• Aged 18-65 years • Healthy with no significant immunosuppressive illnesses. These include but are not limited to: - cancer or treatment of cancer or organ transplantation - treatment of auto-immune or inflammatory conditions such as inflammatory arthritis or inflammatory bowel disease - use of corticosteroid, TNF inhibitor, interleukins, interferons, cyclosporine or other immunosuppressive medications - significant renal or liver disease • Two prior doses of the Pfizer vaccine, the last dose at least 4 months prior to recruitment • No previous significant adverse events to 2 prior doses of the Pfizer vaccine, according to the criteria below. Severe grading indicates the event prevented daily activity, temperature >40.0°C or >10.0 cm in diameter for redness and swelling at the site of injection - severe systemic events (fatigue, headache, fever, muscle or joint pain) - severe local events (pain at injection site, redness and swelling - any event requiring emergency department visit or hospitalisation • No prior anaphylaxis to any cause, including to prior Pfizer vaccine • No prior cardiac inflammatory condition (myocarditis, pericarditis), including to prior Pfizer vaccine • Willing and available to have regular blood and saliva samples taken per the schedule of events • Willing to be randomly assigned to receive a 3rd doses of the Pfizer vaccine either upon enrolment or 3 months later • Willing to provide a signed and dated informed consent form.

• Aged 18-65 years • Healthy with no significant immunosuppressive illnesses. These include but are not limited to: - cancer or treatment of cancer or organ transplantation - treatment of auto-immune or inflammatory conditions such as inflammatory arthritis or inflammatory bowel disease - use of corticosteroid, TNF inhibitor, interleukins, interferons, cyclosporine or other immunosuppressive medications - significant renal or liver disease • Two prior doses of the Pfizer vaccine, the last dose at least 4 months prior to recruitment • No previous significant adverse events to 2 prior doses of the Pfizer vaccine, according to the criteria below. Severe grading indicates the event prevented daily activity, temperature >40.0°C or >10.0 cm in diameter for redness and swelling at the site of injection - severe systemic events (fatigue, headache, fever, muscle or joint pain) - severe local events (pain at injection site, redness and swelling - any event requiring emergency department visit or hospitalisation • No prior anaphylaxis to any cause, including to prior Pfizer vaccine • No prior cardiac inflammatory condition (myocarditis, pericarditis), including to prior Pfizer vaccine • Willing and available to have regular blood and saliva samples taken per the schedule of events • Willing to be randomly assigned to receive a 3rd doses of the Pfizer vaccine either upon enrolment or 3 months later • Willing to provide a signed and dated informed consent form.