Inclusion Criteria:  Healthy adults aged 30-55 years. Able and willing (in the Investigator's opinion) to comply with all study requirements. Willing to allow the investigators to discuss the volunteer's medical history with their General Practitioner and access all medical records when relevant to study procedures. For females only, willingness to practice continuous highly effective contraception (see below) during the study and a negative pregnancy test on the day(s) of screening, bronchoscopy and vaccination. Agreement to refrain from blood donation during the course of the study. Provide written informed consent. Have had a complete COVID-19 vaccination course (as one or two intramuscular injections depending on the authorisation schedule) with the last injection at least 30 days before enrolment Sero-suitable i.e. with evidence of SARS-CoV-2 vaccine-induced antibody responses but no evidence of previous SARS-CoV-2 infection by an authorised serology test. Those with indeterminate levels and no history of laboratory-confirmed SARS-CoV-2 infection may be included or excluded at the PI's discretion on a case-by-case basis.  Exclusion Criteria:  History of laboratory-confirmed SARS-CoV-2 infection. Acute upper respiratory infection (URI or sinusitis) in the past 6 weeks Prior receipt of any vaccine within ≤30 days prior to enrolment or planned receipt of any vaccine within 30 days after the study vaccination Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccine candidate. Inhaled bronchodilator or steroid use within the last 12 months Intranasal steroid use within the last 6 months Any confirmed or suspected immunosuppressive or immunodeficient state, including HIV infection; asplenia; recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids. Any autoimmune conditions, except mild psoriasis, well-controlled autoimmune thyroid disease, vitiligo or stable coeliac disease not requiring immunosuppressive or immunomodulatory therapy. History of allergic disease or reactions likely to be exacerbated by any component of the AZD1222 vaccine. Any history of angioedema. Any history of anaphylaxis. Pregnancy, lactation or willingness/intention to become pregnant during the study. History of cancer (except basal cell carcinoma of the skin and cervical carcinoma in situ). History of serious psychiatric condition likely to affect participation in the study (e.g. ongoing severe depression, history of admission to an in-patient psychiatric facility, recent suicidal ideation, history of suicide attempt, bipolar disorder, personality disorder, alcohol and drug dependency, severe eating disorder, psychosis, use of mood stabilisers or antipsychotic medication). Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture. History of frequent nose bleeds Any other serious chronic illness requiring hospital specialist supervision. Chronic respiratory diseases, including mild asthma (resolved childhood asthma is allowed) Smoking (includes any inhaled product, such as cigarettes and vapes) in the past 6 months OR >5 pack-year lifetime history. Chronic cardiovascular disease (including hypertension), gastrointestinal disease, liver disease (except Gilberts Syndrome), renal disease, endocrine disorder (including diabetes) and neurological illness (excluding migraine) Seriously overweight (BMI≥40 Kg/m2) or underweight (BMI≤18 Kg/m2) Suspected or known current alcohol abuse as defined by an alcohol intake of greater than 42 units every week. Suspected or known injecting drug abuse in the 5 years preceding enrolment. Use of any medication or other product (prescription or over-the-counter) for symptoms of rhinitis or nasal congestion within the last 3 months Any clinically significant abnormal finding on screening biochemistry or haematology blood tests. Grade 1 abnormalities are permissible at investigator discretion. Clinically relevant abnormality on chest X-ray Any other significant disease, disorder or finding which may significantly increase the risk to the volunteer because of participation in the study, affect the ability of the volunteer to participate in the study or impair interpretation of the study data. A history of cerebral venous sinus thrombosis, heparin-induced thrombocytopenia or antiphospholipid syndrome or vaccine-induced thrombosis with thrombocytopenia