January 7, 2011

Double Dose of Chickenpox Vaccine is Best

(Ivanhoe Newswire) -- Two doses of the chickenpox vaccine provide excellent protection in children against the highly-contagious itchy disease, according to a new study.

The Centers for Disease Control and Prevention (CDC) began recommending a single dose of chickenpox vaccine in children aged 1 to 13 years old in 1995. Although the incidence of chickenpox fell by 90 percent after introduction of the vaccine, there was a high rate of breakthrough chickenpox illness in immunized children and continuing outbreaks of chickenpox among children despite high rates of vaccination. Studies also showed that the single-dose vaccine's effectiveness was less than 90 percent. Given the evidence, the CDC began recommending a second dose of the vaccine for children ages 4 to 6 years old in 2006.

This study is the first to assess the clinical effectiveness of two doses of the vaccine in the general population. Eugene D. Shapiro, M.D., and colleagues at Yale University and collaborators at Columbia University, conducted active surveillance in an area in Connecticut and discovered 71 cases of chickenpox in children aged 4 or older. None of the children had received two doses of vaccine, 66 (93 percent) had received one dose, and 5 (7 percent) had received no vaccine.

The investigators compared the effectiveness of two doses of vaccine versus one dose in a case-control study, using 140 matched controls. The effectiveness of one dose in preventing chickenpox was 86.0 percent, while the effectiveness of two doses was 98.3 percent. According to Dr. Shapiro, "The odds of developing varicella (chickenpox) were 95 percent lower in children who had received two doses of the vaccine compared with those who had received only one."

The results suggest that countries immunizing children with only one dose of chickenpox vaccine should consider changing to a two-dose regimen. The authors emphasized, "There should be continued monitoring of the effectiveness of two doses to assure that its high degree of effectiveness is sustained."

SOURCE: Journal of Infectious Diseases, published online January 5, 2010