Observational study of antibody levels after second and third SARS-CoV-2 vaccinations in medical workers.

Background: Countries around the world are actively promoting vaccination against COVID-19. We observed the changes in serum neutralizing antibody titers in medical workers vaccinated with inactivated COVID-19 vaccine, in order to explore the necessity of a third dose of vaccination.

Methods: A total of 62 medical workers in our hospital were observed. Novel coronavirus neutralizing antibody titers in serum were detected by ELISA (enzyme-linked immunoassay). Neutralizing antibody tests followed in four batches according to the different time periods after three vaccinations. Sixty-two observers participated in the first batch of testing for neutralizing antibody, and 18 of them participated in all four batches. Fasting venous blood was taken from all the participants in the morning to detect serum neutralizing antibody titers.

Results: Sixty-two medical workers were divided into age groups of 21-30, 31-40, and >40 years, and the antibody titer in the oldest group was significantly lower than that in youngest group (p = 0.0137). There was a gradual decrease in antibody titers over time at around 1, 3, and 6 months after the second dose of vaccine (p < 0.0001). The antibody positive rate also decreased gradually (p = 0.0003). The neutralizing antibody titer around 1 month after the third dose was significantly increased (p < 0.0001). Unexpectedly, three participants with negative neutralizing antibody after the first and second dose produced neutralizing antibody with a measurable titer after the third dose.

Conclusions: The neutralizing antibody titer in serum increased significantly after the third dose of vaccine. A third immunization even produced neutralizing antibody in previously negative individuals.