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 betweenthe Ad26.COV2.S primary vaccination should preferably be greater than or equal to (>=) 6 months prior to study vaccination on VAC31518COV2008, however a window ofmaximum -20 days is allowed; Cohort 2: Participant completed primary vaccinationwith a 2-dose regimen of BNT162b2 vaccine. The last dose of BTN162b2 shouldpreferably be >=6 months prior to study vaccination on COV2008, however a window ofa 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 toparticipate in the study

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

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

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

Exclusion Criteria:

• Participant has a clinically significant acute illness (this does not include minorillnesses 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 tothe planned study vaccination; randomization at a later date is permitted at thediscretion of the investigator. Please notify the sponsor (or medical monitor) ofthis decision

• Participant has a known or suspected allergy or history of anaphylaxis or otherserious adverse reactions to vaccines or their excipients (including specificallythe excipients of the study vaccine

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

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

• Participant has a history of heparin-induced thrombocytopenia or thrombosis incombination with thrombocytopenia

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

• History of capillary leak syndrome