Association Between Upper Respiratory Tract Infections and Parkinson’s Disease in Korean Populations: A Nested Case–Control Study Using a National Health Screening Cohort

Background: Although several epidemiological studies have suggested a potential association between infections and Parkinson’s disease (PD), relatively few have specifically examined the relationship between upper respiratory tract infections (URIs) and PD, apart from coronavirus disease 2019 (COVID-19). Objectives: We investigated whether a history of URI was associated with the diagnosis of PD among Korean individuals aged ≥40 years, using data from the Korean National Health Insurance Service–Health Screening Cohort. Methods: A total of 5844 patients newly diagnosed with PD were identified and matched with 23,376 control participants at a 1:4 ratio based on age, sex, income, and geographical region. Conditional logistic regression analyses were performed to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for PD, adjusting for potential confounders including smoking, alcohol consumption, body mass index, blood pressure, comorbidity scores, blood glucose, and serum cholesterol levels. Results: Overall, no significant association was found between a history of URI and PD when considering a two-year exposure window. However, in the one-year window analysis, individuals with a history of URI had a modestly reduced odds of PD (≥1, ≥2, or ≥3 episodes: (adjusted OR: 0.93, 95% CI: 0.88–0.97, aOR: 0.91, 95% CI: 0.87–0.96 and aOR: 0.92, 95% CI: 0.87–0.98, respectively). Subgroup analyses revealed that the inverse association was more pronounced among women, older adults (≥65 years), and those with higher comorbidity scores. No clear dose–response trend was observed across increasing frequencies of URI diagnoses. Conclusions: Our findings suggest that the apparent protective association between recent URI history and PD is unlikely to be causal and may instead reflect confounding by medication use or reverse causation related to the prodromal phase of PD. These results should therefore be interpreted with caution and regarded as hypothesis-generating. Further prospective studies incorporating detailed prescription data and long-term follow-up are warranted to clarify the role of infections and anti-inflammatory medications in the pathogenesis of PD.