CHINA – According to a lab study, antibodies triggered by Sinovac Biotech’s COVID-19 vaccine declined 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from a study of blood samples from healthy adults aged between 18-59 in a paper published on Sunday, though it was not peer reviewed.
Among participants who received two doses, two or four weeks apart, only 16.9% and 35.2% respectively still had neutralizing antibodies above what researchers regard as a detectable threshold level six months after the second shot.
The 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.
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.
Turkey had started offering a third dose from either Sinovac or Pfizer to some people who have got Sinovac shots.
As of end-June, Sinovac has delivered more than 1 billion doses of the vaccine, a major vaccination tool in China, Brazil, Indonesia and Chile.
Sinopharm, the vaccine maker has already applied for the emergency use listing of its vaccine in Brazil following a statement by the Brazilian health agency.
The agency said that it will review over the next day whether the documentation submitted by Sinopharm is complete and that it may request additional information to process the emergency-use application.
In the month of June, WHO validated the Sinovac-CoronaVac COVID-19 vaccine for emergency use, giving countries, funders, procuring agencies and communities the assurance that it meets international standards for safety, efficacy and manufacturing.
WHO’s Emergency Use Listing (EUL) is a prerequisite for COVAX Facility vaccine supply and international procurement. It also allows countries to expedite their own regulatory approval to import and administer COVID-19 vaccines.
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