A small Chinese study detailed in The Lancet Infectious Diseases journal showed neutralising antibodies against some Omicron sub-variants were largely undetectable after two doses of a Sinopharm Covid-19 vaccine, with a booster shot only partly restoring them.
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 (300122.SZ), 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.
