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NEW YORK (Reuters Health) -- Kids are less likely to get chickenpox if they get two doses of the chickenpox vaccine instead of just one, suggests a new study.
Most infected people simply feel miserable -- with symptoms including an itchy, blister-like rash, fever, headache and fatigue -- but some do develop serious complications.
While routine vaccination has made a big dent in the number of chickenpox cases in the U.S. over the past 15 years, it doesn't offer perfect protection -- either because the vaccine didn't work the first time, or because its effects can eventually wear off.
Seeking to solve that problem, the U.S. Centers for Disease Control and Prevention (CDC) and the American Academy of Pediatrics (AAP) in 2006 recommended a second dose of the vaccine for kids ages 4 to 6. The first dose is typically given at 12 to 18 months of age.
"Despite the reasonably (high) effectiveness of one dose of the vaccine, there were still outbreaks in schools and daycare centers -- even though there was a pretty good rate of vaccination," Dr. Eugene Shapiro, the study's lead author from the Yale University School of Medicine in New Haven, Connecticut, told Reuters Health.
In the current study, published in The Journal of Infectious Diseases, Shapiro and his colleagues tested the effectiveness of adding the second dose by comparing kids with chickenpox and kids without chickenpox to see how many doses of the vaccine both had received.
Starting right after the CDC updated its recommendation through the beginning of 2010, Shapiro and colleagues found 71 infected children age 4 and up from local pediatricians' offices.
For each of those kids, the researchers found two others who were the same age and saw pediatricians from the same practice. Then they reviewed their medical records for vaccinations.
Of the 71 patients with chickenpox, none had gotten two doses of the vaccine. Sixty-six of them had received one dose, while five hadn't been vaccinated at all.
Out of the 140 kids who had never had chickenpox, 22 of them had been vaccinated twice, 117 once, and only one had never gotten the vaccine.
Based on those findings, the authors calculated that one dose of the vaccine protected 86 percent of kids, while two doses were 98 percent effective.
Although the study was relatively small and didn't directly compare outcomes for kids who had gotten none, one, or two doses of the vaccine, doctors are still optimistic about the results.
"We are now in the second period of (chickenpox) control," an editorial in the journal notes, "and version 2.0 looks promising indeed."
"For most children, one dose (is) enough," said Dr. Margaret Fisher, who was on the AAP committee that recommended adding the extra vaccine dose. "But to really stop the breakthrough cases, we need to add that second dose. I'm delighted now that we have a study that shows that it works."
Two of the study's authors have financial ties to Merck, the company that sells the vaccine. One dose costs about $80.
The chickenpox vaccine sometimes causes soreness or swelling, a fever, or a mild rash, and very rarely more serious reactions. According to the CDC, side effects are more common after the first dose of the vaccine than after the second dose.
Although kids who get chickenpox after receiving a single dose generally have a milder version of the virus than kids who haven't been vaccinated, they can still spread chickenpox to others. And, Shapiro said, those kids may also be at a higher risk of getting shingles, a painful rash that is caused by the same virus as chickenpox, later down the road.
"The recommendation for a second dose ... certainly carries the direct cost of additional vaccine doses," Sarah Clark, of the Child Health Evaluation & Research Unit at the University of Michigan, told Reuters Health in an email.
"But (the vaccine) may be helpful in reducing some of these other costs that are incurred because of breakthrough cases," she added.
Shapiro said that despite the study's convincing results, no one can be sure how the effectiveness of the second dose of the vaccine will hold up many years after vaccination.
When he and his colleagues completed their research, Shapiro said, "It was only three and a half years since the new recommendation was made. It remains to be seen what will happen over the long term."
Both Shapiro and Fisher said the take-away message from the study was that parents should make sure their children get a second dose of the vaccine, if they haven't received it already.
SOURCE: bit.ly/i82eIL and bit.ly/hygWB4 Journal of Infectious Diseases, online January 5, 2011.