In a phase Ⅱ trial, one booster vaccination of an inactivated SARS-CoV-2 vaccine given six months following primary vaccination demonstrated good safety and immunogenicity in healthy adults aged 18 to 59 years. Investigators undertook this study to determine safety and immunogenicity of a booster vaccination with an inactivated SARS-CoV-2 vaccine.

The study included 200 healthy adults aged 18-59 years (pregnant and not breastfeeding females were excluded). The primary vaccination schedule groups (50 subjects in each group) were: (i) 0-14 d 5 μg, (ii) 0-14 d 10 μg, (iii) 0-28 d 5 μg, and (iv) 0-28 d 10 μg, respectively. Subjects who had received two-doses primary vaccination were chosen in ascending order of the study number.

They were vaccinated with a booster dose (similar dosage as primary vaccination) at the 6th month following primary vaccination (thirty-day window period). Collection of blood samples was done prior to and after boosting, followed by testing for the geometric mean titers and seroconversion of live virus neutralizing antibody, receptor-binding-domain IgG antibody, and pseudovirus neutralizing antibody.

Within 28 days after boosting, adverse events were examined. The live virus neutralizing antibody geometric mean titers (95%CI) in group 0-14 d 5 μg, 0-14 d 10 μg, 0-28 d 5 μg and 0-28 d 10 μg elevated from 4.07 (3.30-5.04), 3.75 (3.08-4.55), 8.33 (7.01-11.11) and 7.69 (6.19-9.57) prior to the booster vaccination to 284.84 (215.28-376.86), 233.05 (178.61-304.08), 274.81 (223.64-337.68) and 280.77 (234.59-336.04) in twenty-eight days following booster vaccination, respectively.

In all four groups, the rates of live virus neutralizing antibody seroconversion were 100%. The occurrence of adverse events after booster vaccination were 18% (9 cases), 4% (2 cases), 12% (6 cases), and 12% (6 cases) in the four groups. No serious adverse event was noted and no adverse event was graded as level 3 or worse. Thus, booster vaccination with an inactivated SARS-CoV-2 vaccine exhibits a good safety and immunogenicity profile in healthy people.