SINOVAC Biotech Ltd., a leading provider of biopharmaceutical products in China,
The results indicate that a two-dose schedule generates good immune memory. After a third dose was given 6-8 months after the second dose, strong immune response was induced quickly and the neutralizing antibody titers at day 28 after the third dose increased by three to five times higher than the level recorded on day 28 after the second shot. In addition, the findings brought on evidence showing that longer intervals between the third dose and the second dose result in higher antibody levels.
Seropositive rates in all dose groups were above 90.0% on day 28 after both the second and third doses. The severity of solicited local and systemic adverse reactions reported within 28 days after the third dose were grade 1 to grade 2 in all vaccination cohorts, and no serious adverse reactions were considered as related to the vaccination. A lower adverse reaction rate was observed after the third dose compared to the previous two-dose schedule.
This study is part of the ongoing phase II clinical trial in China, which started on May 3, 2020. This study is a placebo-controlled, double-blind phase II trial in adults aged 18 to 59 years old. The participants were randomly assigned to one of four groups of equal size, each with a different schedule to receive a third dose, 28 days or 6 months after two two-dose regimens, either 14 or 28 days apart. The immunization schedule of three-dose regime for four arms were: (1) days 0, 14, 42; (2) days 0, 14, 254; (3) days 0, 28, 56; and (4) days 0, 28, 268.
In the discussion section, the authors proposed that a booster shot after a two-dose vaccination schedule of CoronaVac® may be necessary; however, policy makers must also consider factors such as the country’s epidemic situation, risk of infection, vaccine supply, etc. At this time, rather than promoting the use of a booster shot, priority should be given to increasing coverage of a two-dose vaccination with CoronaVac®.
 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. doi: https://doi.org/10.1101/2021.07.23.21261026