Abstract

Objective: This study included participants from Hacettepe University 4th, 5th, and 6th-grade students of Medical School and 4th and 5th-grade students of Dental School; and aimed to evaluate the general health status, COVID-19 history, vaccination status, and SARS-CoV-2 antibody levels of the participants to support their physical and social health, during the pandemic period.

Methods: A prospective cohort study was conducted with an integrated, matched, nested case-control study. Sociodemographic characteristics, life habits, COVID-19 history, vaccination status, compliance with mask-distance-hygiene rules, and risks (if any) for COVID-19 were inquired via online questionnaires. Physical examinations, complete blood count, biochemistry tests, and anti-SARS-CoV-2 anti-spike antibody tests were conducted for all consenting participants. All analyses were established using depersonalized data.

Results: Of the 778 participants completing the baseline visit in June-July 2021, the percentages of those vaccinated with at least one, two, and three/more doses of COVID-19 vaccine were 99.1%, 98.0%, and 11.7%, respectively; one had four doses. The median (minimum-maximum) time since the last vaccination was 134 (34-166) days for those vaccinated with two doses [CoronaVac (Sinovac Life Sciences, Beijing, China)] and 25 (14-56) days for those vaccinated with three doses [two doses of CoronaVac and a last dose of Pfizer-BioNTech mRNA vaccine (Comirnaty®). The third dose was applied at a median of 164 (151-202) days after the second dose, and all were heterologous in type. The median (minimum-maximum) antibody level for the overall group was 53.55(0-5680) BAU/mL: 47.19 BAU/mL in those who received two doses, with a more than 100 times increase after a third dose (4943.64 BAU/mL). Of the 522 participants followed up to October 1, 2021, 6 PCR-positive symptomatic participants were diagnosed with COVID-19: the incidence rate was 4/1000 person-months. 

Conclusion: A 100-fold neutralizing antibody level following the third dose demonstrated the importance of a booster dose. Given the time lag between doses, antibody measurements of BioNTech recipients should be repeated in the upcoming months. Booster selection should involve antibody level, variant sensitivity of the vaccine, and individual characteristics of the recipient.