Genetic contribution to severe COVID-19 in adults under 60 years without major comorbidities in the German National Pandemic Cohort Network (NAPKON).

While genome-wide association studies (GWAS) have linked common genetic variants to COVID-19 susceptibility and severity, rare high-impact variants may also contribute to phenotypic heterogeneity. Inborn errors of type I interferon immunity (IFN-I-IEIs), including X-linked TLR7 deficiency, account for ~ 2% of critical COVID-19 cases. In this study, we investigated rare potentially deleterious variants in IFN-I-IEI and GWAS-prioritized genes in young, severely affected COVID-19 patients from the German National Pandemic Cohort Network (NAPKON). Genome sequencing was performed on 110 hospitalized COVID-19 patients, including 82 males and 28 females, all under 60 years of age and without relevant pre-existing medical conditions. Rare potentially deleterious variants in TLR7 and 25 additional IFN-I-IEI genes, as well as 23 GWAS risk genes for COVID-19 severity, were analyzed based on allele frequency, predicted functional impact, and inheritance pattern models and subsequently classified based on the American College of Medical Genetics and Genomics (ACMG) criteria. Polygenic Risk Scores (PRS) were additionally calculated as an exploratory and case-only analysis to assess the contribution of common variant-derived genetic predisposition for severe COVID-19. Consistent with prior findings from other studies in German cohorts, no candidate variants or large deletions were identified in TLR7. However, 7 variants of uncertain significance in IFN-I-IEI genes as well as 13 candidate variants of potential deleterious effect in GWAS risk genes were present in 19 individuals (17.3%). We observed nominally significant differences in PRS distributions, with younger individuals (< 40 years) having higher PRS (p = 0.045) compared to older individuals, and carriers of rare variants having lower PRS compared to non-carriers (p = 0.037). These patterns are consistent with an age-dependent contribution of polygenic risk to severe COVID-19 and a potentially lower polygenic burden among rare-variant carriers, although confirmation in larger well-controlled cohorts will be required. The candidate variants identified in IFN-I-IEI and GWAS risk genes represent targets for further functional studies to clarify their potential contribution to disease risk. These findings highlight the need for future integrative genomic approaches to better understand the joint contribution of common and rare variants to COVID-19 severity.