Chinese researchers reported the findings from a study of blood samples from healthy adults aged between 18 and 59 in a paper published on Sunday, which has not been peer reviewed.

Among participants who received two doses, two or four weeks apart, only 16.9 percent and 35.2 percent respectively still had neutralizing antibodies above what researchers regard as a detectable threshold level six months after the second shot, the paper said.

Those readings were based on data from two cohorts involving more than 50 participants each, while the study gave third doses of the vaccine or placebo to a total of 540 participants.

Researchers said it was unclear how the decrease in antibodies would affect the shot's effectiveness, since scientists have yet to figure out precisely the threshold of antibody levels for a vaccine to be able to prevent the disease.

"In the short-to-medium term, ensuring more people complete the current two-dose schedule of CoronaVac should be the priority," the paper said.

Indonesia and Thailand have already agreed to give a third shot from Moderna and Pfizer respectively for some people who are fully vaccinated with the Sinovac vaccine, amid concerns over its effectiveness against the more transmissible Delta variant of the coronavirus.

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