Clinical and epidemiological evolution of COVID-19 across six pandemic waves in a southern Mexican border city: implications for health system preparedness, 2020-2023

Background The clinical expression and severity of COVID-19 have shifted substantially throughout the pandemic owing to viral evolution, rising population immunity, and the expansion of vaccination. However, in Mexico, few studies have compared these changes across successive waves using primary clinical data from first- and second-level healthcare units. We aimed to characterize temporal changes in symptoms, clinical severity, and outcomes across six COVID-19 waves in a southern border city of Mexico, and to examine their correspondence with national patterns of variant predominance and vaccination coverage. Methods We conducted a descriptive time-series study of confirmed COVID-19 cases treated in first- and second-level units of the Mexican Institute of Social Security in Tapachula, Chiapas, Mexico, from March 2020 to January 2023. Clinical, demographic, and epidemiological data were extracted from medical records and the SINOLAVE. Cases were classified into six epidemic waves. Clinical characteristics and severity indicators were compared across waves using chi-square or Fisher’s exact tests with Bonferroni correction. Predominant variants were inferred through temporal alignment with national genomic surveillance. Results Among 18,319 suspected cases, 6,145 (47.1%) were confirmed. Severe disease, oxygen requirement, intubation, and mortality were concentrated in the first three waves, accounting for 88% of all intubations and 72% of deaths, coinciding with the circulation of unspecified lineages (waves 1–2) and the Delta variant (wave 3). Subsequent waves showed marked reductions in dyspnea, tachypnea, cyanosis, and chest pain, and a relative increase in upper-respiratory symptoms. Beginning with the fourth wave, corresponding to Omicron predominance and increasing vaccination coverage, hospitalizations, intubations, and mortality declined substantially. Conclusions COVID-19 severity and symptom patterns changed markedly across the six waves in Tapachula, reflecting variant replacement and population immunity. These findings provide actionable evidence for health system planning and policy design in resource-limited border settings.