding gathering in closed spaces,” Balicer said.