Chickenpox immunization: Immunization designed to prevent or lessen the severity of the common disease known as chickenpox. The vaccination currently involves only one shot, given at about one year of age. If an older person has not had chickenpox, the shot may be given at any time. There have been few significant reactions to the chickenpox vaccine. All children, except those with a compromised immune system or a known neurological condition, should have the vaccination.

In an outbreak of chickenpox at a day-care center reported in 2002, 17 children who had been vaccinated contracted chickenpox. The effectiveness of previous vaccination was only 44%, well below prior estimates of the effectiveness of the chickenpox vaccine, raising the possibility of routinely administering two doses of chickenpox vaccine to children.