A man receives his third dose of the coronavirus disease (COVID-19) vaccine at the Clalit Healthcare Maintenance Organisation in Jerusalem, August 13, 2021. REUTERS/Ammar Awad

A third dose of Pfizer/BioNTech’s COVID-19 vaccine was found to be 86% effective in people aged over 60, an Israeli healthcare provider said on Wednesday, citing initial results from a study of thousands of members.

Israeli HMO Maccabi, which covers around a quarter of the country’s 9.3 million population, compared results from 149,144 people aged over 60 who received their third dose at least a week ago against those from 675,630 more who had received only two doses, between January and February.

Some 37 people tested positive for coronavirus after their third jab, compared with 1,064 positive cases among those who had received only two doses, Maccabi said in a statement. The comparison groups had similar demographic profiles, it added.

Maccabi did not provide any information on the severity of the 37 positive cases, or whether they had any underlying conditions. The Health Maintenance Organization (HMO) did not immediately respond to a request for comment.

Pfizer has said that its vaccine’s efficacy drops over time, and that a third dose showed significantly higher neutralizing antibodies against the initial SARS-CoV-2 virus as well as against the Beta and highly infectious Delta variants.

Israel began administering third Pfizer doses last month to confront a surge in local infections driven by the Delta variant. Some 1.1 million eligible Israelis — people over 50, healthcare workers, and others — have received their third dose.

The United States and several European countries are expected to begin offering boosters to the elderly and people with weak immune systems, and some are considering whether to make a third dose available more widely.

Nonetheless, Israeli health officials worry that cases will continue to mount given that another 1.1 million Israelis — around 11% of the population — remain unvaccinated. Severe cases have also continued to climb, mostly among the unvaccinated.

Health ministry data released on Wednesday, based on data per 100,000 people, showed 172 serious cases among unvaccinated people over 60, compared with 21 serious cases among vaccinated individuals in the same age group.

Dr. Anat Ekka Zohar of Maccabi said that the third dose “has again proved its effectiveness,” and had “demonstrated protection (against) the Delta variant.”

“The triple dose is the solution to curbing the current infection outbreak,” she said.