Israel sent about 700,000 expiring Pfizer -BioNTech shots to South Korea, in what is the world’s first Covid-19 vaccine swap.

South Korea, which has received just 20% of the 192 million doses it preordered, will send an identical number of shots back to Israel at a later late. The arrangement is likely to be considered by other governments, health experts say.

Israel has fully vaccinated the majority of its roughly 9 million citizens but domestic uptake is stalling ahead of the Pfizer shots’ expiration at the end of July. Recent attempts to swap the vaccines, including with the Palestinian Authority, fell through, partially over how close the shots’ six-month expiration was.

South Korea made for an eager partner. The wealthy country, which is waiting in line as global backlogs slow vaccine distribution, has administered one dose to fewer than a third of its population. The country reported the most daily cases of the year on Wednesday, at more than 1,200.

The vaccines from Pfizer Inc. and partner BioNTech SE arrived at South Korea’s Incheon International Airport Wednesday morning. After quality inspections, the doses should be usable within a week, South Korean health officials said. Based on the country’s recent distribution rates, the 700,000 doses should be administered within a day or two, they said.

The Seoul government is still awaiting delivery of nearly 60 million doses of Pfizer-BioNTech’s Covid-19 vaccine. It will send 700,000 of those shots back to Israel in September or October, the two governments said.

The majority of Israel’s population is fully vaccinated against Covid-19.

The majority of Israel’s population is fully vaccinated against Covid-19.

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

“This is a win-win deal,” Israeli Prime Minister Naftali Bennett said.

Mr. Bennett said the deal between Israel and South Korea was authorized by Pfizer CEO Albert Bourla following several discussions between the two countries in recent days. Pfizer is committed to ensuring the vaccine is administered effectively including through intergovernmental collaborative efforts such as the Israel-South Korea swap, a company spokeswoman said in a statement.

Such deals can help straighten out imbalanced Covid-19 vaccine supply around the world—at least among wealthy countries.

For now, most of the unused doses are being donated. Last month, President Biden said the U.S. will donate 500 million doses through the Covax program, an initiative financed mostly by Western governments to help lower-income countries with vaccines. The U.K. and Canada each recently vowed to eventually donate about 100 million shots. European countries including Germany, France and Italy pledged to collectively share 75 million doses.

Vaccine swaps, which bring back a haul of shots in the future, carry benefits for the country sending the shots as fast-spreading variants of the coronavirus continue to emerge, and may ultimately require booster shots for individuals already fully inoculated, said Daniel Rhee, a research scientist at the International Vaccine Institute in Seoul.

“It balances out the risk for countries that need vaccines now and countries that need longer expiration dates for unused doses,” Mr. Rhee said.

Israel had been looking for a partner to swap vaccines with after the Palestinian Authority pulled out of a deal in mid-June. Israel had originally offered up to 1.4 million doses to the Palestinian Authority. But following an outbreak of the Delta variant that started in Israel in early June, the government has embarked on a fast-paced vaccination drive for those between the ages of 12 and 16. More than 100,000 individuals in that age bracket have received their first doses in recent days, the country’s healthy ministry said.

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Seoul had proposed a similar exchange with the U.S., before South Korean President Moon Jae-in’s visit to the White House in May. South Korea, which had held off on mass purchases of vaccines over potential side effects and unclear efficacy, was further back in the line when Covid-19 vaccines became available late last year.

The South Korean government has purchased enough doses to vaccinate its roughly 52 million people twice. Seven in 10 South Koreans said they would get vaccinated, according to a government survey in May.

For now, vaccine waste isn’t an issue in South Korea. Nearly everyone shows up for their vaccination appointment, with a no-show rate of just 2%. Leftover vaccines can be booked via mobile apps—and have been snapped up within seconds.

By the end of September, South Korea expects to get another 80 million doses from Pfizer, AstraZeneca PLC, Johnson & Johnson and Moderna Inc.

The country remains on track to vaccinate 70% of the population by September. Once it hits that threshold, South Korea will consider swapping its surplus vaccines with other countries in need, said Jung Eun-kyeong, commissioner of the Korea Disease Control and Prevention Agency, at a briefing Tuesday.

“Acceptance rates are high, allowing us to distribute the vaccines in a short amount of time,” Ms. Jung said.

Write to Dasl Yoon at dasl.yoon@wsj.com