Calendar-Period Variation in the Association Between Documented Complete Rotavirus Vaccination and Laboratory-Confirmed Gastroenteritis: An 11-Year Test-Negative Study

Background/Objectives: Rotavirus vaccines have been available in Türkiye as self-financed products since 2006, but nationally representative uptake data and long-term data on their association with laboratory-confirmed rotavirus gastroenteritis remain limited. We evaluated calendar-period variation in the association between documented complete rotavirus vaccination and laboratory-confirmed rotavirus gastroenteritis among children younger than 5 years. Methods: This retrospective test-negative study included children aged 0–59 months who underwent qualitative stool rotavirus antigen testing during clinically evaluated acute gastroenteritis at a tertiary hospital between January 2015 and June 2026. Cases were rotavirus-positive episodes and controls were rotavirus-negative episodes. The institutional vaccination registry identified documented completion of either the two-dose Rotarix or three-dose RotaTeq schedule before testing, it did not separately identify partial schedules. Cluster-robust logistic regression adjusted for age group, sex, treatment setting, calendar year, and test month. Results are reported as odds ratios (ORs) with 95% confidence intervals (CIs). Results: The analysis included 2788 episodes among 2242 children: 534 rotavirus-positive cases and 2254 test-negative controls. Documented complete vaccination was recorded in 81 cases (15.2%) and 539 controls (23.9%). The adjusted OR for rotavirus positivity was 0.47 (95% CI, 0.36–0.62) for episodes with documented complete vaccination versus episodes with no documented complete series. Period-specific adjusted ORs were 0.85 (95% CI, 0.56–1.29) in 2015–2019, 0.64 (95% CI, 0.20–2.06) in 2020–2021, and 0.22 (95% CI, 0.13–0.38) in 2022–2026; between-period heterogeneity was significant (Q = 15.36; p < 0.001). The adjusted OR among hospitalized episodes was 0.42 (95% CI, 0.27–0.66). Among documented complete schedules, 448 episodes involved Rotarix and 172 involved RotaTeq; product-specific analyses were exploratory. Conclusions: The observed association varied substantially by calendar period; therefore, the pooled OR should not be interpreted as a stable national measure of vaccine performance. Within this tested clinical population, documented complete vaccination was associated with lower odds of laboratory-confirmed rotavirus gastroenteritis, including in the hospitalized test-negative analysis. Because the comparison group may have included partially vaccinated children, these findings are not conventional vaccine-effectiveness estimates. Nationally representative surveillance with complete dose-level vaccination histories is needed.