A lot of efforts have been made to investigate the immune response after the homologous BBIBP-CorV booster vaccination (10, 16C19). The IgG against the RBD of the SARS-CoV-2 S protein seroconversion rate increased from 42.27% before the third dose to 100% 1 month after the third dose and then slightly decreased to 98.97% Isradipine 5 months later. Positive IgM against the RBD of the SARS-CoV-2 S protein was rare and was observed in only one participant at month 1 after the third dose. The neutralizing antibody levels at month 1 and month 6 after the third Isradipine dose increased 63.32-fold and 13.16-fold compared with those before the third dose, and the Isradipine positive rate for neutralizing antibody was still 100% at month 6 after the third dose. Importantly, the antibody responses induced by the vaccine and immune persistence were not affected by sex or age. No serious adverse reactions were reported. Total antibody and IgG against the RBD of the SARS-CoV-2 S protein were highly correlated with neutralizing antibody, suggesting that total antibody and IgG against the RBD of the SARS-CoV-2 S protein could be used as predictors for neutralizing antibody. In conclusion, the third dose of homologous BBIBP-CorV inactivated vaccine induced a robust antibody response and moderate immune persistence. These finding are of great significance for development future vaccination strategies. Keywords: SARS-CoV-2, inactivated vaccine, booster vaccination, immune persistence, antibody response 1.?Introduction The ongoing coronavirus disease 2019 (Covid-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused serious damage to global public health and the economy (1). Although the disease is mild in most cases, it progresses to a Sirt6 severe form in some patients and may lead to deaths, especially in the elderly and people with comorbidities. According to the World Health Organization (WHO), as of October 3, 2022, SARS-CoV-2 resulted in more than 615 million laboratory-confirmed cases and over 6.5 million deaths (2). Vaccines are considered as an economical and effective means for prevention and control of SARS-CoV-2. The vaccines against SARS-CoV-2 have been proven Isradipine to be safe and reduce symptomatic infections and asymptomatic infections of SARS-CoV-2 as well as the adverse results (3C7). Its well worth noting that vaccine-induced antibody titers and protecting effectiveness of Covid-19 vaccines declined over time, no matter vaccine type (6, 8, 9). Additionally, variants of SARS-CoV-2 have spread globally, causing resurgences of infections actually in countries and areas with successful mass-vaccination campaigns (6). Due to the decrease of vaccine effectiveness coinciding with the quick spread of SARS-CoV-2 variants, a booster vaccination has been implemented in many countries, in which a third dose of Covid-19 vaccine was given in people who experienced received a second dose. Inactivated SARS-CoV-2 Isradipine disease vaccines have been widely applied in China and many additional countries. As an inactivated vaccine, the BBIBP-CorV vaccine has shown a vaccine effectiveness of 78.1% in adults in the phase III clinical trial having a two-dose routine and was proven to be safe and well tolerated in people aged 3-17 years, people aged 18-59, people aged 60, and individuals with comorbidities (4, 10C12). The BBIBP-CorV vaccine has been authorized for conditional use in China and is included in the WHO emergency use listing (13) (14). Given that the vaccination was cost-effective in low- and middle-income countries (15), a vaccination strategy with BBIBP-CorV inactivated vaccine would bring important economic benefits in these countries. A third dose of homologous BBIBP-CorV vaccine showed a satisfying security profile and induct powerful humoral reactions against SARS-CoV-2 illness (10, 16C20). However, the humoral response and immune persistence of a third dose of homologous BBIBP-CorV vaccine have not been fully explored, as earlier studies focused within one month after the administration of the third dose (10, 16C20). The 6-month durability of the humoral immune response in vaccine recipients was still unfamiliar and immune persistence was still under investigation. In this study, 97 participants who experienced no earlier SARS-CoV-2 illness and received three doses of BBIBP-CorV inactivated vaccine were enrolled. The antibody response and immune persistence within 6 months as well as the security within.