Background: Immunologic alterations caused by psoriasis and/or its therapies may affect the risk of serious infections. Objective: For patients with psoriasis, we explored the overall and therapy-related risk of contracting an infectious disease (ID) requiring hospitalization in a large population-based cohort. Methods: The incidence of ID was compared between patients with psoriasis and a randomly selected cohort (ratio 1:5) using hospital and pharmacy databases covering 2.5 million Dutch residents between 1997 and 2008. First and multiple IDs were defined and categorized into 20 groups based on primary International Classification of Diseases, Ninth Revision, Clinical Modification discharge diagnoses. Multivariate Cox regression and Poisson event-count models were used to test the risk difference of IDs between patients with psoriasis and reference cohort. Results: A total of 25,742 patients with psoriasis and 128,710 reference subjects were followed up for approximately 6 years. The likelihood of IDs in patients with psoriasis was twice as high as the reference population (908 vs 438 events/100,000 person-years, crude hazard ratio 2.08, 95% confidence interval 1.96-2.22). In a multivariate model the hazard ratio decreased to 1.54 (95% confidence interval 1.44-1.65). This risk was highest for patients with more severe psoriasis (adjusted hazard ratio 1.81, 95% confidence interval 1.57-2.08), but was not associated with recent systemic antipsoriatic drug dispensing. Respiratory tract, abdominal, and skin infections occurred most frequently in patients with psoriasis. Multiple event analysis that counted the total number of infectious discharge diagnoses gave similar results. Limitations: No data were available on lifestyle factors. Conclusion: The risk of severe infections was significantly higher for patients with psoriasis compared with control subjects and could not be explained by exposure to systemic antipsoriatic drugs.

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Keywords cohort study, comorbidity, epidemiology, psoriasis, respiratory tract infection, serious infectious diseases, systemic drugs
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Wakkee, M., de Vries, E., van den Haak, P., & Nijsten, T.E.C.. (2011). Increased risk of infectious disease requiring hospitalization among patients with psoriasis: A population-based cohort. American Academy of Dermatology. Journal, 65(6), 1135–1145. doi:10.1016/j.jaad.2010.08.036