ARM 1 1. Previous diagnosis of COVID-19 (microbiologically proven, either symptomatic or asymptomatic) 2. Have been exposed to a known COVID-19 case within the last 72 hours, defined by the current Department of Health and Human services such as household contact, 15 minutes of face to face exposure, 2 hours in close space. 3. Any contra-indication to intra-nasal IFN-a such as severe nasal bleeding requiring intervention, nasal malignancy, nasal deformity, radiotherapy to the nasopharynx and/or oropharynx 4. Pregnant or breast-feeding women, or women who wish to become pregnant during the course of the study 5. Participant unable to return for regular follow-up 6. Life expectancy of less than 4 months 7. Participant already included in another intervention study on the prevention of COVID-19 8. Currently unwell with influenza-like symptoms – if participant is found to be COVID-19 negative and becomes asymptomatic, they can be reconsidered for participation

ARM 1 1. Previous diagnosis of COVID-19 (microbiologically proven, either symptomatic or asymptomatic) 2. Have been exposed to a known COVID-19 case within the last 72 hours, defined by the current Department of Health and Human services such as household contact, 15 minutes of face to face exposure, 2 hours in close space. 3. Any contra-indication to intra-nasal IFN-a such as severe nasal bleeding requiring intervention, nasal malignancy, nasal deformity, radiotherapy to the nasopharynx and/or oropharynx 4. Pregnant or breast-feeding women, or women who wish to become pregnant during the course of the study 5. Participant unable to return for regular follow-up 6. Life expectancy of less than 4 months 7. Participant already included in another intervention study on the prevention of COVID-19 8. Currently unwell with influenza-like symptoms – if participant is found to be COVID-19 negative and becomes asymptomatic, they can be reconsidered for participation