Efficacy of antibody response following the vaccination of SARS-CoV-2 infected and noninfected healthcare workers by two-dose inactive vaccine against COVID-19.

Sinovac is an inactive vaccine produced against Coronavirus Disease 2019 (COVID-19) for almost a year. No sufficient information is available concerning pro-vaccine immunogenicity. We investigated the efficacy of antibody response following vaccination of SARS-CoV-2-infected and noninfected healthcare workers by a two-dose inactive vaccine against COVID-19. The immunogenicity acquired on the 27th day and 42nd day after the first dose of vaccine (corresponding to Day 14 after the second dose) were compared by the demographics, immunosuppression, comorbidities, postvaccination reaction, and IgG levels of 120 subjects. The overall rate of second postvaccine seropositivity was 97.5% (n = 117) of all individuals, and 44 of these were seropositive after the first dose. The percentage of having a previous COVID-19 (59.1%) among seropositive individuals before 2nd vaccination was significantly higher than those of seropositive individuals (10.96%) after second vaccination (p < 0.0001). In our study, 35 healthcare workers stated that they had previously had a COVID-19 infection. Anti-SARS-CoV-2 antibody responses in people infected with SARS-CoV-2 follow a classical pattern, with a rapid increase within the first 3 weeks after the appearance of symptoms. Although the titers decreased thereafter, the ability to detect anti-SARS-CoV-2 IgG antibodies supports the view that the majority of subjects previously screened as positive for virus remain intact with confirmed neutralizing activity for up to 6 months.