A small Chinese study detailed in The Lancet Infectious Diseases journal showed neutralizing 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 eradicating all outbreaks while many countries have adopted an approach of learning to live with the virus.

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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 neutralizing 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.

Neutralizing activity against those sub-variants was observed in just 24-48 percent 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 percent, 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.

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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.

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