A Study of Ad26.COV2.S in Adults (COVID-19)

Exclusion Criteria

• •Participant has a clinically significant acute illness (this does not include minor illnesses such as diarrhea or mild upper respiratory tract infection) or temperature greater than or equal to (\>=) 38.0 degree Celsius within 24 hours prior to the planned first dose of study vaccine; randomization at a later date is permitted at the discretion of the investigator and after consultation with the sponsor
• •Participant has a history of malignancy within 5 years before screening (exceptions are squamous and basal cell carcinomas of the skin and carcinoma in situ of the cervix, or malignancy, which is considered cured with minimal risk of recurrence)
• •Participant has a history of any neurological disorders or seizures including Guillain-Barre syndrome, with the exception of febrile seizures during childhood
• •Participant has a positive diagnostic test result for SARS-CoV-2 infection confirmed by polymerase chain reaction (PCR) at screening
• •Participants with comorbidities that are or might be associated with an increased risk of progression to severe COVID-19, that is, participants with moderate-to-severe asthma; chronic lung diseases such as chronic obstructive pulmonary disease (COPD) (including emphysema and chronic bronchitis), idiopathic pulmonary fibrosis and cystic fibrosis; diabetes (including type 1 or type 2); serious heart conditions, including heart failure, coronary artery disease, congenital heart disease, cardiomyopathies, and (pulmonary) hypertension or high blood pressure; obesity (BMI \>= 30 kg/m\^2); chronic liver disease, including cirrhosis; sickle cell disease; thalassemia; cerebrovascular disease; neurologic conditions (dementia); smoking end stage renal disease; organ transplantation; cancer; HIV infection and other immunodeficiencies; hepatitis B infection; and sleep apnea. Applicable to Cohort 3 only: Participants may have hypertension of mild severity (according to the Toxicity Grading Scale), as long as it is stable and medically controlled as defined by no change in medication over the past 6 months (except for issues of tolerability or use of similar drug with same mechanism of action, for example, thiazides, Beta blockers, Alpha blockers at the same effective dose)
• •Applicable to Cohorts 1 and 3 only: Participant currently working in an occupation with a high risk of exposure to SARS-CoV-2 (for example, health care worker or emergency response personnel) or considered at the investigator's discretion to be at increased risk to acquire COVID-19 for any other reason