A single dose of the chicken pox virus prevented about 50,000 hospital visits in the U.S. between 2000 and 2006. Now a new study published in the Journal of Infectious Diseases finds that the treatment is more effective in those who get two shots before the age of six. The chicken pox vaccine was 98.3 percent effective in those who had received two doses, as opposed to 86 percent effective in children who'd gotten just one shot. "...[T]he odds of developing varicella were 95 percent lower in children who had received two doses of the vaccine compared with those who had received only one," explained lead researcher Dr. Eugene Shapiro of Yale University. Report