Major depressive disorders increase the susceptibility to self-reported infections in two German cohort studies

INTRODUCTION: In several claims-based studies, major depressive disorder (MDD) has been associated with increased risk of hospitalization due to acute infections. It remains unclear if this is a causal effect, and if it generalizes to an increased susceptibility to infections. METHODS: We used data of the BiDirect (n = 925) and the HaBIDS (n = 1007) cohort studies to estimate the effect of MDD on self-reported infections, which were assessed with identical infection susceptibility questionnaires in both studies. We used the Center for Epidemiologic Studies Depression Scale (CES-D) to examine if there was a dose–response relationship between depressive symptom severity and self-reported infections. RESULTS: BiDirect participants with MDD diagnosis (48%) had a higher risk of lower respiratory tract infections (incidence rate ratio 1.32, 95% confidence interval [1.00–1.75]), gastrointestinal infections (1.68 [1.30–2.16]) and fever (1.48 [1.11–1.98]) after adjusting for confounders identified by a directed acyclic graph approach. There was a dose–response relationship, i.e. individuals with higher CES-D scores reported more infections. Effect sizes were similar in HaBIDS (4% individuals with MDD). CONCLUSION: We found increased risks of mild infections in patients with MDD diagnosis and a dose–response relationship between depressive symptom severity and infection frequency. While causal immunological pathways remain unclear, the results of our study might contribute to a change in prevention strategies, e.g. by recommending vaccination against influenza and S. pneumoniae to MDD patients because observed effect sizes in our study are similar to those of patients with cardiovascular and metabolic diseases for which the respective vaccinations are recommended. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00127-022-02328-5.