Immune Response to Sinovac-CoronaVac Vaccine


Sinovac's CoronaVac COVID-19 vaccine was given emergency use authorization for a two-dose schedule, with an interval of 14 to 28 days between doses. As has been seen with other COVID-19 vaccines, a longer interval between doses gives a stronger antibody response. 

Published results of Chile's mass vaccination campaigns, in which approximately 10.2 million people were vaccinated with two doses of CoronaVac, showed a vaccine effectiveness in the 18-year-old and older population of 65.9% against symptomatic infection, 87.5% against hospitalization, 90.3% against ICU admission, and 86.3% against death. Similar results were observed in Uruguay.

In a recent pre-print article, results of a study conducted in China with 540 participants indicate that two doses of the CoronaVac vaccine, at intervals of 14 and 28 days, produce a very strong immune response. However, levels of neutralizing antibodies against COVID-19 decline over time, and six to eight months after the second dose, the level of antibodies could be considered insufficient to maintain protection (a minimum level of protection against COVID-19 has not yet been established), though there may be protection through T-cell- and B-cell-mediated immunity. According to this study, a third dose administered six months after the second dose appears to be highly effective in replenishing the SARS-CoV-2 specific immune response, with a significant increase in antibodies. There were fewer adverse reactions to the third dose than in previous studies.

Although it appears that there will be a need for a third dose, the timing for administering it will depend on several factors, such as the local epidemiological situation, vaccine supply, disease risk, and other relevant factors, bearing in mind that at present the priority is to vaccinate as many people as possible with the two-dose schedule.


 Pan H, Wu Q, Zeng G, et al. Immunogenicity and safety of a third dose, and immune persistence of CoronaVac vaccine in healthy adults aged 18–59 years: interim results from a double-blind, randomized, placebo-controlled phase 2 clinical trial. (2021) medRxiv 2021.07.23.21261026; doi:

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