The principal aim of the COMMUNITY study is to investigate immune responses following SARS-CoV-2 infection and/or vaccination in healthcare workers and hospitalized COVID-19 patients. The study aims to enroll a total of 5,000 healthcare workers and 500 COVID-19 patients. All participants are enrolled at Danderyd Hospital in Stockholm, Sweden. Participants will be followed every four months with blood and mucosal sampling in addition to demographic and clinical data provided by the participants and from national registries. Primary outcome: \- Antibody responses in blood and mucosa following SARS-CoV-2 infection and/or vaccination Secondary outcomes: * Cellular immune responses following SARS-CoV-2 infection and/or vaccination * Antibody responses in blood and mucosa following following Influenza A/B infection and/or vaccination * Antibody responses in blood and mucosa following following RSV A/B infection Blood and mucosal samples are collected every 4 months and at tighter intervals in sub studies. PCR testing is conducted upon symptoms of respiratory infections and with regular intervals regardless of symptomatology in sub studies. Participants provide detailed demographic and clinical data through a smart phone application-based questionnaire. Data on infection and vaccination history is collected from national registries