That comes as 30 of 37 cases in Hong Kong of post-jab heart inflammation occurred after second jabs.

Experts from two scientific committees said one BioNTech jab will offer sufficient protection.

And they also said young adolescents, especially boys, could suffer from possible adverse side effects.

The experts urged authorities to gather more data from Beijing pharmaceutical firm Sinovac - maker of the other vaccine available in the SAR - to speed up approval of lowering the age requirement from the current 18 years old to 12 so that families have a choice.

Authorities in June lowered the age requirement for BioNTech from 18 to 12.

In a briefing yesterday, Lau Yu-lung, chairman of the Scientific Committee on Vaccine Preventable Diseases, said statistics showed that myocarditis -inflammation of the heart muscle - is a side effect of the BioNTech vaccine.

Hong Kong has recorded 37 cases of myocarditis after vaccination - 32 boys and five girls, Lau said, and 30 of them had heart inflammation after the second jab.

So experts believe one shot should be enough for youngsters after balancing the benefits and risks.

Lau added it had not been a mistake to allow youngsters to get two BioNTech shots as "all the decisions made at that time were based on the best available evidence."

And youngsters from 12 to 17 should be allowed to receive Sinovac jabs as they could develop more antibodies than adults, he said.

Statistics showed the risk of youngsters having serious side effects after vaccination is the same as adults, Lau added.

Also yesterday, David Hui Shu-cheong, a Chinese University of Hong Kong respiratory medicine expert, said a third jab may be necessary, especially for those who have had organ or bone marrow transplants or who have a compromised immune system.

Hui said there is no urgency to administer a third jab, but he recommended authorities consider arrangements to administer them two months before Hong Kong reopens the border with the mainland.