The purpose of this study is to assess how well-tolerated GSK's investigational varicella vaccine (VNS Vaccine) is, in comparison to an already approved varicella vaccine (VV) known as Varivax. The study will be conducted on healthy children aged 12 to 15 months, and who have neither contracted varicella nor received a varicella vaccination.

Inclusion Criteria:

• Participant's parent(s)/Legally acceptable representatives (LAR[s]), who, in the opinion of the investigator, can and will comply with the requirements of the protocol (e.g., completion of the eDiaries, return for follow-up visits).
• Written or witnessed/thumb printed informed consent obtained from the participant's parent(s)/LAR(s) prior to performance of any study-specific procedure.
• Healthy participants as established by medical history and clinical examination before entering the study.
• A male or female between, and including, 12 to 15 months of age (i.e., from the day of 1 year birthday until the day before 16 months of age) at the time of the administration of study interventions.
• Only for children in countries where PCV is recommended at 12 to 15 months of age as per national immunization schedule and provided as part of the study interventions: participant who previously received the primary series of PCV in the first year of life with last dose at least 60 days prior to study entry.

Exclusion Criteria:

• History of any reaction or hypersensitivity likely to be exacerbated by any component of the study interventions.

• Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination (no laboratory testing required).

• Hypersensitivity to latex.

• Recurrent history of uncontrolled neurological disorders or seizures.

• History of varicella disease.

• Active untreated tuberculosis

• Any other clinical condition that, in the opinion of the investigator, might pose additional risk to the participant due to participation in the study.

• Use of any investigational or non-registered product (drug, vaccine or invasive medical device) other than the study interventions during the period beginning 30 days before the dose of study interventions administration (Day -29 to Day 1), or their planned use during the study period.

• Planned administration of a vaccine in the period starting 30 days before the dose and ending 43 days after the dose of study interventions administration (Visit 2), with the exception of inactivated influenza vaccine which may be given at any time during the study and administered at a different location than the study interventions. Any other age-appropriate vaccine may be given starting at Visit 2 and anytime thereafter.

• Chronic administration of immune-modifying drugs (defined as more than 14 consecutive days in total) and/or planned use of long-acting immune modifying treatments at any time up to the end of the study.

  • Up to 90 days prior to the study intervention administration:

    • For corticosteroids, this will mean prednisone equivalent >= 0.5 mg/kg/day with maximum of 20 mg/day for pediatric participants. Inhaled and topical steroids are allowed.
    • Administration of immunoglobulins and/or any blood products or plasma derivatives.

  • Up to 180 days prior to study interventions administration: long acting immune modifying drugs including among others immunotherapy (e.g., tumor necrosis factor-inhibitors), monoclonal antibodies (except the ones not interfering with the immune response to the study vaccines, e.g., nirsevimab), antitumoral medication.

• Previous vaccination against measles, mumps, and rubella.

• Previous vaccination against hepatitis A virus.

• Previous vaccination against varicella virus.

• Only for children in countries where PCV is recommended at 12 to 15 months of age as per national immunization schedule and provided as part of the study interventions, participant who previously received a booster dose of any PCV.

• Concurrently participating in another clinical study, at any time during the study period, in which the participant has been or will be exposed to an investigational or a non-investigational intervention (drug/invasive medical device).

• Child in care.

• Any study personnel's immediate dependents, family, or household members.

• Participants with the following high-risk individuals in their household:

  • Immunocompromised individuals.
  • Pregnant women without documented history of varicella.
  • Newborn infants of mothers without documented history of varicella.
  • Newborn infants born less than (<) 28 weeks of gestation.

Study Sponsor will assess requests from qualified researchers for anonymized individual patient-level data and related study documents. Data sharing is subject to certain criteria, conditions, and exceptions. For further information, refer to https://www.gsk-studyregister.com/About\_GSK\_Patient\_Level\_Data\_Sharing\_Final\_13July2023.pdf

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or asset(s) with development terminated across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months, but an extension may be granted, when justified, for up to 6 months.

Observer-blind study.