Inclusion Criteria:  Age >/=16 years (Sites may choose to only enrol adults based on their provincial age of majority) HIV positive for HIV group; For HIV negative group, individuals should be immunocompetent and generally in good health (i.e. participants should not have a condition associated with immunodeficiency nor be receiving immunosuppressant medication) Receiving at least 1 dose of COVID-19 vaccine, or have received 1 or 2 doses of a COVID-19 vaccine Able to provide signed, informed consent Able to attend study visits  Exclusion Criteria:  Signs or symptoms of active COVID-19 infection For HIV-uninfected persons: immune-compromising conditions or on medication which suppresses the immune response