Antibodies triggered by Sinovac Biotech’s COVID-19 vaccine fell below a key threshold starting at about six months after most recipients received a second dose, although a third shot had a strong booster effect, a laboratory study found.
Chinese researchers reported the findings from a study of blood samples from healthy adults aged 18 to 59 in a paper published on Sunday, which has not been peer reviewed.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.
Photo: Antara Foto / Fransisco Carolio / via Reuters
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 a placebo to 540 participants.
Researchers said that it was unclear how the decrease in antibodies would affect the shot’s effectiveness, as scientists have yet to determine the antibody level that a vaccine needs 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 has agreed to give a third Moderna shot and Thailand a third Pfizer-BioNTech shot for some people who are fully vaccinated with the Sinovac vaccine, amid concerns over its effectiveness against the more transmissible Delta variant of SARS-CoV-2.
Turkey is offering a third Sinovac or Pfizer dose to some people who have gotten Sinovac shots.
As of the end of last month, Sinovac had delivered more than 1 billion doses of the vaccine, a major vaccination tool in China, Brazil, Indonesia and Chile.
Philippine Secretary of Health Francisco Duque said that the country’s panel of vaccine experts had yet to decide on whether to give a booster shot, but they were discussing the possibility.
The study said that participants in some cohorts who received a third dose of the Sinovac shot about six months after the second showed about a three to fivefold increase in antibody levels after 28 more days, compared with the levels seen four weeks after the second shot.
Researchers cautioned that the study did not test the antibodies’ effect against more transmissible variants, saying that further research was needed to assess antibody duration after a third shot.
The study was conducted by researchers at disease control authorities in Jiangsu Province, China, as well as Sinovac and other Chinese institutions.
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