TEL AVIV—Israel began administering a booster shot of Pfizer Inc.’s Covid-19 vaccine Monday to people with weak immune systems, as it confronts an outbreak of the virus’s highly contagious Delta variant.

Israel’s Health Ministry said a third dose of the Pfizer vaccine would be offered to immunocompromised individuals, such as those who have undergone organ transplants. The step follows evidence that the additional shot might increase such patients’ antibody counts, according to a Health Ministry letter sent to Israeli healthcare providers.

Israel is one of the first globally to approve a booster shot after fully inoculating more than 80% of adults in the country with two doses of the Pfizer vaccine since starting its campaign in December. Pfizer said last week it planned to seek clearance from U.S. regulators to distribute a booster shot.

Eshel Mordehai shows off his vaccination certificate Monday after getting a third dose of the Pfizer Covid-19 vaccine at a medical center near Tel Aviv.

Eshel Mordehai shows off his vaccination certificate Monday after getting a third dose of the Pfizer Covid-19 vaccine at a medical center near Tel Aviv.

Photo: jack guez/Agence France-Presse/Getty Images

The Israeli Health Ministry said the recommended time between a second and third shot would be eight weeks, with a minimum gap of four weeks. The government said it was weighing whether to offer a third shot to the wider population.

Israel’s decision to give a third shot comes amid an outbreak of the Delta variant. New cases in Israel have risen to a seven-day average of 450 on Monday from around 10 a day for most of last month.

The decision on the booster shot followed a recently published Israeli Health Ministry study that showed that the Pfizer vaccine protected 64% of inoculated people from any type of infection during an outbreak of the Delta variant, down from 94% before. The vaccine still provided 94% protection against severe illness, compared with 97% before.

The number of cases in Israel is relatively low by global standards. Other studies have shown that the Pfizer vaccine continued to offer a much higher protection against symptomatic infections.

U.S. federal health officials have signaled they would take a cautious view toward authorizing booster shots.

“Americans who have been fully vaccinated do not need a booster shot at this time,” according to a joint statement issued Thursday by the Food and Drug Administration and Centers for Disease Control and Prevention. “FDA, CDC, and NIH [National Institutes of Health] are engaged in a science-based, rigorous process to consider whether or when a booster might be necessary.”

Companies are working on coronavirus booster shots, as some early studies suggest antibody levels against Covid-19 wane with time, making boosters more necessary. We explore what that means for individual consumers. Illustration: Laura Kammermann/The Wall Street Journal The Wall Street Journal Interactive Edition

Meanwhile, Sheba Medical Center, in central Israel, said Monday it began giving the third shot to heart-transplant patients.

“It’s very important now because we have the emerging Delta virus and waning immunity, so I think it’s really urgent to do it now,” said Galia Rahav, head of the hospital’s infectious-diseases unit. Prof. Rahav was one of the authors of a study published in April that found just 18% of heart-transplant patients developed antibodies following two doses of the Pfizer vaccine.

Israel has said it was focusing on the number of cases of severe illness that develop from the virus, rather than the number of new infections.

On Thursday, the total cases of severe illness in Israel stood at 47. Israeli health officials say many of the cases are people who are fully inoculated but are also from the elderly at-risk population, something to be expected since the vaccine isn’t 100% effective and most of the population has been immunized.

Nachman Ash, the director general of the Israeli Health Ministry, said Monday that Israel would decide in the coming days about giving a booster shot to those who are 65 years old and above following an in-depth study.

“It is something we are really considering and examining as part of the defense of the elderly population against severe illness,” Prof. Ash said.

To prevent another surge in infections, Israel’s government has recently reimposed an indoor mask requirement and other measures after finding that about 90% of the new infections were likely caused by the Delta variant. Israel’s total population is around 9.3 million.