A Study to Evaluate Different Dose Levels of Ad26.COV2.S in Healthy Adolescents From 12 to 17 Years Inclusive

The primary purpose of this study is to assess the safety, reactogenicity, and humoral immune response of Ad26.COV2.S administered intramuscularly (IM) as a 1-dose schedule or as a 2-dose schedule (56-day interval) in adolescents.

Available safety, immunogenicity and efficacy data from the Ad26.COV2.S vaccine development program supports initiating evaluation of Ad26.COV2.S in the pediatric population. Ad26.COV2.S will be evaluated in the pediatric population through a dose-confirmation approach. Ad26.COV2.S (also known as Ad26COVS1, VAC31518, JNJ-78436735) is a monovalent vaccine composed of a recombinant, replication-incompetent human adenovirus type 26 (Ad26) vector, constructed to encode the severe acute respiratory syndrome coronavirus(-2) (SARS-CoV-2) spike (S) protein, stabilized in its prefusion conformation. The study duration from screening until the last follow-up visit will be, excluding the 28-day screening phase, 8 months (Groups 4-6) to 12 months (Groups 1-3), consisting of 12-month study duration comprising a study period (6-months) including vaccination with a 1 active dose and a placebo vaccination (56-day interval), followed by a booster vaccination at 6 months and follow-up (safety and immunogenicity) until at least 6 months after booster vaccination (Groups 1-3) and 8 month study duration comprising 2 active doses (56-day interval) and follow-up (safety and immunogenicity) until at least 6 months after second vaccination (Groups 4-6). Assessments like immunogenicity (such as humoral and cellular immune responses), safety and reactogenicity (such as adverse events \[AEs\] monitoring) will be performed in this study. Other safety assessments include vital signs measurements (heart rate, supine systolic and diastolic blood pressure, respiratory rate, and body temperature) and physical examinations. The overall study duration from enrolment of the first participant until study completion is expected to be up to 1 year 11 months.

Eligibility Criteria:
Inclusion Criteria:

* Participant's age is 12 to 17 years of age at the time of first vaccination
* Participant must be healthy, in the investigator's clinical judgement, as confirmed by medical history, physical examination, and vital signs performed at screening, and must not have comorbidities related to an increased risk of severe coronavirus disease-2019 (COVID-19)
* 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
* Participant and/or parent(s)/legal guardian(s) are available and willing to participate for the duration of the study visits and follow-up
* Each participant or participant's parent(s)/legal guardian(s) must have access to a consistent means of contact either by telephone contact or email/computer

Exclusion Criteria:

* Participant has a history of malignancy, bone marrow transplant, or solid organ transplant within 5 years before screening
* Participant has a known or suspected allergy, history of anaphylaxis, or other serious adverse reactions, related to vaccines or their excipients (including specifically the excipients of the study vaccine)
* Use of systemic corticosteroids at an immunosuppressive dose (treatment duration more than 14 days for one course or recurrent use) within 6 months before administration of study vaccine and during the study
* Participants with a history of illness or with an ongoing illness that, in the opinion of the investigator, may pose additional risk to the participant if he/she participates in the study
* Any serious, chronic, or progressive disease (example: diabetes, cardiac disease, hepatic disease, progressive neurological disease or seizure disorder; autoimmune disease, acquired immunodeficiency syndrome \[AIDS\] infection, blood dyscrasias, bleeding diathesis, signs of cardiac or renal failure, or severe malnutrition)