A Study of Ad26.COV2.S Administered as Booster Vaccination in Adults Who Have Previously Received Primary Vaccination With Ad26.COV2.S or BNT162b2

Inclusion Criteria:

- Cohort 1: Participant received Ad26.COV2.S in VAC31518COV3001. The interval between
the Ad26.COV2.S primary vaccination should preferably be greater than or equal to (>=)
6 months prior to study vaccination on VAC31518COV2008, however a window of maximum
-20 days is allowed; Cohort 2: Participant completed primary vaccination with a 2-dose
regimen of BNT162b2 vaccine. The last dose of BTN162b2 should preferably be >=6 months
prior to study vaccination on COV2008, however a window of a maximum of -20 days is
allowed

- Participant must provide consent indicating that he or she understands the purpose,
procedures and potential risks and benefits of the study, and is willing to
participate in the study

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

- Participant must be willing to provide verifiable identification, has means to be
contacted and to contact the investigator during the study

- Participant must be able to read, understand, and complete questionnaires in the
electronic clinical outcome assessment (eCOA) (that is, the Coronavirus disease
(COVID-19) signs and symptoms surveillance question, the e-Diary, and the electronic
patient-reported outcomes (ePROs). Participants with visual impairment are eligible
for study participation and may have caregiver assistance in completing the eCOA
questionnaires

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
>= 38.0 degree Celsius (C) (100.4 degree Fahrenheit [F]) within 24 hours prior to the
planned study vaccination; randomization at a later date is permitted at the
discretion of the investigator. Please notify the sponsor (or medical monitor) of this
decision

- Participant has a known or suspected allergy or history of anaphylaxis or other
serious adverse reactions to vaccines or their excipients (including specifically the
excipients of the study vaccine

- Participant received treatment with immunoglobulins (Ig) in the 3 months or exogenous
blood products (autologous blood transfusions are not exclusionary) in the 4 months
before the planned administration of the study vaccine or has any plans to receive
such treatment during the study

- Participant has a known history of confirmed severe acute respiratory syndrome
coronavirus-2 (SARS-CoV-2) infection

- Participant has a history of heparin-induced thrombocytopenia or thrombosis in
combination with thrombocytopenia

- Participant has a history of acute polyneuropathy (example. Guillain-Barre syndrome)

- History of capillary leak syndrome