Inclusion Criteria:  Adult, ≥ 18 years at the time of signing the informed consent. Cohort specific inclusion criteria:  Solid organ transplant  Participants with heart, lung, kidney, or liver transplant, and are stable on immunosuppressants (defined as no change in dose in the previous 4 weeks).  Hematopoietic stem cell transplant  Participants with autologous (up to 6 months after transplantation) or allogeneic stem cell transplant who are immunosuppressed, with no evidence of active graft-versushost disease, at least one month after the procedure.  Cancer patients on chemotherapy  Participants with solid tumors (except breast cancer), histologically diagnosed, who were undergoing intravenous cytotoxic chemotherapy within the last 6 months, who received at least 1 cycle prior to cytotoxic chemotherapy, and have a life expectancy of longer than 3 months.  Chronic inflammatory conditions  Participants with chronic inflammatory conditions, including those on immunosuppressant medications, can be recruited. The following conditions are specifically excluded: multiple sclerosis and peripheral demyelinating disease.  Primary immune deficiency  Examples include combined granulomatous disorder, SCID, common variable immunodeficiency.  Immunocompetent:  No confirmed or suspected immunosuppressive or immunodeficient state. No use of immunosuppressant medication within the past 1 month (≥ 20 mg per day of prednisone or its equivalent, given daily or on alternate days for ≥ 15 days within 30 days prior to administration of AZD1222). The following exceptions are permitted: topical/inhaled steroids or short-term oral steroids (course lasting ≤ 14 days). No receipt of immunoglobulins and/or any blood products within 3 months prior to administration of AZD1222 or expected receipt during the period of study follow up. No severe and/or uncontrolled cardiovascular disease, respiratory disease, gastrointestinal disease, liver disease, renal disease, endocrine disorder, and neurological illness, as judged by the Investigator.  Exclusion Criteria:  History of allergic disease or reactions likely to be exacerbated by any component of AZD1222. Active infection with SARS-CoV-2 as confirmed locally by RT-PCR. Known past laboratory-confirmed SARS-CoV-2 infection. Significant infection or other acute illness, including fever (temperature > 37.8°C) on the day prior to or day of first dosing. Thrombocytopenia with platelet count ≤ 75,000 x 109/microliter based on complete blood count test at screening visit. HIV-positive participants based on a positive ELISA test performed at screening visit. History of cerebral venous sinus thrombosis (CVST). Receipt of any vaccine (licensed or investigational) within 30 days prior to and after administration of AZD1222.