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

The purposes of this study are to demonstrate the non-inferiority (NI) of the neutralizing antibody response to the original strain 14 days after booster vaccination with Ad26.COV2.S at the different dose levels, administered greater than or equal to (\>=) 6 months after single-dose primary vaccination with Ad26.COV2.S, compared to the neutralizing antibody response to the original strain induced by single-dose primary vaccination with Ad26.COV2.S; To demonstrate the NI of the neutralizing antibody response to the leading variant of high consequence or concern 14 days after booster vaccination with Ad26.COV2.S at the 5\*10\^10 virus particle (vp) dose level, administered \>= 6 months after single-dose primary vaccination with Ad26.COV2.S (5\*10\^10 vp dose level), compared to the neutralizing antibody response to the leading variant of high consequence or concern induced by single-dose primary vaccination with Ad26.COV2.S at the 5\*10\^10 vp dose level, if feasible; To demonstrate the NI of the neutralizing antibody response to the original strain 14 days after booster vaccination with Ad26.COV2.S at the different dose levels administered \>=6 months after completing a 2-dose primary vaccination with Pfizer BNT162b2, compared to the neutralizing antibody response to the original strain induced by 2-dose primary vaccination with Pfizer BNT162b2; To demonstrate the NI of neutralizing antibody response to the leading variant of high consequence or concern 14 days after booster vaccination with Ad26.COV2.S at the 5\*10\^10 vp dose level, administered \>= 6 months after completing a 2-dose primary vaccination with Pfizer BNT162b2, compared to the neutralizing antibody response to the leading variant of high consequence or concern induced by 2-dose primary vaccination with Pfizer BNT162b2, if feasible.


Eligibility Criteria:
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