Israel has started to administer fourth Covid vaccine shots to people with weakened immune systems, amid a surge in cases driven by Omicron.
It is one of the first countries in the world to do so.
Heart transplant patients at a hospital near Tel Aviv were among the first to get the jab.
The Israeli health ministry has also approved the fourth shot for residents in retirement homes and geriatric patients.
Israel rolled out its Covid-19 vaccine programme at the world's fastest rate and was the first country to routinely offer a booster shot in the summer.
The BBC's Tom Bateman in Jerusalem has previously said the evidence favouring a fourth shot has not yet been released, and one expert from a national pandemic committee said there was a "paucity of data" on whether immunity from the third shot was waning.
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