Impact of the COVID-19 Pandemic on Routine Adult Vaccination in Primary Care: A Multicenter Interrupted Time Series Analysis
Background/Objectives: This study evaluated changes in routine adult vaccination across the COVID-19 pandemic period in Istanbul primary care using interrupted time series (ITS) analysis, disentangling immediate from gradual effects and comparing patterns across socioeconomically distinct centers, through a multicenter retrospective analysis of routine (non-COVID-19 vaccination records). Methods: The full descriptive dataset comprised 33,016 adult vaccination records. After excluding 25,805 COVID-19 vaccination records, 7211 routine (non-COVID-19) vaccination records from nine family medicine units in three Istanbul training family health centers—Şişli-Gülbahar, Kağıthane, and Gaziosmanpaşa—were included in the routine vaccine and ITS analyses and were analyzed over 97 monthly observations (June 2017–August 2025), divided into three 672-working-day periods: pre-pandemic, pandemic, and post-pandemic. Segmented negative binomial ITS regression with a log (working days) offset was the primary analysis; Newey-West HAC-robust Poisson regression served as sensitivity analysis. Catchment districts were characterized using the 2022 Socioeconomic Development Ranking (SEGE-2022). Results: No significant immediate level change occurred in total routine vaccination at pandemic onset. Pneumococcal vaccination showed the most pronounced signal: an upward pretrend (annual slope IRR = 2.583; p = 0.001), a level increase at onset (IRR = 3.589; p = 0.013), then a rapid within-pandemic slope decline (annual slope IRR = 0.227; p < 0.001). Tetanus-diphtheria (Td) vaccination fell sharply at onset (IRR = 0.686; p = 0.002) and continued declining post-pandemic (annual slope IRR = 0.805; p = 0.008). Versus Gaziosmanpaşa (rank 85/973), Kağıthane (rank 38) showed the highest total routine vaccination rate (excluding COVID-19 vaccination) (IRR = 1.539; p < 0.001), exceeding Şişli (rank 1; IRR = 1.269; p < 0.001). Conclusions: Changes in routine adult vaccination across the pandemic period were vaccine-specific: pneumococcal vaccination surged then declined; Td vaccination showed continued post-pandemic decline. Center-level vaccination patterns were not consistently aligned with district-level socioeconomic ranking.