This is a prospective, longitudinal cohort study in people with Cystic Fibrosis (pwCF) that involves repeated serial sampling of participants. This study design was chosen to provide comprehensive information on SARS-CoV-2 seroprevalence changes over time and the subsequent clinical impact on pwCF. The study will be conducted at participating CF centres over a 3-year period. Study participants will include paediatric and adult pwCF. For the UK section of the study, UK investigators in the European Cystic Fibrosis Society-Clinical Trials Network (ECFS-CTN) will be invited to participate. Participating investigators can enrol all eligible pwCF over a 12-month period. Participants are then followed up for 24 months. Participants will donate blood samples at their routine clinic visits. Blood samples will be collected at Day 0 (baseline), at Months 6, 12, 18 and 24 (to coincide with routine clinical reviews). Additional blood samples will be taken opportunistically every time the participant visits the clinic for blood draws. These blood samples could be related to, routine care, annual review visits, pulmonary exacerbations (PEx), CF complications or when initiating new treatments (e.g. CFTR modulators). Serum from blood samples will be shipped to a central laboratory (Queen's University Belfast) for standardized measurement of SARS-CoV-2 antibodies. Alongside the blood samples the investigators will also collect clinical data from the patient's health records and will input this data into the case report form (CRF). Clinical data will be collected in conjunction with routine care visits, according to local clinical practice. Investigators will collect data elements from information routinely recorded in the patients' medical records. Data will be collected at baseline, month 6, 12, 18 and 24 as per the study schedule, and at additional blood sampling timepoints as previously explained above. Data collection will include routine data available from CF clinic follow-ups including background demographic information, CF medical history, medications, exacerbation information, sputum microbiology and clinical and lung function parameters. Information on SARS-CoV-2 infection history and vaccine receipt will also be collected. The maximum follow-up duration of participation in the study for each patient will be 24 months. This study duration (24-month follow-up) is justified as it provides sufficient time to observe changes in antibody prevalence over the course of the COVID-19 pandemic as well as sufficient time to determine long term clinical outcomes for pwCF who are SARS-CoV-2 seropositive. Furthermore, investigators anticipate the 2-year study follow-up period will provide sufficient time to observe the impact of vaccination on antibody levels given that a number of vaccines are now commercially available. The investigators will compare the level of antibody responses between natural COVID-19 infection and vaccination in pwCF and how this varies over time. This will be achieved by analyzing seroprevalence and antibody levels according to natural infection and vaccination status and according to time of sample post infection or post vaccination, if known. Optional Study sample collection: For participants who consent, a second blood sample will also be drawn into EDTA tubes (plasma). Consent to this optional study sample would allow this sample and any remaining serum (following antibody testing) to be stored for future analysis and allow further research to be carried out on related studies to COVID-19 and CF.