The purpose of this study is to to evaluate the safety and immune response of COVI-VAC given as a single booster dose in healthy adults previously vaccinated against COVID-19 with an authorised mRNA or adenovirus-vectored vaccine. Approximately 30 participants who have been fully vaccinated ≥ 3 months ago will be enrolled and receive one dose of COVI-VAC. COVI-VAC is administered by drops into each nostril. To assess the safety of the vaccine, each participant will record symptoms and oral temperature in a diary daily for 7 days after receiving COVI-VAC or placebo.

During the full study safety laboratory tests, physical exams, and vital signs (including oxygen saturation) will be conducted periodically for safety. Adverse events and medication use will be recorded.

Blood samples and intranasal samples will be collected periodically to assess the immune response from the vaccine.

Inclusion Criteria:

1. Men and women aged 18 to 65 years of age, inclusive, on the day of signing the ICF
2. Body mass index (BMI) ≤ 35 kg/m2
3. In good health with no history, or current evidence, of clinically significant medical conditions with particular reference to, but not restricted to, thromboembolic disorders, coronary heart disease, chronic obstructive lung disease, and no clinically significant test abnormalities that will interfere with participant safety, as defined by medical history, physical examination, vital signs (including oxygen saturation), and safety laboratory tests as determined by the Investigator
4. Previous receipt of an authorised mRNA or adenovirus-vectored COVID-19 vaccine with completion of vaccine regimen ≥ 3 months before Day 1

Exclusion Criteria:

1. Residents of residential care facilities
2. Pregnant or lactating women
3. Inadequate venous access for repeated phlebotomy
4. History of confirmed or suspected SARS-CoV-2 infection