This happens when enough of a population has protection against an infection that it stops being able to spread - and even people who don't themselves have immunity are indirectly protected.

For Covid the estimated threshold for herd immunity is at least 65%-70%.

But scientists in the UK are more cautious.

Dr Sarah Pitt, a virologist at the University of Brighton, urged "extreme caution" in concluding that herd immunity had been reached - something she believes will be difficult even at high vaccination rates.

 She said it was still too early to tell: "We need to see whether the cases in Israel continue to fall and stay at low levels."  Reaching this level of population immunity is important to protect people who can't be vaccinated or whose immune system is too weak to produce a good, protective response.

In Israel, more than half (5.3 million) its residents have been vaccinated and an additional 830,000 people have tested positive for the virus in the past, which should give them some natural immunity.

That works out as roughly 68% of the population who are likely to have antibodies in their blood which can fight off the virus.

Prof Eyal Leshem, a director at Israel's largest hospital, the Sheba Medical Center, said herd immunity was the "only explanation" for the fact that cases continued to fall even as more restrictions were lifted.

"There is a continuous decline despite returning to near normalcy," he said.

"This tells us that even if a person is infected, most people they meet walking around won't be infected by them."

And cases are falling in all age groups including children, even though under-16s are not generally being vaccinated.

(Adapted from https://www.bbc.com/news/health-56722186)