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Introduction: Bacille Calmette–Guérin (BCG) vaccination has shown promising therapeutic effects for type 1 diabetes (T1D). According to recent studies, immunometabolism modification and regulation of T lymphocytes constitute the proposed mechanisms by which BCG vaccination may delay T1D onset. Clinical trial evidence from Turkey supports that two to three doses of the BCG vaccine in childhood, with the first dose administered in the first year of life, may prevent T1D. In the same study, one or zero vaccinations appeared to have no effect in T1D onset prevention. In Greece, the BCG vaccine was administered in a single dose at the age of 9 years in elementary school. BCG vaccination was not performed on a mandatory basis, creating one BCG vaccinated and one non-vaccinated population. The aim of our study was to investigate the possible effect of a single dose of BCG vaccine, at the age of 9 years, on the time of T1D onset, in a population of BCG vaccinated and non-vaccinated patients with diagnosed T1D. Methods: To test this hypothesis, a survey through the Pan-Hellenic Federation of People with Diabetes (PFPD) was performed. In this observational, retrospective study, participating patients provided information regarding age, gender, time of diagnosis, and BCG vaccination status. Patients diagnosed with T1D before the age of 9 years were excluded from the analysis. Results: The final sample included 196 patients (73 male and 123 female) with a mean age of 42.2 ± 14.3 years and a mean duration of diabetes of 16.8 ± 12.9 years. Mean age of T1D diagnosis in the BCG vaccinated group was 24.0 ± 19.0 years, while the mean age of T1D diagnosis in the BCG non-vaccinated group was 21.5 ± 14.3 years (p = 0.03). No interaction was found between gender and the age of diagnosis for BCG vaccinated and unvaccinated patients (p = 0.86). Conclusion: The results of our study suggest that a single dose of BCG vaccine, performed at the age of 9 years, may delay the onset of T1D by 2.5 years. Additional studies of children receiving multiple doses of BCG should be conducted to possibly prove prolongation of the disease-free interval. [ABSTRACT FROM AUTHOR]