An Israeli hospital administered fourth Covid-19 vaccine doses on Monday to a test group made up of 150 medical staff.
The Sheba Medical Centre study in Ramat Gan outside Tel Aviv "will zero in on efficacy of the vaccine in producing antibodies, and safety, in order to ascertain if a fourth vaccine is needed in general," a spokesman said.
It comes as the country considers approving a fourth dose for vulnerable populations, in a bid to outpace a surge in infections fuelled by the Omicron variant.
A health ministry expert panel last week recommended that Israel become the first country to offer a fourth vaccine dose - also known as a second booster - to those aged over 60, those with compromised immune systems, and medical workers.
The proposal was welcomed by the Israeli government, which has struggled against a plateauing of turn-out for vaccines.
Final approval
But the fourth shot awaits final approval by the ministry's director-general, Nachman Ash, a medical doctor whose decision, officials say, will be made without government intervention.
Given concern about the lack of test data, Dr Ash may amend the eligibility criteria by raising the age threshold to 70 and dropping medical workers from the roster, Israeli media said.
The health ministry has neither confirmed nor denied that, nor said when Dr Ash's decision is due.
Some 63 per cent of Israel's 9.4 million population have received the first two vaccine doses, according to ministry data. Almost 45 per cent have also received a third dose, or booster shot. Close to 2,000 confirmed or suspected Omicron cases have been logged.
Israel was the fastest country to roll out initial vaccines a year ago, and became one of the first to observe that immunity waned over time, responding with a widespread booster programme.
