The experimental vaccine and placebo were both manufactured by Sinovac Research & Development Co., Ltd. A total of 744 subjects will be enrolled, with 144 at phase Ⅰ, and 600 at phase Ⅱ. Participants will be assigned to receive two doses of experimental vaccine or placebo on the schedule of day 0,14 or day 0,28.

All of the participants in phaseⅠwill be assigned to receive two doses of experimental vaccine or placebo on the schedule of day 0,14 or day 0,28.300 participants in phase Ⅱ will be assigned to receive two doses of experimental vaccine or placebo on the schedule of day 0,14 or day 0,28 and one dose of booster immunization will be given 6 months after primary immunization of day 0,14 or day 0,28.

The other 300 participants in phase Ⅱ will be assigned to receive three doses of experimental vaccine or placebo on the schedule of day 0,14,42 or 0,28,56 .

Results

On July 23, 2021, medRxiv published additional results for this clinical trial.

Conclusions A third dose of CoronaVac administered 6 or more months after a second dose effectively recalled specific immune response to SARS-CoV-2, resulting in a remarkable increase in antibody levels, and indicating that a two-dose schedule generates good immune memory. Optimizing the timing of a booster dose should take into account immunogenicity, vaccine efficacy/effectiveness, local epidemic situation, infection risk, and vaccine supply.

Interim Results

The Lancet on February 1, 2021, published interim results for this clinical trial.

Findings: Between April 16 and April 25, 2020, 144 participants were enrolled in the phase 1 trial, and between May 3 and May 5, 2020, 600 participants were enrolled in the phase 2 trial. 743 participants received at least one dose of the investigational product (n=143 for phase 1 and n=600 for phase 2; safety population).

In the phase 1 trial, the incidence of adverse reactions for the days 0 and 14 cohort was seven (29%) of 24 participants in the 3 ug group, nine (38%) of 24 in the 6 μg group, and two (8%) of 24 in the placebo group, and for the days 0 and 28 cohort was three (13%) of 24 in the 3 μg group, four (17%) of 24 in the 6 μg group, and three (13%) of 23 in the placebo group.

The seroconversion of neutralizing antibodies on day 14 after the days 0 and 14 vaccination schedule was seen in 11 (46%) of 24 participants in the 3 μg group, 12 (50%) of 24 in the 6 μg group, and none (0%) of 24 in the placebo group; whereas at day 28 after the days 0 and 28 vaccination schedule, seroconversion was seen in 20 (83%) of 24 in the 3 μg group, 19 (79%) of 24 in the 6 μg group, and one (4%) of 24 in the placebo group.

In the phase 2 trial, the incidence of adverse reactions for the days 0 and 14 cohort was 40 (33%) of 120 participants in the 3 μg group, 42 (35%) of 120 in the 6 μg group, and 13 (22%) of 60 in the placebo group, and for the days 0 and 28 cohort was 23 (19%) of 120 in the 3 μg group, 23 (19%) of 120 in the 6 μg group, and 11 (18%) of 60 for the placebo group.

Seroconversion of neutralising antibodies was seen for 109 (92%) of 118 participants in the 3 μg group, 117 (98%) of 119 in the 6 μg group, and two (3%) of 60 in the placebo group at day 14 after the days 0 and 14 schedule; whereas at day 28 after the days 0 and 28 schedule, seroconversion was seen in 114 (97%) of 117 in the 3 μg group, 118 (100%) of 118 in the 6 μg group, and none (0%) of 59 in the placebo group.

Interpretation

Taking safety, immunogenicity, and production capacity into account, the 3 μg dose of CoronaVac is the suggested dose for efficacy assessment in future phase 3 trials.