People with weak immunity and others facing a high risk from the coronavirus should get a third Covid-19 shot, experts from two Department of Health scientific committees recommended.
And the experts said people who received the Beijing-made Sinovac vaccine can switch to German-made BioNTech shots for the booster.
Government adviser David Hui Shu-cheong said patients with weak immune systems, including cancer patients, HIV/Aids patients and those who have undergone organ transplants, can receive the third jab at least four weeks after the second dose.
"This is because these patients will face a higher risk of death after infection," Hui said.
Citizens aged above 60 and high-risk groups including medical workers, staff at the airport and quarantine hotels and cross-border truck drivers can also receive a booster shot six months after their second dose, Hui said.
He also said people who have been infected with Covid-19 and received one jab can choose to get a second jab six months after their first jab after consulting doctors.
Although recovered patients had higher antibody levels after receiving the first shot, he said, the level of antibodies drops over time.
People who had opted for the Sinovac vaccine can switch to BioNTech for a booster shot as studies have shown the BioNTech jabs can produce a higher antibody level, he explained.
"But for people who have received BioNTech shots already, we don't suggest they switch to Sinovac," Hui said. "You all know that mRNA vaccines are stronger in stimulating the immune system and giving a higher antibody level, and the antibodies also tend to last for longer.
"If the subject is already receiving two doses of BioNTech jabs, there is no reason why you want to switch to a weaker [vaccine]. There is no scientific reason to do that."
Hui stressed that both types of shots can be used as a booster, but "BioNTech is slightly better." Those who have serious allergic reactions after receiving BioNTech vaccines can now choose to get Sinovac jabs, he said.
Hui also said studies showed that more people who received BioNTech vaccine suffered more side effects including pain in the vaccination area and tiredness than did Sinovac takers.
But those side effects are mild and the symptoms should disappear within a few days, he added.
Experts also suggested the entire population - people who can get vaccinated now, aged 12 and above --should get a booster shot. But priority will be given to people with weak immunity and high-risk groups so the vaccination program can be implemented step by step.
"If high-risk groups received the vaccine before and their antibodies drop to a low level, they will be at risk," Hui said. "And that's the reason why we have to have a list of immuno-compromised individuals, who will be given top priority to receive the booster dose."
The Centre for Health Protection will calculate how many shots will be needed after people start to get the booster jab and also consider whether the SAR will need to purchase more vaccine.
The chairman of the Scientific Committee on Vaccine Preventable Diseases, Lau Yu-lung, said people should receive the Covid-19 and flu vaccine 14 days apart.
The World Health Organization earlier said people could receive both jabs at the same time, but Lau said Hong Kong can adopt a more cautious approach.
"We still have time and we are not in a rush, so we can adopt a more cautious measure so that we can distinguish the different side effects caused by the two vaccines," Lau said This is a more safe and conservative suggestion."
Lau added that there should be a minimal interval of at least 14 days between the administration of Covid-19 vaccine and any other vaccines.
But people with urgent needs, including those who need a tetanus shot or HPV vaccine, can still get it at the same time as Covid-19 jabs, he said.
Meanwhile, American experts have endorsed kid-size doses produced by US pharmaceutical company Pfizer for children aged five to 11.
A Food and Drug Administration advisory panel voted unanimously, with one abstention, that the benefits of the vaccine outweigh its potential risks.
The FDA will decide whether it will approve the vaccine for the children within days.