An immunocompromised condition can be due to inherited disorders, infections, underlying diseases, therapy, and complications. The number of immunocompromised individuals is constantly increasing. About 10 years ago, the proportion of the adult population reported to be immunocompromised was 2.6–2.8% in the USA (seven million individuals). In a recent USA report based on a survey of more than 20.000 participants, the self-reported proportion instead was 6.6% [1]. Similar estimates in the UK and the EU countries gave at least 500.000 individuals and at least eight million individuals, respectively, as being immunosuppressed. Several factors contribute to this increase, such as better treatment results for underlying diseases, allowing more patients to live longer, and thereby increasing the prevalence of many conditions in the population. Furthermore, new drugs with immunosuppressive properties were introduced to patients previously treated differently or not at all, an aging population frequently having comorbidities, and new methods were introduced to allow for the diagnosis of previously unrecognized inherited disorders. These individuals are at increased risk for severe disease from vaccine-preventable infections. This can be due to infections circulating in the community, such as pneumococci or annual influenza, and was strongly emphasized when new emerging infectious diseases appeared, such as the experience gathered during the last two pandemics, H1N1 “swine flu” and COVID-19, both resulting in higher morbidity and mortality in immunocompromised individuals compared to healthy, age-matched adults.