A CDC report shows that more than half of students who developed chicken pox during an outbreak at a Maine elementary school were not vaccinated against the disease, researchers say.

The findings, published in the April issue of Pediatrics, emphasize the importance of childhood chickenpox (varicella) vaccinations, and encourage catch-up vaccinations for older children and adolescents.

"This outbreak was attributed to primary failure to vaccinate," the researchers write. Catch-up vaccination in susceptible children is especially important to prevent severe infection seen in this group, they add.

Chicken pox vaccine is part of recommended routine immunizations for children aged 12-18 months. Older children and susceptible adults should also receive a catch-up vaccination since the likelihood of a severe infection increases with age.

Vaccine Rate Decreases by Grade

While chicken pox vaccine coverage overall was 74%, there was a decreased rate of vaccinated students by increasing grade.

Researchers from the CDC surveyed the parents of students involved in the outbreak. They found that about two-thirds of the students had received a chicken pox vaccine, but the vaccination coverage decreased by grade.

Ninety percent of kindergarteners were vaccinated against chicken pox compared with 60% of third-graders. As a result, the third-grade class had more than 2.5 times the number of chicken pox cases than those in kindergarten.

The illness was particularly severe in unvaccinated studentsevere in unvaccinated students; 22% of the unvaccinated students reported severe infection and one child was hospitalized for a severe skin infection.

The chicken pox vaccine is not 100% effective; vaccinated children who contract the disease usually have a mild form. None of the vaccinated students reported severe infection.

There has been a dramatic decrease in the number of chickenpox casesdramatic decrease in the number of chickenpox cases since the chicken pox vaccine's introduction in 1995.