The rate of chickenpox, which causes death in about 100 people and puts about 12,000 Americans in the hospital each year, has fallen 85 percent in the United States since the introduction of Merck's chickenpox vaccine in 1995. Although many pediatricians believe the vaccine requires a booster shot to remain effective as a child gets older, parents should get their children vaccinated to ensure protection. A recent report in the New England Journal of Medicine found that about 75 percent of children in the United States are receiving the vaccination, with 33 states requiring it for entry into school or day care. Some vaccinated children do contract chickenpox, but the severity of the illness they experience is generally lower than that for children who are not vaccinated. Following an outbreak at a day-care center in New Hampshire that proved the chickenpox vaccine is only 44 percent effective, as reported in the New England Journal of Medicine, the Centers for Disease Control and Prevention is collecting data and will study the issue of booster shots this year.