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"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."
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% 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% and 90% 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.
What the study found
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 US National Institute of Neurological Disorders and Stroke.
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 found.
Only 2% 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% reduced risk of shingles.
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 noted.
Dr Kenneth Bromberg, director of the Vaccine Research Center at the Brooklyn Hospital Center in New York City, commented favourably on the study.
"These researchers answered important questions about efficacy with one dose; it's about 90%, and the effectiveness of two doses seems to be about 100%," 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."
More information
Learn more about chickenpox and the vaccine from the US
Centers for Disease Control and Prevention.
