Impact of SARS-CoV-2 vaccination on cancer trends: a nationwide interrupted time series study

A recent population-based cohort study of Seoul residents in South Korea reported increased incidence of several cancers within one year of SARS-CoV-2 vaccination. We examined whether a short-term, population-level change in cancer incidence trends was detectable at the national vaccination rollout, using a design that does not compare vaccinated with unvaccinated individuals. We conducted an interrupted time series analysis of Korea National Health Insurance Service data for 36,024,394 individuals born between 1955 and 2004, covering February 2011 to September 2025 (176 months). The intervention point was August 2021, when cumulative first-dose coverage exceeded 50%. Monthly incidence of 13 cancer categories defined by ICD-10 code ranges and of four site-specific cancers (stomach, colorectum, lung and prostate) was compared with counterfactual forecasts from ARIMA/SARIMA models, and segmented regression estimated immediate level and trend changes (significance threshold P < 0.01). The placement of the intervention point was varied from 0 to 18 months, and the resulting estimates were compared with those obtained at 30 placebo points in the pre-pandemic period. Overall, 89.3% of the population received at least one dose. No immediate level change (relative risk 0.923–1.072) or trend change (0.995–1.005) reached significance for any of the 13 categories, and none reached significance for the four site-specific cancers. Across the lag grid, 2.0% of estimates met the threshold, compared with 4.1% at placebo points. The 99% confidence limits excluded immediate increases larger than 8.9% to 23.0% across the 13 categories and 8.1% to 23.4% across the four site-specific cancers. The national SARS-CoV-2 vaccination rollout in South Korea was not accompanied by a detectable short-term, population-level change in cancer incidence trends over the 50 months that followed. Not applicable.