During an outbreak of chickenpox in Minnesota in the fall of 2002, more than half the children who became infected had been immunized with the varicella vaccine, according to a new report.

Evidently, booster shots may be required to provide stronger protection against chickenpox.

Dr. Brian R. Lee, at the Minnesota Department of Health in Minneapolis, and his colleagues investigated the outbreak that involved 55 children among 319 attending an elementary school in northern Minnesota.

According to the team’s report in the Journal of infectious Diseases, 29 of the affected children had been vaccinated, and 6 had apparently had chickenpox previously - which usually prevents another infection.

The primary case in the outbreak was a vaccinated 6-year-old boy.

The investigators found that 25 percent of vaccinated children came down with chickenpox, as did 56 percent of unvaccinated students; among those with a history of varicella, the infection rate was 6 percent.

Lee’s group estimates that the effectiveness of the vaccine in warding off infection was 56 percent. However, immunized children did have fewer lesions, less fever, and fewer sick days than their non-immunized kids.

The risk of catching chickenpox was more than two-fold higher for those vaccinated 4 or more years before the outbreak, compared with those vaccinated more recently - so protection wanes over time.

These findings underscore the importance of keeping any child with chickenpox out of school, regardless of their vaccination status, Lee’s team states.

They also highlight the importance of continuing “to maintain and improve the national level of vaccination against varicella zoster and to consider whether a booster dose will offer additional protection,” the researchers conclude.

SOURCE: Journal of infectious Diseases, August 1, 2004.