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

A Randomized, Double-blind, Phase 2 Study to Evaluate the Immunogenicity, Reactogenicity and Safety of Ad26.COV2.S Administered as Booster Vaccination in Adults 18 Years of Age and Older 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.

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