Jul 28, 2021 05:41 AM

Third Dose of Sinovac’s Covid-19 Vaccine Shows Booster Effect, Study Shows

Sinovac’s Covid-19 vaccine has been listed by WHO for emergency use.
Sinovac’s Covid-19 vaccine has been listed by WHO for emergency use.

A third, booster shot of the Covid-19 vaccine produced by Chinese drugmaker Sinovac Life Sciences Co. Ltd. resulted in significant increases in antibody levels, a study showed, but it wasn’t clear whether a booster would be effective against variants.

The study involved 540 participants since May 2020 and was published Sunday by Jiangsu Provincial Center for Disease Control and Prevention, Fudan University and Sinovac, on medRxiv for peer review.

Antibodies in participants who received two doses declined below a key threshold around six months after the second dose, the study found. Neutralizing antibody titers in those who received two doses 14 days apart dropped to 4.1 geometric mean titers (GMTs) after six months or longer, while those vaccinated 28 days apart dropped to 6.7. Readings below 8 GMTs are considered antibody negative.

The proportion of participants who tested antibody positive also declined over time. The data showed that only 16.9% of those vaccinated with two doses 14 days apart and 35.2% of those vaccinated with two doses 28 days apart tested positive for neutralizing antibodies six months after the second dose.

When a third dose was given six months after the second dose, GMTs assessed 14 days later increased to 137.9 among the group vaccinated with two doses 14 days apart and to 143.1 among the group vaccinated with two doses 28 days apart, the study showed.

The researchers also found that giving a third dose too early induced much lower antibody levels. If participants received a third dose only 28 days after the second dose, their GMTs after six months fell below 50, the data showed.

Optimizing the timing of a booster dose should take into account immunogenicity, vaccine efficacy, local epidemic situation, infection risk and vaccine supply, the paper suggested.

The paper said a two-dose vaccination schedule generates good immune memory, meaning the immune system quickly and specifically recognizes the virus and initiates a corresponding immune response. But researchers are not clear about the relationship between decreased antibody GMTs and efficacy rate, the researchers said.

As the study used the original Covid-19 strain, the efficacy of a booster injection against the new variants of the virus is unclear. Participants in the study were healthy adults ages 18 to 59, so it isn’t clear how a third dose would perform in older people who are more vulnerable to the virus.

Sinovac’s Covid-19 vaccine was listed by the World Health Organization for emergency use. Indonesia is considering providing a booster shot using the Sinovac shot, Reuters reported.

Contact reporter Denise Jia ( and editor Bob Simison (

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