An Israeli hospital administered fourth Covid-19 vaccine doses to a test group of health workers on Monday, in what it called the first major study into whether a second round of boosters will help contend with the fast-spreading Omicron variant.

Results of the trial, likely to be closely watched internationally, will be submitted to Israel's Health Ministry in about two weeks, said a spokesperson for Sheba Medical Centre near Tel Aviv.

Israel was the fastest country to roll out initial vaccinations a year ago and became one of the first to launch a booster programme after observing that immunity waned over time...