This study is evaluating the immunogenicity and safety of intramuscular administration of the investigational MMRVNS vaccine compared to intramuscular administration of the MMRV (Merck's measles, mumps, rubella and varicella) vaccine (ProQuad) that is already licensed for this route, when administered as a second dose in children 15 months to 6 years of age who were previously primed with a first dose of any combination of measles, mumps, rubella and varicella-containing vaccine(s).

Inclusion Criteria:

• Participants' parent(s)/legally acceptable representative(s) (LAR(s)) who, in the opinion of the investigator, can and will comply with the requirements of the protocol.
• Written or witnessed/thumb printed informed consent obtained from the participants parent(s)/LAR(s) prior to performance of any study specific procedure.
• Informed assent obtained from the participants in line with local rules and regulations.
• Healthy participants as established by medical history and clinical examination at screening.

For countries where the second dose of measles, mumps, rubella, and varicella vaccination is administered between 4 and 6 years of age:

• A male or female participant between and including 4 and 6 years of age at the time of the study intervention administration, and in accordance with local regulations.
• Participant who previously received a first dose of varicella-containing vaccine in the second year of life.
• Participant who previously received a first dose of measles, mumps, rubella-containing vaccine in the second year of life.

For other countries:

• A male or female participant between and including 15 months to 6 years of age at the time of study intervention administration, and in accordance with local regulations.
• Participant who previously received a first dose of varicella-containing vaccine in the second year of life, or earlier at 11 months of age in accordance with national vaccination schedule following official recommendation, at least 3 months before study entry.
• Participant who previously received a first dose of measles, mumps, rubella-containing vaccine in the second year of life, or earlier at 11 months of age in accordance with national vaccination schedule following official recommendation, at least 1 month before study entry.

Exclusion Criteria:

Medical conditions

• History of any reaction or hypersensitivity likely to be exacerbated by any component of the study interventions, including hypersensitivity to neomycin or, gelatin.
• Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination.
• Hypersensitivity to latex.
• Unstable chronic conditions as determined by medical history and physical examination.
• Major congenital defects, as assessed by the investigator.
• History of measles, mumps, rubella, or varicella/zoster disease, as evaluated by the investigator.
• Recurrent history or uncontrolled neurological disorders or any neuroinflammatory, congenital neurological conditions, encephalopathies, or seizures.
• History of febrile seizures, for participant under 4 years of age.
• Active untreated tuberculosis.
• Condition that, in the judgement of the investigator, would make intramuscular injection unsafe.
• Any other clinical condition that, in the opinion of the investigator, might pose additional risk to the participant due to participation in the clinical study.

Prior/Concomitant therapy

• Use of any investigational or non-registered product other than the study interventions during the period beginning 30 days before the dose of study intervention, or their planned use during the study period.

• Administration of immunoglobulins or other blood products or plasma derivates during the period starting 90 days before the study intervention or planned administration during the study period.

• 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 interventions administration: systemic corticosteroid.
  • Up to 180 days prior to study intervention administration: long-acting immune-modifying drugs including among others immunotherapy, monoclonal antibodies, antitumoral medication.

• Previous vaccination with a second dose of varicella containing vaccine or measles, mumps, rubella-containing vaccine.

• Use of salicylates or salicylate-containing products or its planned use during the period of 6 weeks following study intervention administration.

• Planned administration/administration of a vaccine not foreseen by the study protocol in the period starting 30 days before the dose and ending 43 days after the dose of study intervention administration, 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 intervention, and

• Diphtheria, tetanus, acellular pertussis-containing vaccines may be administrated according to the local immunization practices of each participating country. It has to be administered on the same day as the study intervention administration at a different location. However, other than protocol-specified DTaP brand and diphtheria, tetanus, whole cell pertussis-containing vaccines are not allowed.

If emergency mass vaccination for an unforeseen public health threat is recommended and/or organized by public health authorities outside the routine immunization program, the time period described above can be reduced provided it is used according to the local governmental recommendations and sponsor is notified.

Prior/Concurrent clinical study participation

• Concurrently participating in another clinical study, at any time during the study period,

Other exclusion criteria

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

• Child in care.

• 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 <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.

Study is observer-blind, participants care providers investigators and outcomes assessors are blinded but Investigational Medicinal Product administrators are unblinded.