By Dov Lieber

Early data from Israel suggests a booster shot of Pfizer Inc.’s Covid-19 vaccine can significantly improve immunity in those aged 60 and above, as the U.S. and other countries plan additional doses to increase protection against the highly infectious Delta variant.

Israel was one of the first countries late last month to authorize a third Pfizer dose for the elderly who were fully vaccinated with the recommended two shots, after indications that vaccine protection against severe illness has waned.

The booster shot reduced the risk of infection in the 60-plus age group by 86% and against severe infection by 92%, according to an observational study by Israel’s second largest healthcare provider, Maccabi Health Services, released Wednesday.

Both Pfizer and Moderna Inc. have said their own studies showed a booster shot would improve protection. The Maccabi findings are based on real-world data from a relatively large group, which could help inform other countries that are planning their own vaccine rollout strategies.

The Biden administration said Wednesday it was preparing to offer booster shots beginning the week of Sept. 20. U.S. health authorities said people 65 and older and individuals in chronic-care facilities are expected to get boosters initially. The program would eventually cover more than 155 million people in the country.

U.S. authorities said the current protection against severe disease, hospitalization and death could diminish in the months ahead, especially among those who are at higher risk or were vaccinated during the earlier phases of the vaccination rollout. “For that reason, we conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability,” they said.

The booster shot would be administered to Americans about eight months after the second dose of the vaccine. Israeli healthcare providers offered a third dose to the elderly at least five months after their second shot.

Israel, a relatively small country, has fully inoculated more than 62% of its roughly nine-million population since launching its campaign in December.

An Israeli Health Ministry study published in July found those who had two shots were just 39% protected against infection between June 20 and July 17 amid an outbreak of the Delta variant in the country. Data shared later with medical experts advising Israel’s government showed that protection against severe illness for vaccinated people aged 60 and up had dropped to 81% from 97% in mid-April. Israeli health officials say it remains unclear how much of the loss of protection is due to waning immunity or the rise of the Delta variant.

Maccabi said its latest study compared test results of 149,144 people who had received a booster after at least seven days, with 675,630 people matched by age, gender, socioeconomic status and population who had received two doses between January and February of this year. Just 37 of the former group tested positive, compared with 1,064 of the latter group, the study found.

“These results are highly encouraging. They suggest that the third booster may restore the vaccine efficacy to its original levels,” said Eran Segal, a computational biologist at the Weizmann Institute of Science and a key adviser to the Israeli government on the pandemic.

Israel this month lowered the minimum age required for a booster shot to 50 and is now considering offering it to those aged 40 and above, as more and more local healthcare providers say it is already showing results and few have reported any serious side effects.

More than a million Israelis and over half of those eligible for the booster shot have since received it, according to authorities.

The Israeli government authorized the booster in the hopes that the unproven measure at the time could help stem a serious Delta outbreak, which also forced it to bring back mask mandates, impose limitations on gathering and travel, and to consider another lockdown. The country had a seven-day average of new confirmed cases of nearly 6,300 and a total of 578 cases of severe illness as of Wednesday, nearly triple what it was at the start of August.