Israel on Sunday began offering a COVID-19 booster to children as young as 12, and its prime minister said a campaign that began a month ago among seniors has slowed a rise in severe illness caused by the Delta variant.

Announcing the decision, top Israeli health officials said the effectiveness of the second dose of the Pfizer-BioNTech vaccine waned six months after administration, making a booster necessary.

In a news conference, the head of public health at Israel's Health Ministry Sharon Alroy-Preis stated that the third dose should bring people to the level of protection achieved by the second dose, when it was fresh, and that this will mean that people are 10 times more protected after the booster.

Those eligible for the third shot can receive it provided at least five months have passed since their second jab - a timeframe shorter than an eight-month interval in effect in the United States, which is considering cutting the waiting time.

Hoping to curb the spread of the highly contagious Delta variant, Israel began administering the booster to its older population a month ago and has been gradually lowering the age of eligibility. It stood at 30 before Sunday's announcement.

So far 2 million people out of a population of 9.3 million have received three doses.