Objective: This study aimed to describe the timing and recorded reasons for early withdrawal among participants in four TURKOVAC vaccine clinical trials and to explore differences according to sex, age group, study design, and follow-up duration.
Materials and Methods: This retrospective, descriptive secondary analysis used aggregate withdrawal data from TURKOVAC Primary Phase III (n=1,290), Hybrid COV-Booster TR Phase II (n=222), Inactivated Booster Phase III (n=4,340), and TURKOVAC after BNT162b2 Phase IIb (n=65). Reasons for withdrawal were classified into standardized categories. Descriptive statistics and exploratory chi-square or Fisher's exact tests were used.
Results: Of the 5,917 enrolled participants, 656 (11.1%) withdrew early. Withdrawal rates were 9.5%, 14.4%, 11.5%, and 1.5% across the four studies, respectively (p=0.005). The most frequent reasons were withdrawal of consent (296/656, 45.1%) and loss to follow-up (281/656, 42.8%). Sex-specific withdrawal rates were generally similar; however, the Inactivated Booster Phase III study showed a higher withdrawal rate among male participants (12.3% vs. 9.8%, p=0.015). Younger participants had higher withdrawal rates in the TURKOVAC Primary Phase III study and in the pooled analysis. Across studies, the proportion of withdrawals occurring within 6 months ranged from 47.5% to 100.0%.
Conclusion: Early withdrawal was mainly associated with withdrawal of consent, loss to follow-up, and pandemic-related external vaccination. One adverse-event-coded withdrawal (1/656, 0.2%) was recorded; the aggregate dataset did not identify this event as vaccine-related, and causality could not be assessed in this secondary analysis. Overall, attrition did not appear to be predominantly safety-driven. Literature-informed implications for participant retention include close attention to early follow-up, clear communication, feasible visit procedures, and contingency planning for external vaccination.