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

Inclusion Criteria:

- Participant must have a body mass index (BMI) less than (<) 40.0 kilograms per meter
square (kg/m^2)

- Contraceptive (birth control) use by women should be consistent with local regulations
regarding the acceptable methods of contraception for those participating in clinical
studies. Before randomization, participants who were born female must be either (a)
not of childbearing potential; (b) of childbearing potential and practicing a highly
effective method of contraception (failure rate of less than (<) 1 percent (%) per
year when used consistently and correctly) and agrees to remain on such a method of
contraception from signing the informed consent until 3 months after the last dose of
study vaccine. Use of hormonal contraception should start at least 28 days before the
first administration of study vaccine. The investigators should evaluate the potential
for contraceptive method failure (example, noncompliance, recently initiated) in
relationship to the first vaccination. Highly effective methods for this study
include: (1) hormonal contraception: (i) combined (estrogen and progestogen
containing) hormonal contraception associated with inhibition of ovulation (oral,
intravaginal, or transdermal); (ii) progestogen-only hormonal contraception associated
with inhibition of ovulation (oral, injectable, or implantable); (2) intrauterine
device (IUD); (3) intrauterine hormone-releasing system (IUS); (4) bilateral tubal
occlusion/litigation procedure; (5) vasectomized partner (the vasectomized partner
should be the sole partner for that participant; (6) sexual abstinence. Applicable to
Cohort 2 only: Before randomization, a woman must be (a) postmenopausal
(postmenopausal state is defined as no menses for 12 months without an alternative
medical cause) or permanently sterile; and (b) not intending to conceive by any
method.

- All female participants of childbearing potential must: have a negative highly
sensitive urine or serum beta-human chorionic gonadotropin (hCG) pregnancy test at
screening; have a negative highly sensitive urine beta-hCG pregnancy test immediately
prior to each study vaccine administration

- A male participant must agree not to donate sperm for the purpose of reproduction for
a minimum 28 days after receiving the dose of study vaccine

- Participant agrees to not donate bone marrow, blood, and blood products from the first
study vaccine administration until 3 months after receiving the last dose of study
vaccine

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 investigators 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 previously received a coronavirus vaccine

- Participant has a positive molecular test result for severe acute respiratory syndrome
coronavirus-2 (SARS-CoV-2) infection, confirmed by polymerase chain reaction (PCR) at
screening

- Participants who are at increased risk of severe coronavirus disease-2019 (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, type 2, or gestational); serious heart conditions, including heart failure,
coronary artery disease, congenital heart disease, cardiomyopathies, and pulmonary
hypertension; severe obesity; chronic kidney disease being treated with dialysis;
participants who are immunocompromised (as outlined in other exclusion criteria);
chronic liver disease, including cirrhosis; and participants who live in a nursing
home or long-term care facility

- Participant currently working in an occupation with a high risk of exposure to
SARS-CoV-2 (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