TEL AVIV (FINANCIAL TIMES) - Israel is weighing further lockdowns and the possibility of extending booster vaccine shots to those over 50 years old after the number of new coronavirus infections surged to the highest levels since February.

Almost 80 per cent of the adult population has been fully vaccinated, one of the highest rates in the world, but more than 6,000 people tested positive for Covid-19 on Monday (Aug 9), according to the Health Ministry. Nearly 650 people were in hospital and 400 of them were in serious condition, it said, without indicating how many had already been vaccinated.

Just under 5 per cent of coronavirus tests were coming back positive, in a sign that the nation was in the midst of a major Covid-19 increase, warned Dr Salman Zarka, the government's coronavirus czar. Israel was at "a critical point for our health, for our lives and for our economy", he said.

As the first country to launch a mass inoculation drive and the first to reopen its entire economy after fully vaccinating nearly 70 per cent of its adult population with the BioNTech-Pfizer vaccine by early April, Israelis have enjoyed a period of post-pandemic freedom.

Now, with studies showing the efficacy of the vaccine fading among the over-60s, some of whom received their first shots as early as last December, the health authorities have taken the lead in administering booster injections.

Israel did not wait for the United States Food and Drug Administration to approve boosters of the Pfizer vaccine but administered third shots to about 2,000 immunocompromised people weeks ago, before extending them to all those over 60 on Aug 1.

As many as 600,000 older Israelis have now received a third shot, which could soon be offered to the over-50s too.

Israel's experience with the Delta coronavirus variant and Pfizer's messenger RNA vaccine is being closely watched by the US and Europe as they consider authorising third shots for their older populations.

Prime Minister Naftali Bennett on Tuesday said that 90 per cent of new infections were in people aged 50 and above. "I ask every Israeli citizen over 50 to be very careful in the coming weeks."

The rapidly rising caseload threatens to unravel the government's plans to keep the economy open. Israeli officials are considering a lockdown over the Jewish High Holidays next month, to break the chain of infection.

During these holidays, families tend to visit their grandparents.

A lockdown is seen as a last resort, and restricting it to the holidays would mean it has less impact on the economy, said a Finance Ministry official.

Health Ministry data shows that the possibility of serious illness is as much as six times higher in the unvaccinated than in those who have had their two shots.

"If people get vaccinated as quickly as possible, then there won't be any need for any lockdown," said the official, who asked not to be identified. "We look at the charts daily, and right now there are enough vaccines, more than enough vaccines for everyone who wants one, or two, or three."

But the rapid pace of new infections makes it likely that new restrictions will have to be imposed, he said.