Doesn't Ensure Protection
NEW YORK (Reuters Health) - An outbreak
of chickenpox among a group of children in New Hampshire shows
that the virus that causes chickenpox can be highly infectious
even among those who have been vaccinated, according to a new
Dr. Karin Galil of the Centers
for Disease Control and Prevention in Atlanta, Georgia and colleagues
report their findings in the December 12th issue of The New England
Journal of Medicine.
The researchers evaluated an outbreak
of chickenpox, which is caused by the varicella virus, at a daycare
center in New Hampshire. A total of 88 parents returned a questionnaire
that aimed to gauge prior chickenpox illness and vaccination among
the children. In all, 25 children came down with chickenpox between
December 2000 and January 2001. The researchers sourced the outbreak
to a 4-year-old child who had been vaccinated for chickenpox 3
years prior to contracting the illness.
The child infected about half of
his classmates who had no prior history of chickenpox infection.
At the time of the outbreak, roughly 73% of kids old enough for
chickenpox vaccine had received it, the report indicates.
"The effectiveness of the vaccine
was 44% against disease of any severity and 86% against moderate
or severe disease," write Galil and colleagues. Experts have estimated
that the chickenpox vaccine is between 71% to 100% effective at
preventing varicella infection.
Children who had been vaccinated
3 years or more before the outbreak were at greater risk of vaccination
failure than those who had been vaccinated more recently, they
On the surface it appears that
immunity against chickenpox weakened as time passed after vaccination.
However, the authors note that "the reasons for the poor performance
of the vaccine are not apparent.
"Although policy cannot be established
on the basis of one outbreak, the findings in this investigation
raise concern that the current vaccination strategy may not protect
all children adequately," the authors write.
Nonetheless, the investigators
point out that the illness is much less of a threat today than
it was before the era of chickenpox vaccination, when there were
roughly 11,000 hospitalizations and 100 deaths from the disease
"Vaccination remains the most effective
strategy for protecting children and adults against illness and
death due to varicella," Galil and colleagues conclude.
Current guidelines call for one
dose of chickenpox vaccine for children between the ages of 1
and 12 years and two doses of vaccine for people over 13.
"It has long been known...that
'breakthrough' varicella may nevertheless develop in 10 to 15
percent of vaccinated persons," Dr. Anne A. Gershon of Columbia
University in New York City writes in an accompanying editorial.
Gershon suggests that a second
dose of chickenpox vaccine "should decrease the number of children
who have...vaccine failure and might also prevent waning immunity,
if it does indeed currently occur."
What's more, Gershon points out
that it eventually took the routine administration of two doses
of measles vaccine to control measles in the US.
"The time for exploring the possibility
of routinely administering two doses of varicella vaccine to children
seems to have arrived," Gershon concludes.
SOURCE: The New England Journal
of Medicine 2002;347:1909-1915, 1962-1963.
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