An Israeli hospital launched a clinical trial Monday on the impact of a fourth shot of the Pfizer-BioNTech coronavirus vaccine, as the health ministry weighs a national fourth jab rollout.

The study being conducted by Sheba Medical Centre involves 150 volunteers from the hospital's staff, who are at least four months removed from their third vaccine shot and have signs of waning antibodies.

Director of the Infectious Disease Epidemiology Unit at Sheba, Gili Regev-Yochay, said the study would assess the potential antibody boost from a fourth shot, the existence of adverse effects and whether an additional booster reduces the risk of infection.

Participants will be monitored for six months, she added.

Israeli health ministry spokeswoman Gal Rotem Golan told AFP that the ministry had approved the trial to "check the efficacy" of the fourth shot.

The study's launch comes as Ministry of Health Director General Nachman Ash weighs the recommendation of an Israeli expert panel to begin administering a fourth vaccine shot to medical staff and everyone above 60 years old.

Prime Minister Naftali Bennett hailed the recommendation, which came earlier this month, citing the risk of the highly transmissible Omicron coronavirus variant.

He said that Israel, which was among the first countries in the world to offer a third shot to the general public, would be a trailblazer for the fourth jab.

But the rollout is on hold, pending the health ministry's approval.

Ash reportedly wants more time to assess the recommendation, as he awaits further data on the severity of infections caused by Omicron.

Ran Balicer, chairman of Israel's national expert panel on Covid-19, supported the trial as Israel is "the first (country) to have a large national cohort of third dose vaccinated people with substantial waning immunity".

Heart transplant surgeon Jacob Levee, who is participating in the trial for the fourth shot, told reporters: "I did not want to expose the heart transplant patients I am taking care of to the potential of getting the Omicron variant."

Some 4.2 million of Israel's estimated 9.3 million citizens have received three doses of coronavirus vaccine.