This study is testing a new vaccine called **MMRVNS** to see how well it works and how safe it is. The **MMRVNS** vaccine is for measles, mumps, rubella, and varicella (chickenpox). It is given as a shot in the muscle (intramuscular). The study compares this new vaccine to an already approved vaccine called **MMRV (ProQuad)**. **Key Points:** - Children aged 15 months to 6 years who had their first measles, mumps, rubella, and varicella vaccine dose can join. - The study involves getting one more vaccine shot and having a health check-up. - Safety is a priority, and children with certain health conditions or sensitivities cannot participate. Children should be healthy, without allergies to vaccine ingredients like neomycin or gelatin. Kids with immune system problems or certain medical histories are not eligible. The study will not include those participating in other clinical trials or who plan to take certain medications or vaccines during the study. Participants cannot live with high-risk individuals, such as immunocompromised people or newborns in certain conditions.