Typhoid Fever in the Republic of Korea, 2001-2024: Long-Term Decline, Residual Annual Periodicity, and Southeastern Coastal Spatial Clustering in 24 Years of Nationwide Surveillance.

Abstract

Typhoid fever (Salmonella Typhi) has declined dramatically in Korea over three decades, but residual seasonality, demographic patterns and provincial spatial concentration have not been systematically reanalysed. Using all 3535 typhoid cases reported to the Korea Disease Control and Prevention Agency between 2001 and 2024 (3051 domestic and 484 imported [13.7%]), we quantified the long-term annual trend with the Hamed-Rao modified Mann-Kendall test (τ = -0.667, p = 0.0011) and a negative-binomial generalized linear model (GLM) yielding a Sen slope of -7.3%/year (95% CI -9.9 to -4.4); a parametric-bootstrap analysis of the runner-up 1-breakpoint GLM identified 2018 as the most likely structural change point (95% CI [2009, 2022]). A continuous Morlet wavelet transform of the weekly series revealed annual periodicity that exceeded a 1000-simulation AR(1) red-noise null by a factor of 2.60. Direct age standardization and age-band-specific Mann-Kendall tests showed statistically significant declines in every age stratum. Provincial spatial analysis (16 sido) gave a global Moran's I of 0.403 (p = 0.0083, 9999 permutations) under a Queen + k-nearest-neighbor-2 baseline scheme, with the southeastern coastal cluster (Gyeongnam-Busan-Ulsan-Gyeongbuk axis) preserved across 12 alternative weighting schemes after empirical Bayes shrinkage of sigungu-level rates and across two equal-length time strata; one province (Ulsan) survived Benjamini-Hochberg FDR adjustment of the 16 LISA p-values. Korean typhoid has continued its long-term decline; residual incidence concentrates in the southeastern coastal corridor, and imported cases-including ciprofloxacin-resistant H58 strains from South Asia-now form a substantial fraction of notifications, motivating pre-travel counseling and post-travel vigilance.