According to a lab study, antibodies triggered by Sinovac Biotech’s COVID-19 vaccine dropped below a key threshold from around six months after a second dose for most recipients, although a third shot had a strong booster effect.

Chinese researchers reported the findings from a study of blood samples from healthy adults aged between 18-59.

Among participants who received two doses, two or four weeks apart, only 16.9 percent and 35.2 percent respectively still had neutralising 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 drop 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 inoculated with the Sinovac vaccine, amid concerns over its effectiveness against the more transmissible Delta variant of the coronavirus.

Turkey had started offering a third dose from either Sinovac or Pfizer to some people who have got Sinovac shots.