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OBJECTIVE. A refrigerator-stable formulation of ProQuad has been developed to expand the utility of ProQuad to areas in which maintenance of a frozen cold chain (-15[degrees]C or colder) during storage and transport may not be feasible. The objective of this study was to demonstrate that the immunogenicity and safety profiles of a refrigerator-stable formulation of ProQuad are similar to the recently licensed frozen formulation. METHODS. In this double-blind, randomized, multicenter study, healthy 12- to 23-month-old children with negative vaccination and clinical histories for measles, mumps, rubella, varicella, and zoster were vaccinated with either the refrigerator-stable formulation of ProQuad (N = 1006) or the frozen formulation of ProQuad (N = 513). Patients were followed for 42 days after vaccination for adverse experiences. Immunogenicity was evaluated 6 weeks after vaccination. RESULTS. The refrigerator-stable formulation of ProQuad was generally well tolerated. The incidence of adverse experiences was similar between groups. No vaccine-related serious adverse experiences were reported. For both groups, the response rate was [greater than or equal to]97.7% for measles, mumps, and rubella, and the percentage of patients with a varicella zoster virus antibody titer of [greater than or equal to]5 U/mL glycoprotein antigen-based enzyme-linked immunosorbent assay after vaccination was [greater than or equal to]88.8%. The geometric mean titers for all antigens were numerically slightly higher in patients who received the refrigerator-stable formulation. CONCLUSIONS. The refrigerator-stable formulation of ProQuad is generally well tolerated, highly immunogenic, and noninferior in terms of postvaccination antibody responses. This refrigerator-stable formulation may improve ease of vaccine administration, increase use of the vaccine throughout the world because of its improved storage conditions, and replace the frozen formulation of ProQuad or any dose of M-M-RII and Varivax in routine practice. KEYWORDS. measles, mumps, rubella, varicella, vaccine, ProQuad, Varivax, M-M-RII, immunization. PEDIATRICS 2007;119:e1299-e1305. URL: www.pediatrics.org/cgi/doi/10.1542/peds.2006-2283