Serum neutralizing antibody following the vaccination of inactivated SARS-CoV-2 vaccines

Objective To determine the difference of serum neutralizing antibody levels in healthy persons following the vaccination of inactivated SARS-CoV-2 vaccines.

Methods Health care workers that received inactivated SARS-CoV-2 vaccines were enrolled in Sichuan provincial people's hospital from January to December 2021. All participants were classified into four groups according to the number and time of vaccination， which were groups of 28 days after the second dose， 180 days after the second dose， 28 days after the third dose and 150 days after the third dose. Serum neutralizing antibody was quantitatively detected by chemiluminescence method. Furthermore， the serum neutralizing antibody levels were compared within and between groups by gender， age and body mass index（BMI）.

Results A total of 349 participants were enrolled in this study， including 113 males and 236 females. The positive rates of neutralizing antibody in the groups of 28 days after the second dose， 180 days after the second dose， 28 days after the third dose and 150 days after the third dose were 74.0%， 25.7%， 100.0% and 100.0%， respectively. In the four groups， neutralizing antibody levels were 10.38 （5.76， 24.00） AU·mL^-1， 4.18 （3.00， 6.18） AU·mL^-1， 41.18 （25.80， 116.21） AU·mL^-1 and 33.33 （18.09， 69.12） AU·mL^-1， respectively. The positive rate of neutralizing antibody significantly differed across the groups （P<0.001）. Furthermore， neutralizing antibody level in the third dose groups were significantly higher than that in the second dose groups （P<0.001）. Neutralization antibody level in young people （<45 years old） was significantly higher than that in middle-aged and elderly people （≥45 years old） in the groups of 180 days after the second dose and 28 days after the third dose （P<0.05）. Additionally， neutralization antibody level in normal weight participants was significantly higher than that in overweight and obese participants in the group of 28 days after the second dose （P<0.05）. However， no significant difference was found in all groups by gender （P>0.05）.

Conclusion Compared with two doses of inactivated SARS-CoV-2 vaccine， three doses can significantly induce higher neutralizing antibody and stronger immune protection. Age and BMI have certain effect on the neutralizing antibody.