Bookings will open on November 5.
Cancer patients, transplant recipients, and those with AIDS can get their booster as soon as four weeks after their second dose.
Medical workers and airport and hotel staff can get a third shot six months after their second jab, as can the over 60s.
Government experts have recommended that people opt for BioNTech for their third dose for a better immune response, though people can still choose Sinovac if they want.
Officials estimate that 1.86 million people are included in the priority groups, though they admit they only have 1.7 million BioNTech doses left.
“Right now we still have enough stock, but we will monitor the situation. In case it is running low, obviously we will procure enough vaccines for people who need them,” Health Secretary Sophia Chan said.
Asked if everyone would be offered a third dose in future, the health chief said authorities will keep monitoring the situation.
“The joint scientific committee has already said, for [all] people who have two doses of Sinovac, they are recommended to have the third dose. Our first phase now is for those high-risk groups to take it first,” Chan said.
“For those who have taken BioNTech but are not in the high-risk groups, we will look into more data. The joint scientific committee and our experts will look into [it] before any further recommendations.”
The minister also said she expects the public response for the third dose to be good, saying many people have been asking for booster shots.
The University of Hong Kong’s Chair of Epidemiology, Ben Cowling, questioned the timing of the booster shots, given the SAR’s zero-Covid strategy.
“If we’re confident that we can keep case numbers very, very low if a fifth wave does occur then I think it may be premature to do third doses now,” he said.
“It may be better to wait, so that we can have more benefit from the protection from the third doses when we really need it, which is when Covid is circulating in the community.”
He urged the government to work on boosting the vaccination rate among the elderly population and assess the risks of a potential outbreak, warning that the virus will find its way back to Hong Kong “sooner or later”.