The main purpose of this study is to assess immune response and safety of various potencies of a measles, mumps, rubella, and varicella (MMRVNS) vaccines given to healthy children of 4 to 6 years of age.

This study is designed to evaluate the immunogenicity and safety of the investigational measles, mumps, rubella, varicella vaccine (referred to as the MMRVNS vaccine) compared with the licensed measles, mumps, rubella, varicella vaccine, ProQuad (referred to as the MMRV vaccine), when given as a second dose to children 4 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).

This study will evaluate immunogenicity and safety using 3 MMRVNS formulations which vary for some or all the individual virus potencies. The 3 MMRVNS formulations (designated as MMRV(H)NS vaccine, MM(H)RVNS vaccine and M(L)M(L)R(L)V(L)NS vaccine) will be compared with the MMRV vaccine. To ensure representative data on the comparator, participants enrolled in the MMRV group will be randomized to two different lots (designated as MMRV_Lot 1 and MMRV_Lot 2). Throughout the study, the two lots will be analyzed as a pooled group.

Inclusion Criteria:

• Healthy participants as established by medical history and clinical examination before entering into the study.
• A male or female between, and including, 4 years and 6 years of age (i.e., from 4 year birthday until the day before the 7-year birthday) 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.
• Participant who previously received a single dose of measles-, mumps-, rubella-containing vaccine in the second year of life.
• Written informed consent obtained from the participants' parent(s)/legally acceptable representative(s) (LAR[s]) prior to performance of any study-specific procedure (participant informed assent will be obtained from participants in line with local rules and regulations).
• Participants' parent(s)/LAR(s), who, in the opinion of the investigator, can and will comply with the requirements of the protocol (e.g., completion of electronic diaries [eDiaries], return for follow-up visits).

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 (no laboratory testing required).
• Hypersensitivity to latex.
• Major congenital defects, as assessed by the investigator.
• History of measles, mumps, rubella, or varicella disease.
• Recurrent history of or uncontrolled neurological disorders or seizures.
• Acute disease at the time of enrollment. Acute disease is defined as the presence of a moderate or severe illness with or without fever. Fever is defined as body temperature >=38.0 degrees Celsius (°C) (100.4 degrees Fahrenheit [°F)] by any age-appropriate route. All study interventions can be administered to participants with a minor illness such as diarrhea, mild upper respiratory infection without fever.
• Participant with history of coronavirus disease 2019 (COVID-19) who is still symptomatic.
• Any other clinical condition that, in the opinion of the investigator, might pose additional risk to the participant due to participation in the study.

Prior and Concomitant Therapy

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

• Chronic administration (defined as more than 14 days in total) of immunosuppressants or other immune-modifying drugs during the period starting 90 days prior to the study intervention administration. For corticosteroids, this will mean prednisone equivalent >= 0.5 mg/kg/day or 20 mg/day whichever is the maximum dose for pediatric participants. Inhaled and topical steroids are allowed.

• Administration of immunoglobulins and/or any blood products or plasma derivatives during the period starting 180 days before the dose of study interventions or planned administration during the study period.

• Administration of long-acting immune-modifying drugs at any time during the study period (e.g., infliximab).

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

• Administration or planned administration of a vaccine not foreseen by the study protocol in the period starting 30 days before the dose and ending at 43 days after the dose of study interventions administration* (Visit 3), with the exception of:

  • inactivated influenza (flu) vaccine which may be given at any time during the study and administered at a different location than the study interventions and,
  • routinely recommended licensed childhood DTPa-containing vaccines which can preferably be co-administered according to the local immunization practices of the participating country.

• Any other age-appropriate vaccine may be given starting at Visit 3 and anytime thereafter.

  • In case an emergency mass vaccination for an unforeseen public health threat (e.g., a pandemic) is recommended and/or organized by public health authorities outside the routine immunization program, the time period described above can be reduced if necessary for that vaccine, provided it is used according to the local government recommendations and that GSK/IQVIA is notified accordingly.

Prior/Concurrent Clinical Study Experience

• 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).

Other Exclusions

• 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 <28 weeks of gestation.

GSK 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.

IPD will be made available within 6 months of publishing the results of the primary endpoints, a key secondary endpoints and safety data of the study.

Access is provided after a research proposal is submitted and has received approval from the 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 can be granted, when justified, for up to another 12 months.

Single-blind study. Recipients and care providers will be unaware of vaccine administered.