NEW YORK (Reuters Health) - 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.
