Serological evaluation of immune effect of inactivated COVID-19 vaccine in adult population in Deqing County， Huzhou City， Zhejiang Province

Objective To investigate the specific anti-SARS-CoV-2 antibody in adults and above after initial vaccination with inactivated COVID-19 vaccine， and determine the influencing factors.

Methods In this study， residents aged 18 and above who had completed two doses of inactivated COVID-19 vaccine in Deqing County， Huzhou City， Zhejiang Province were included. Information such as gender， age， type of vaccine and vaccination time were collected， and serum specimens were sampled. Anti-SARS-CoV-2 receptor binding domain （RBD） antibody was quantitatively examined by enzyma-linked immunosorbent assay （ELISA） and influencing factors were determined.

Results The median concentration of anti-SARS-CoV-2 IgG antibody in the residents vaccinated with an inactivated booster vaccine was higher than that in those vaccinated with only two doses of COVID-19 vaccine or single dose （P<0.05）. The median concentration of IgG antibody in males was 9.73 （4.01‒23.70） RU‧mL^-1， lower than 17.76 （7.07‒49.23） RU‧mL^-1 in females （P<0.05）. The median concentration in the residents vaccinated with BBIBP-CorV （Sinopharm） was 6.53 （0.97‒13.69） RU‧mL^-1， which was lower than that in those vaccinated with CoronaVac （Sinovac） that was 17.29 （8.54‒43.73） RU‧mL^-1 （P<0.05）. The median concentration in those with BBIBP-CorV was also lower than 12 （5.45‒40.06） RU‧mL^-1 in those with heterologous booster vaccine （P<0.05）. The median concentration was 9.73 （3.83‒23.63） RU‧mL^-1 in the residents with an interval of more than 6 months from the second dose， which was lower than 14.66 （6.36‒35.98） RU‧mL^-1 in those with an interval of 3‒6 months （P<0.05）. Moreover， immune effect was better in females （χ²=16.464， P<0.05）， 18‒45 years （χ²=7.158， P<0.05）， and those vaccinated with CornaVac （χ²=49.637， P<0.05）， while decreased in those with an interval of more than 6 months from the second dose （χ²=8.447， P<0.05）.

Conclusion Gender， age， and type of vaccine may affect the effect of immunization. The COVID-19 vaccination shows an acceptable immunogenicity in adults； however， it declines in 6 months after vaccination. It warrants strengthening the booster vaccination to maintain the immune response.