Chickenpox , also known as varicella, is a common childhood disease. While chickenpox is usually mild—characterized by itchy spots, a fever, and tiredness—it can be serious, especially in infants and adults. In fact, in the United States, about 12,000 people are hospitalized and 100 people die as a result of chickenpox each year.

The chickenpox vaccine became available in the United States in 1995. Since then, the incidence of chickenpox has decreased substantially. But recent reports of chickenpox outbreaks in groups with high immunization rates have raised questions about the effectiveness of the chickenpox vaccine.

A new study in the February 18, 2004 issue of the Journal of the American Medical Association found that the effectiveness of the chickenpox vaccine decreases significantly within the first year following vaccination. In addition, the vaccine was found to be less effective if it was administered to children younger than 15 months of age.

About the Study

This study included 339 children ages 13 months to 16 years who were diagnosed with chickenpox. These children were age-matched with 669 children who had not had chickenpox (the control group).

A research assistant visited the homes of children with suspected chickenpox 3–5 days after they developed symptoms. The research assistant rated the severity of each child’s illness, based on number and type of lesions, severity of fever and other symptoms, and a subjective assessment. Chickenpox was diagnosed only if virus was detected in skin rashes by a specialized laboratory technique known as polymerase chain reaction (PCR). For all children, the researchers recorded if and/or when the child had been vaccinated for chickenpox.

From this information, the researchers calculated the effectiveness of the vaccine, taking into account the time since vaccination and the child’s age at vaccination.

The Findings

Overall, the chickenpox vaccine was 87% effective. In other words, vaccinated children were 87% less likely than the unvaccinated children to develop chickenpox. Furthermore, chickenpox was significantly more severe in unvaccinated children. Most cases of chickenpox in vaccinated children were mild.

In the first year after vaccination, the vaccine’s effectiveness was 97%. But in year two, the effectiveness dropped significantly—to 86%. And by years 7–8 after vaccination, the effectiveness of the vaccination was down slightly to 81%—a fall that the researchers reported might have been due to chance.

The effectiveness of the vaccine was related to the child’s age at vaccination. When the vaccine was administered to children younger than 15 months, it was 73% effective during the first year, compared with 99% effective when administered to children older than 15 months.

How Does This Affect You?

These findings suggest that while the chickenpox vaccine is highly effective overall, some vaccinated children still develop chickenpox. Additionally, the vaccine’s effectiveness drops in the first year after immunization and may continue to fall slightly for five or more years. Further studies will be needed to determine how much protection extends into adolescence and adulthood—times of life when chickenpox infection may prove most dangerous.

Currently, the Centers for Disease Control and Prevention (CDC) recommends that children should ideally get the chickenpox vaccine between 12 and 18 months of age. The current study supports previous evidence that children who get the vaccine before 15 months may be at an increased risk of developing chickenpox despite being vaccinated. But before this recommendation is changed, researchers will have to weigh the benefits of increasing the age recommendation against the risk of leaving children unvaccinated for an additional three months. Another solution, as the study’s authors point out, may be to administer a second dose of vaccine, but the benefits (if any) and timing of a second dose remain to be evaluated.

The hope is that chickenpox will eventually be eliminated in the U.S. Until then, complications and deaths from chickenpox will continue to occur, although—based on the authors' findings of less serious cases in vaccinated children—hopefully at a lower rate among varicella-immunized children.

Immunizations aren't perfect, but they're among the greatest accomplishments in the on-going fight against disease. Be sure yours and your children's vaccinations are up to date as recommended. Chickenpox in healthy children is usually only a mild illness, but in teenagers and adults it can be extremely serious. If you aren't sure whether you had chickenpox as a child, talk to your doctor. Testing may be indicated, and if susceptible, you may be a candidate to receive the chickenpox vaccination. It could save your life.