Professor David Hui says fourth booster shot may be needed for those who had earlier taken Sinovac jab, as antibody levels may be lower.
Professor Wallace Lau raises idea of annual jabs, also citing waning antibodies.
Hong Kong government vaccine experts have said they do not rule out the possibility of a fourth Covid-19 jab for residents, with the city rolling out third shots under its booster programme next week.
The idea, raised by Professor David Hui Shu-cheong, a government pandemic adviser, came on the heels of official advice issued by the Macau government last week, indicating that the third dose should be given at least 28 days after the second shot, while the fourth should come six months following the third jab.
Authorities of the casino hub also said the extra shots should cover immunocompromised people aged 12 or above.
Hong Kong will start offering booster shots to the elderly and those under high-risk groups from next Thursday. Those who have received the Chinese-produced Sinovac vaccine can also get a third jab even if they are not in the vulnerable category, although such cases will be handled on a discretionary basis by on-site staff.
On Saturday, Hui said the amount of antibodies in a person vaccinated with the Sinovac shot could drop to a low level six to eight months after their second jab. But this could be boosted to 97 per cent from about 30 per cent if they opt for the German-made BioNTech jab as their third dose, compared with 58 per cent for another Sinovac shot, according to the expert.
“If the third dose is Sinovac, you may need a fourth shot six or eight months later due to the fall in antibody levels,” Hui told a radio programme. “By contrast, if you opt for BioNTech as your third dose, the antibody levels will shoot up much higher and last longer even as they wane.”
On a separate programme, Professor Wallace Lau Chak-sing, convenor of the advisory panel on Covid-19 vaccines, said although not much data was available to support the necessity of a fourth or fifth dose, it was possible annual vaccinations were needed for residents in the future as antibody levels waned.
Hui also rebutted comments by city leader Carrie Lam Cheng Yuet-ngor, who said she would opt for a Sinovac booster shot because “a higher level of antibodies does not mean a vaccine is better, having enough is fine”.
“It is just [her] opinion,” he said. “We do not know what level of antibodies can provide enough protection, but the risk of experiencing severe infection is lower if the level is higher, especially for frontline medical workers and people who have weak immune systems.”
On Saturday, Hong Kong confirmed seven coronavirus infections, all imported, bringing the local tally to 12,367, with 213 related deaths. The new cases included two domestic helpers from the Philippines, with the remaining patients being arrivals from Nepal, Pakistan, Britain and the United States.
About 24,500 people had signed up online for the third vaccine dose as of 8pm on Friday, the first day bookings became available. Hui said the number was “not small”, adding that he believed more would sign up.
Asked whether priority for the third shot should also be given to people who had been vaccinated earlier, Hui said those with weaker immune systems should be targeted instead.
“We need an orderly roll out of the vaccination scheme. If you are a healthy person who received the BioNTech jab, there are enough antibodies for you after six to 10 months, you do not need to rush,” he said.
“For those who had received the Sinovac vaccine, they may need to take the booster shot earlier, but we still need to prioritise in the end.”
On reopening the border with mainland China, Hui said the National Health Commission might send representatives to the city to study its Covid-19 preventive measures before proceeding with the move. —SCMP