The Hong Kong government is warning it may soon bin most coronavirus vaccine doses ordered earlier this year amid a lack of demand and the jabs' approaching expiry date.

More than 3 million Pfizer-BioNTech doses have arrived in the city, but just over 1 million jabs have been used so far and unless takeup improves quickly, authorities will be forced to throw away the remaining supply when they expire later this year.

City residents "only have a three month window," Thomas Tang, member of Hong Kong's vaccine task force, said during a radio interview.

"The vaccines all have expiry dates (and) cannot be used after that date. Community vaccination centers for BioNTech will, according to present plans, cease operating after September," the former Centre for Health Protection controller added.

Pfizer-BioNTech doses have a six month shelf life and must be stored at freezing temperatures. On the other hand, Sinovac vaccines - roughly 2 million of which were delivered to Hong Kong - can last up to one year and require less resource-intensive storage methods.

Among locals, mistrust of the government paired with few vaccination incentives in the nearly virus-free city has sent inoculation rates spiraling down.

Public opinion of the city's vaccination program further sank lower earlier this year when Hong Kong fast-tracked approval of the mainland-developed Sinovac vaccine, which has yet to be backed by the World Health Organization.

To date, a slim 19% of the Hong Kong population have received their first Pfizer-BioNTech or Sinovac dose, while just 14% have taken both doses. Even among medical workers, hesitancy persists. Only a third of health care professionals have been vaccinated so far.

"It's just not right," said Tang, that the city should hoard unused doses even as the rest of the world "is scrambling for vaccines." Hong Kong is one of the only places globally to have received enough vaccine doses for the entire population.

In a bid to use the remaining supply of COVID vaccines, Hong Kong authorities announced plans to expand the inoculation program to nonresidents including roughly 40,000 permit-holding mainlanders and 13,000 refugees.

Earlier this year, the vaccine eligibility threshold was lowered first to 30 years old and then again to 16 as the number of people signing up to the vaccination program dwindled.