TEL AVIV (BLOOMBERG) - A year after rolling out the world's fastest coronavirus vaccination programme, Israel again finds itself a global test case as it begins widely delivering fourth doses amid a surge in the Omicron variant.

Although the number of serious cases and deaths remain well below last year's peak, infections have skyrocketed to a record level and could once again threaten to overwhelm hospitals if the new daily caseload reaches 50,000 as Israeli Prime Minister Naftali Bennett has warned.

Many of the newly infected were vaccinated at least twice.

The surge is gripping Israel as its leaders adopt strategies that have confused the public.

And as lines at testing sites get longer, officials have also begun acknowledging they might have to resort to what Mr Bennett decried so vigorously when he was an opposition lawmaker: Another lockdown.

Israel's pandemic experience has been studied around the world because of aggressive steps it took to contain the virus at the outset, its botched reopening after the first lockdown, and its early administration of vaccines and boosters.

Israel went ahead this week with a fourth dose for people age 60 and over despite concerns about the lack of efficacy data, in an effort to protect its most vulnerable.

Professor Galia Rahav, a member of the team of experts advising the government, said Israel shouldn't consider expanding eligibility until it has more data on waning immunity, and cautioned against other countries rolling out a fourth shot too soon.

Israel began administering third doses in August.

"I don't push for a fourth vaccine in countries that were vaccinated with the booster two months ago, or even three months ago," said Prof Rahav, head of the infectious disease unit at Sheba Medical Center outside Tel Aviv.

"But more than 3 months ago, yes."

With less than 70 per cent of the population fully vaccinated, Israel's no longer among the world's most-vaccinated places, figuring No 67 on a Bloomberg ranking.

That's partly because it has a large population of young children, whose parents have often resisted vaccination.

On Tuesday (Jan 4), new infections approached 12,000 in the nation of 9.5 million, breaking an early-September record set just weeks after Israel's world-beating booster drive began.

Almost 59 per cent of the newly infected had been vaccinated with a second shot within the past six months or had recovered from the virus and received one dose.

An additional 8 per cent were last vaccinated more than six months ago.

Of the 125 seriously ill, 84 weren't vaccinated at all.

Testing has become so pervasive, overwhelming laboratories, that beginning Friday, the government will start limiting access to the more accurate PCR tests to people over 60 and at-risk populations.

Antigen tests, including for home use, will fill in the gap.

Plans to go ahead with a fourth vaccine ran into turbulence last month because of the lack of solid data on Omicron or the efficacy of yet another shot. Yet after a clinical trial of a fourth inoculation began, and infection numbers began shooting up, the government went ahead with its plan.

Preliminary data suggested that the fourth dose offered a much higher level of protection against both infection and severe morbidity, and a fivefold increase in the number of antibodies around a week after, Mr Bennett said.

With the caseload soaring, Israel may find itself in lockdown No 4, coronavirus czar Salman Zarka warned this week, defending the government's shifting regulations as a response to changing conditions and new information about Omicron.

He's set 1,500 seriously ill as a red line. If hospitals are swamped by people in intensive care units and heart-lung machines, "then we will have no choice and have to enter a lockdown," he said.

"But we don't have to reach that point. All of us can together make the effort to put on masks, get vaccinated and stay away from crowds."