“The priority to receive an additional dose of Sinopharm is for the senior citizens and people with chronic diseases,” it said.

Bahrain issued a similar statement soon after, saying booster shots would be given to recipients of the Sinopharm jabs, starting with priority groups.

Around the world, countries are studying whether to give a booster shot to extend immunity. Britain officially started a trial on Wednesday to see whether people who completed the immunisation regime would need a boost shot, and which vaccine to give them if they did.

Executives of Pfizer-BioNTech and Moderna also said last month that people who received both doses of their vaccines would probably need a booster shot within 12 months, and possibly annually.

“It is a concern that these inactivated vaccines appear to have very weak immunogenicity such that they need a third dose just six months after the second dose is given,” said Nikolai Petrovsky, a vaccine developer and professor at Flinders University in Australia, adding that the problem might stem from the small amount of spike protein that inactivated vaccines contain or a weaker kind of adjuvant used to boost the vaccine’s performance.

Taiwan’s low coronavirus vaccination rate under scrutiny as it battles spike in cases

Better immunity could be achieved by mixing vaccines that used different technologies, rather than administering a third shot, he said.

“In my experience with other vaccines such as hepatitis B, little benefit can be expected from subsequent boosting so what is needed is a new approach,” Petrovsky said.

“We will inevitably see more use of mix and match regimens as adenovirus vaccines [the technology behind the Oxford-AstraZeneca product] cannot be used for further boosts due to anti-vector immunity, so a different vaccine, such as recombinant protein will be needed to boost the hundreds of millions who have already been vaccinated with adenovirus vaccines in due course.”

A Chinese vaccine expert, who declined to be named due to the sensitivity of the matter, said the industry had been studying the antibody levels of vaccinated and recovered patients and was working on ways to improve immunisation.

“Vaccination using different technologies for their advantages is better, but a third dose of an inactivated vaccine is acceptable if no other options are available,” the person said, adding that the Chinese authorities would arrange ways to boost the immune response in the future if it were needed.

The UAE hosted the largest final-stage clinical trials of the Sinopharm product in July last year. It is now being produced in Abu Dhabi through a joint venture of China National Biotec Group – Sinopharm’s vaccine subsidiary – and its trials partner G42 Healthcare.

The UAE has one of the highest inoculation rates in the world, with nearly 74 per cent of those eligible for jabs having received them. However, it continues to report more than 1,000 new infections every day. It has remained open for business and visitors during its vaccination drive.

Taiwan raises alert level for entire island after 267 more local cases reported

The Chinese embassy in the UAE last week warned Chinese citizens there to wear masks and avoid using public transport and joining large gatherings.

Chinese researchers are conducting a trial to see if administering the adenovirus-vectored vaccine made by CanSino Biologics, followed by a protein subunit vaccine produced by Anhui Zhifei Longcom Biopharmaceutical can boost immunity. The preliminary results are expected next month.