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SINOVAC: Third Dose Vaccine Induces A Significant Increase in Antibodies

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[Photo: The Edge Markets]

SINOVAC Biotech Ltd., a leading provider of biopharmaceutical products in China, announced the publication of two papers on 10 August 2021, which are available on Medrxiv. The publications detail findings from clinical studies on the immunogenicity, safety, and immune response of CoronaVac® in two- and three-dose schedules. The results indicate that a third booster dose of CoronaVac® induces a strong immune response in healthy adults with no serious adverse reactions related to the vaccine. This research provides important scientific data and supplement existing research to help policymakers develop strategies for vaccine rollout and timing of booster doses.

The first paper, titled 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, includes published data on the immunogenicity and safety profile of a two-dose schedule, as well as interim results on immune persistence and the immunogenicity and safety of a third dose of CoronaVac® in healthy adults aged 18 to 59 years old.

The study is part of the placebo-controlled, double-blind phase II clinical trial in healthy adults aged 18 to 59 years old, conducted in China. Participants were randomly assigned (1:1:1:1) to one of four schedules to receive a third dose of vaccine, 28 or 6 months after two two-dose regiments, 14 or 28 days apart. The immunization schedule of the three-dose regimens for the four arms were: (1) days 0, 14, 42; (2) days 0, 14, 54; (3) days 0, 28, 56; and (4) days 0, 28, 268.

The results indicated that a two-dose schedule generates good immune memory. A third dose given 6 to 8 months after the second dose quickly induced a strong immune response, and the neutralizing antibody titers at day 28 after the third dose increased by three to five times higher than the levels recorded on day 28 after the second injection. In addition, the findings revealed evidence that show longer intervals between the second and third doses will 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. No serious adverse reactions were considered to be related to vaccination, and a lower rate of adverse reaction was observed after the third dose compared to the two-dose schedule.

The second paper, titled A booster dose is immunogenic and will be needed for older adults who have completed two doses vaccination with CoronaVac: a randomised, double-blind, placebo-controlled, phase 1/2 clinical trial, includes published data on the immunogenicity and safety profile of a third dose of CoronaVac® in healthy adults aged 60 and above, as well as research on the immune persistence of two priming doses.

The results showed that a third dose of the vaccine administered 8 months after the second dose can induce a significant increase of neutralizing antibodies at day 7 post-injection. In addition, a third dose was found to be well-tolerated in elderly patient populations with no vaccine-related serious adverse events reported.

The study is part of randomized, double-blind, placebo-controlled, phase 1/2 clinical trial conducted in China in healthy adults aged 60 and above, conducted in China. The study recruited 303 participants that who were assigned to groups and given a third dose of 3μg, 6μg or placebo eight months after the second dose. An additional arm of 1.5μg per dose was added to the study.

The results indicate that, in all 1.5μg, 3μg, and 6μg dose vaccine groups, the level of neutralizing antibodies increased significantly on day 7 after the third dose of vaccine. In the group of participants who received the 3μg vaccine, which is approved for emergency use, the geometric mean titer (GMT) of serum neutralizing antibodies increased to 305 on day 7 after the third dose of vaccine, which was approximately 7-fold as compared to the levels at day 28 after the second dose. The GMT levels continued to increase on days 14 and 28 after the third dose; however, there were no significant differences in the levels recorded days 14, 28 and 7 after the administration of a third dose. Furthermore, no significant differences in neutralizing antibody levels were observed among different age groups (60 to 64 years old, 65 to 69 years old, 70 years old and above).

The safety data showed that, within 28 days after the subjects received the third dose of vaccine, there was no significant difference in incidence of adverse reactions between the different arms vaccine and the placebo groups. Any reported adverse reactions were grade 1 or 2, the most common reaction being injection-site pain. No serious vaccine-related adverse reactions were observed.

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