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

The COMMUNITY study is designed to longitudinally follow healthcare workers and recovered COVID-19 patients to assess SARS-CoV-2 immune responses over time. Follow-ups are conducted every four months, including serum, PBMC, and mucosa (saliva and nasal secretions) samples. Sub studies with additional samplings and PCR screenings are conducted with regular intervals to assess vaccine responses and correlates to protection against infection.

Analyses include:

• Quantification and characterization of antibody responses
• Evaluation of T and B-cell responses
• RT-qPCR detection of viral infection
• Mucosal microbiome evaluation

Clinical and demographic data, including information on hospital workplace, exposure, immunosuppressive treatments or conditions, and results from SARS-CoV-2 Rapid Diagnostic Tests, are collected at each follow-up through a smartphone-based app system. Data on SARS-CoV-2 infections and vaccinations are obtained from national registries. In addition, data from home based PCR-screenings are continuously being monitored for SARS-CoV-2, Influenza and RSV infections.

All samples are stored in Biobank Sweden.

Inclusion Criteria:

• Health care workers: Employment at Danderyd hospital at time of enrollment
• COVID-19 patients: Admitted to Danderyd Hospital with confirmed SARS-CoV-2 infection at time of enrollment

Exclusion Criteria:

• No exclusion criteria

Danderyd Hospital employs approximately 4,500 healthcare workers. All HCWs are invited to participate in the study through information disseminated via the hospital intranet, email, and word of mouth.

Under consideration