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

Background Large-scale vaccination is being implemented globally with CoronaVac, an inactivated vaccine against coronavirus disease 2019 (COVID-19). Immunogenicity and safety profiles of homologous two-dose schedules have been published. We report interim results of immune persistence, and the immunogenicity and safety of a third dose of CoronaVac. Methods In this ongoing, placebo-controlled, double-blind phase 2 trial in 18-to-59-year-olds, we randomly assigned subjects, 1:1:1:1, to one of four schedules to receive a third dose, 28 days or 6 months after two two-dose regimens (14-day or 28-day apart): schedule 1: days 0, 14, 42; schedule 2: days 0, 14, 194; schedule 3: days 0, 28, 56; schedule 4: days 0, 28, 208. For each schedule, participants were randomly assigned to either a medium-dose group (3 g per 0.5 mL of aluminum hydroxide diluent per dose), a high-dose group (6 g), or a placebo group (2:2:1). The primary outcome was geometric mean titers (GMTs) of neutralizing antibody to live SARS-CoV-2. Results Overall, 540 participants received a third dose. In the 3 g group, neutralizing antibody titers induced by the first two doses declined after 6-8 months to below the seropositive cutoff (GMT: 4.1 [95%CI 3.3-5.2] for Schedule 2 and 6.7 [95%CI 5.2-8.6] for Schedule 4). When a third dose was given 6-8 months after a second dose, GMTs assessed 14 days later increased to 137.9 [95%CI 99.9-190.4] for Schedule 2, and 143.1 [95%CI 110.8-184.7] for Schedule 4, approximately 3-fold above Schedule 1 and Schedule 3 GMTs after third doses. Similar patterns were observed for the 6 g group. 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. None of the fourteen serious adverse events were considered to be related to vaccination. 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. (Funded by the National Key Research and Development Program, Beijing Science and Technology Program and National Science Fund for Distinguished Young Scholars; ClinicalTrials.gov number, NCT04352608.)