A Nationwide Population-based Cohort Study of the Incidence of Severe and Rare Infections Among Adults With Psoriasis in Denmark

Nikolai Loft; Lone Skov; Craig Richardson; Vivek Trivedi; Ivette Alarcon; Alexander Egeberg


The British Journal of Dermatology. 2022;187(3):353-363. 

In This Article

Abstract and Introduction


Background: Patients with psoriasis have a high risk for multiple comorbid conditions. However, few studies have examined the association between psoriasis and severe and rare infections. This study reports the incidence of severe and rare infections (considered as rare in Denmark) among Danish patients with psoriasis, compared with the general population.

Objectives: The objectives of this study were to assess the incidence and risk of severe and rare infections in Danish patients with psoriasis and the matched general population, and to compare this risk for patients with severe or mild psoriasis with that of the general population.

Methods: Data for individuals aged ≥18 years who were alive and resident in the source population were collected from the Danish National Patient Register between 1 January 1997 and 31 December 2018. Individuals with any of the investigated chronic infections prior to inclusion were excluded. Patients with psoriasis were matched (1 : 6) for age and sex with general population controls. Severe infections were defined as infections requiring treatment in a hospital setting and rare infections included HIV, hepatitis B and C, and tuberculosis infections. Incidence rates (IRs) were reported per 100 000 person-years of exposure. Severe psoriasis was defined according to previous or active use of systemic or biological treatment. Patients who never received biological and/or systemic treatment were categorized as having mild psoriasis.

Results: A total of 94 450 patients with psoriasis were matched with 566 700 controls. The respective IRs were higher for patients with any psoriasis compared with controls; IR 3104·9 [95% confidence interval (CI) 3066·6 to 3143·7] and IR 2381·1 (95% CI 2367·6 to 2394·6) for any infection, IR 3080·6 (95% CI 3042·5 to 3119·3) and IR 2364·4 (95% CI 2350·9 to 2377·9) for severe infections, and IR 42·9 (95% CI 38·89 to 47·4) and IR 31·8 (95% CI 30·34 to 33·3) for rare infections, respectively. Patients with severe psoriasis had higher IRs of severe or rare infections (IR 3847·7, 95% CI 3754·3 to 3943·4) compared with patients with mild psoriasis and controls.

Conclusions: As the severity of psoriasis increases, so does the risk of severe and rare infections. Therefore, clinicians should be aware of the increased risk of severe and rare infections in patients with severe psoriasis so that early investigation and treatment can be initiated.


Psoriasis is a common chronic, immune-mediated inflammatory disease that affects 2 to 4% of the population in Western countries.[1,2] Psoriasis is characterized by lesional skin exhibiting red plaques with silver or white scales and disease pathogenesis is driven by a network of leucocytes and cytokines.[3] The global burden of psoriasis is large, both in terms of impact on quality of life for patients[4] and healthcare costs.[5–7]

Patients with psoriasis are known to be at higher risk for multiple comorbid conditions, including psoriatic arthritis, nonalcoholic steatohepatitis, cardiovascular disease and certain types of cancer.[8–17] Previous studies have also found that patients with psoriasis have an increased risk of serious infections (associated with hospitalization, pneumonia and herpes zoster).[18–20] A UK population-based cohort study from 2020 describes the risk of hospitalization and death resulting from infection in people with psoriasis and found associations with small increases in risk for any infection, respiratory infections and soft tissue and skin infections.[21] Additionally, patients with psoriasis are often treated with immunosuppressive therapies, which may lead to or aggravate infections. Thus, the potential association between psoriasis and risk of incidence of infections is important. However, few studies have assessed the incidence and risk of severe infections and rare infections, the latter of which, in Denmark, includes tuberculosis (TB), hepatitis B (HBV) and hepatitis C (HCV) and HIV, among patients with psoriasis with different disease severities.[17–20]

There is currently limited real-world evidence regarding the occurrence and risk of severe and rare infections among patients with psoriasis. Furthermore, certain treatments for psoriasis may aggravate infections. Therefore, the current nationwide population-based cohort study aimed to investigate Danish nationwide administrative longitudinal registries for descriptive data on the incidence of severe infections and infections considered rare in Denmark among individuals with different psoriasis disease severities and matched individuals from the general population. Additionally, the risk for all patients with mild or severe psoriasis was compared with that of the general population.