Hacettepe University COVID-19 Vaccinated's Health Cohort- Students of Health Sciences

The study was planned, and ethical approval procedures were initiated in late January, 2021. However, serial official approval processes delayed the initiation of the study beyond the pre-planned initiation date (late in March 2021, to catch the 28th day after the second dose of CoronaVac vaccine). Thus, the study plan was revised, and so were the approval processes.

The planned prospective cohort study started its recruitment process in June, with the first enrollment on June 21, 2021. The study is an open cohort; vaccination times vary, some students participated in the first visit, but have/can leave the study before May 2023 (the planned closure time for testing); some students did not participate in the first visit (in June-July 2021), yet preferred to be involved in the study bythe time of the second visit (October 2021) and so.

Participants are being recruited from students of medical school (grades 4,5, and 6) and dental school (grade 4 and %), and are involved in the study for all/some of the procedures offered. The study aimed to:

1. Investigate the sociodemographic characteristics, health status, life habits, quality of life (via validated indices), COVID-19 status, vaccination status for SARS-CoV-2, use of non-pharmaceutical preventive measures against COVID-19. These will be accomplished through online, depersonalized, self-completed standardized questionnaire and use of validated scales.;
2. Complete anthropometric measures and physical examination at each visit;
3. Check blood levels for complete blood count, renal function tests, liver enzymes, D-dimer, vitamin D, HbA1c levels, and lipid profile;
4. Check vaccine-induced immunity using quantitative, Anti-SARS-CoV-2 Receptor Binding Domain kit (Abbott Laboratories) at enrolment and each subsequent visits;
5. Evaluate the durability of vaccine-induced immunity, as measured by antibody levels periodically (at 3rd, 6th, 12th and 24th month after full immunization following full vaccination, i.e. 2 doses plus 28 days);
6. The cohort will be followed up to 24 months (through September 2023) for detecting COVID-19 infection and/or related hospitalization, intensive care unit (ICU)-admission or death; also, to study the potential association with personal risk factors and/or vaccine-induced immunity;
7. A nested, test-negative case-control study is integrated into the the cohort, such that each time a new COVID-19 case is identified (with a PCR-positivity), two gender and school/grade matched controls will be invited for PCR testing. Blood will be drawn from PCR test negative controls at the day (or within 1 day later) of the PCR-positivity of the case detected. This nested case-(test-negative) control design will enable to study the association between infection and current Anti-Spike1 RBD antibody levels, controlling for personal risk factors;
8. Given the considerable amount of asymptomatic infections among vaccinated individuals, 150 participants will be selected from among volunteers for 6 consecutive PCR-testing, with 15 days intervals between each testing.

Three study designs will be used to reach study objectives; namely, a large prospective cohort (for evaluating changes in general health status and neutralizing antibody levels, as measured by SARS-CoV-2-RBD Abs); a subcohort for periodical PCR testing to detect asymptomatic infection rates;

Over the course of the planned study, Pfizer-Biontech (R) m-RNA vaccination has been introduced for mass vaccination, as 2 doses, 28 days apart. Also, the Ministry of Health promoted a third dose of COVID-19 vaccination for health personnel in particular, due to rapidly decreasing immunity for COVID-19 among 2-dose CoronaVac vaccinated individuals. This change in mass vaccination introduced variation in our exposure status. Accordingly, the objectives of the study extended to calculation and comparison of the incidence-rates of COVID-19 infection and related complications among those vaccinated with either of the 2 vaccines (namely, CoronaVac and Biontech), controlling for the first doses and boosters, including mix-and-matches.

The four visits of the main cohort was completed as of July 7, 2023 (as planned). Preliminary findings have been published in a Turkish peer-reviewed medical journal. Due to administrative issues, periodic PCR-based asymptomatic case detection was postponed- will be resumed in Fall 2023 (if case numbers increase). The nested case-control sub-study will continue through November 1, 2023, as PCR-confirmed cases appear. So far, we have a total of 194 cases since the initiation of the cohort; 154 of these were detected by the cohort team, others were detected outside the cohort facility, but all were confirmed (upon students' approval) through personal electronic health records. The study is anticipated to finish by december 7, 2023. The unexpected 3 months of delay at initiation of the study (due to the time lag for the ethical approval process at the Ministry of Health) led to an expansion in data analysis and final reporting of the study (registered as December 7, 2023, by the University Scientific Research Unit, which is the sole funder of the study.)