BEIJING: China is giving COVID-19 booster vaccines using technologies different from the initial injections, in an effort to improve immunisation strategies amid concerns that its most-used jabs appeared to be weaker against variants such as Omicron.

Boosting population immunity could be crucial to preparing China to eventually reopen its borders and pivot from its "dynamic zero" strategy, which involves travel curbs and mass testing following dozens of local infections. Experts are watching whether combined Chinese doses would lead to higher effectiveness.

Adults injected with a vaccine developed by Sinopharm or Sinovac at least six month earlier can now receive their booster doses with a vaccines using different technologies, produced by CanSino Biologics (CanSinoBIO) or a unit of Chongqing Zhifei Biological Products, National Health Commission official Wu Liangyou said on Saturday.

Around one-third of China's 1.4 billion people had received boosters using vaccines of the same technologies as their primary doses as of Feb. 7.

Data showed that boosters of either the same or different technologies as primary vaccinations can improve immunity, Wu told a news briefing. He did not contrast the two approaches.

A small-sample Hong Kong research showed that Sinovac's CoronaVac shot, boosted with a third dose about two to five months after the second, failed to produce neutralising antibody responses to Omicron in most recipients.

The Sinovac booster-triggered antibody response was also weaker against Delta than a CanSinoBIO booster in a clinical trial.

Tested against Omicron in a Chinese study, a third BBIBP-CorV shot produced by Sinopharm elicited lower antibody level than a booster of Zhifei's vaccine in those who already received two BBIBP-CorV shot.

Among close contacts of infected people in an earlier Omicron outbreak in Chinese cities of Tianjin and Anyang, receiving a booster reduced Omicron breakthrough infection rate more than threefold compared with the primary vaccination, said Shao Yiming, who is on an expert team for China's COVID vaccine development work group, at Saturday's news briefing.

Shao did not provide complete data or specify which boosters were analysed for the readings.

Products from Sinopharm and Sinovac approved in China are inactivated vaccines that contain inactivated or "killed" coronavirus. Zhifei's vaccine contains parts of the coronavirus protein, while the CanSinoBIO shot uses a modified human cold virus to ferry genetic material from the coronavirus protein.