TUESDAY, April 2 (HealthDay News) -- The chickenpox vaccine is
very effective at preventing the disease, and its protection
doesn't wane over time, new research finds.
"This is a really good vaccine," said the study's lead author, Dr. Roger Baxter, co-director of the Vaccine Study Center at Kaiser Permanente in Oakland, Calif. One dose is enough to protect against most cases and severe infection, he said, while "the second dose just wipes it out."
The study results were released online April 1 in advance of
publication in the May print issue of
Chickenpox, an infection caused by the varicella virus, was
commonplace in childhood until the introduction of the vaccine in
the United States in 1995. Prior to the vaccine's introduction,
more than 90 percent of children contracted chickenpox by the time
they were 20 years old, according to the study. Although usually
mild, the disease, which causes itchy blisters all over the body,
was responsible for about 100 deaths a year.
When the vaccine was introduced, it wasn't clear if one dose
would be sufficient, or if protection would wear off over time.
Consistent protection was important because chickenpox infection in
older teens and adults can be much more serious than it generally
is in childhood, Baxter said.
Initial studies suggested the vaccine was between 80 percent and
90 percent effective, but sporadic outbreaks of chickenpox still
occurred. In 2006, the Advisory Committee on Immunization Practices
recommended that a second dose of varicella vaccine be given when
children are between 4 and 6 years old.
The current study included 7,585 children vaccinated with
varicella vaccine in 1995 when they were 2 years old. The
researchers followed the children's health for 14 years, looking
for cases of chickenpox or herpes zoster, which is more commonly
known as shingles. Shingles is another type of infection caused by
the chickenpox virus that tends to affect people later in life,
according to the U.S. National Institute of Neurological Disorders
In that group of children, 2,826 also received a booster dose of
varicella vaccine between 2006 and 2009.
Slightly more than 1,500 cases of breakthrough cases of
chickenpox occurred during the study period. All of the cases
occurred after the first dose of the vaccine. No breakthrough cases
were reported after the second dose of vaccine, the study
Only 2 percent of the breakthrough cases were severe. Overall,
the rate of chickenpox in vaccinated children was about 10 times
lower than it would have been had they not been vaccinated,
according to the study.
Shingles is unusual in children, but researchers estimated the
children who were vaccinated had a 40 percent reduced risk of
Because breakthrough cases occurred soon after vaccination, it's
possible that the second dose would be more effective if given
earlier than the recommended age of 4 to 6 years, the authors
Dr. Kenneth Bromberg, director of the Vaccine Research Center at
the Brooklyn Hospital Center in New York City, commented favorably
on the study.
"These researchers answered important questions about efficacy with one dose; it's about 90 percent, and the effectiveness of two doses seems to be about 100 percent," Bromberg said.
Both experts said it looks like this vaccine will continue to
protect these children into adulthood, although no studies have yet
confirmed these findings because the vaccine has only been
available for 17 years.
With most vaccines, effectiveness wanes a little bit over time,
said Baxter. "But with the varicella vaccine, it got more and more
effective over time," he noted.
Will adults need another dose of vaccine? From this study,
Bromberg said, "it looks promising that the answer will be no."
Learn more about chickenpox and the vaccine from the
U.S. Centers for Disease Control and
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