The study comes as China, which has approved only locally developed COVID shots including the Sinopharm vaccine, strives to improve vaccination rates, maintaining a "dynamic zero COVID" policy aimed at eradicate all outbreaks while many countries have adopted an approach of learning to live with the virus.
The vaccine, BBIBP-CorV, is one of the two Sinopharm COVID shots approved for use in China, and is also the main shot that the state-owned firm has exported.
Among 25 individuals who received two doses of BBIBP-CorV vaccine, the neutralising activity against sub-variants such as BA.2.12.1 and BA.4/BA.5 "was not or only minimally detectable", researchers said in correspondence published on Monday.
Neutralising activity against those sub-variants was observed in just 24-48% of subjects who received a BBIBP-CorV booster shot after the two-dose product, researchers said, citing results from a group of 25 participants.
The rate improved slightly, to 30-53%, for those who received a third shot made by a unit of Chongqing Zhifei Biological Products, another vaccine approved for use in China, according to data from another group of 30 subjects.
The study did not discuss the boosters' efficacy, a rate that reflects how well they could lower the risk of COVID disease or death, which is usually observed in large clinical trials.
(Reporting by Roxanne Liu and Ryan Woo; Editing by Miyoung Kim and Kenneth Maxwell)