Abstract
BACKGROUND/OBJECTIVES: Methicillin-resistant Staphylococcus aureus (MRSA) remains a major cause of both community-onset and hospital-acquired infections, yet longitudinal data from Latin American hospitals spanning the COVID-19 pandemic are scarce. We characterized temporal trends, seasonality, and the impact of the COVID-19 pandemic on MRSA prevalence and incidence density among clinical S. aureus isolates at a tertiary-care hospital in western Mexico over 9.5 years. METHODS: We analyzed 6625 non-duplicate clinical S. aureus isolates (6609 with valid resistance data) from June 2016 to December 2025. Temporal trends were assessed using Mann-Kendall tests, Theil-Sen estimation, and binomial generalized linear models. Seasonality was evaluated through STL decomposition, generalized additive models, and Fourier analysis. An interrupted time series (ITS) model with GLS-AR(1) and Newey-West corrections compared three COVID-19 phases: pre-pandemic (2016-2020), high viral circulation (2020-2022), and post-peak stabilization (2022-2025). Exposure-adjusted incidence densities (per 1000 patient-days) were analyzed in parallel. RESULTS: MRSA prevalence declined from 28.1% pre-pandemic to 14.0% post-peak (Mann-Kendall z = -9.03, p < 0.001; OR = 0.85 per year, 95% CI: 0.829-0.871). MRSA incidence density decreased by 50%, from 1.27 to 0.63 per 1000 patient-days, while aggregate S. aureus incidence density remained stable (z = -0.17, p = 0.868). The ITS joint Wald test confirmed a significant cumulative shift in MRSA trajectory post-pandemic (p = 0.019 counts; p = 0.012 incidence density), with a significant post-peak level drop (p = 0.008). S. aureus exhibited moderate seasonality peaking in May-July (GAM edf = 7.26, p < 0.001), whereas MRSA showed only marginal seasonal variation. CONCLUSIONS: MRSA declined markedly across the study period, with the steepest reduction following the Omicron peak. The decline persisted after adjustment for pandemic-related fluctuations in hospital volume, supporting periodic reassessment of empiric anti-MRSA prescribing policies in similar settings.