BACKGROUND. In the United States, children receive primary doses of M-M-RII (Merck & Co, Inc, West Point, PA) and Varivax (Merck & Co, Inc) beginning at 12 months, often at the same health care visit. Currently a second dose of M-M-RII is given to 4- to 6-year-old children, to increase vaccination rates and to reduce the number of individuals without detectable antibodies. A second dose of a varicella-containing vaccine may result in similar benefits.

OBJECTIVES. To demonstrate that ProQuad (measles, mumps, rubella, and varicella virus vaccine live; Merck & Co, Inc) may be given in place of a second dose of M-M-RII or second doses of M-M-RII and Varivax for 4- to 6-year-old children.

METHODS. Four- to 6-year-old children who had been immunized previously with M-M-RII and Varivax were assigned randomly to receive either ProQuad and placebo (N = 399), M-M-RII and placebo (N = 195), or M-M-RII and Varivax (N = 205) and were then monitored for safety and immunogenicity.

RESULTS. ProQuad was generally well tolerated. Similarity (noninferiority) was demonstrated in postvaccination antibody responses to measles, mumps, and rubella between recipients of ProQuad and all recipients of M-M-RII and in responses to varicella between recipients of ProQuad and recipients of Varivax. Postvaccination seropositivity rates for antibodies against all 4 viruses were nearly 100% in all 3 groups. Small fold increases were observed for measles, mumps, and rubella antibody titers. In contrast, substantial boosts in varicella antibody titers were observed among recipients of a second dose of varicella vaccine, administered as ProQuad or Varivax.

CONCLUSIONS. ProQuad may be used in place of a second dose of M-M-RII or second doses of M-M-RII and Varivax for 4- to 6-year-old children.

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