Varicella vaccine neutralizes chicken pox
The study, which was conducted by the Kaiser Permanente Vaccine Study Center, followed 7,585 children who were vaccinated in 1995 between 12 and 23 months of age. The incidence rate of chickenpox in the children was nine to 10 times lower than corresponding rates of unvaccinated children of the same age in the era before the vaccine.
The varicella vaccine was licensed in the United States in 1995 and recommended shortly thereafter by the Advisory Committee on Immunization Practices.
"Clearly, the vaccine is a very effective tool in preventing or limiting the severity of chickenpox in young people," Randy Bergen, the chief of outpatient pediatrics at Kaiser Permanente's Walnut Creek Medical Center, said. "As with any vaccine, though, the rate of vaccination has a huge impact on effectiveness. The more children vaccinated, the more effective the vaccine is for the entire community. At Kaiser Permanente, our use of a comprehensive electronic health record, Kaiser Permanente HealthConnect, enables us to quickly identify children in the targeted age ranges who have not been vaccinated, and to reach out to their parents to ensure they get the shots. Keeping vaccination rates high confers benefit on the community as a whole because there are fewer children who can contract and spread the virus."
There were a total of 1,505 breakthrough cases of chickenpox within the study cohort in the 14 years following varicella vaccination. Breakthrough cases occur when children contract chickenpox despite receiving the vaccine. Only 28 of the children with chickenpox experienced severe symptoms. There were no cases among children who received two doses of the vaccine.
The breakthrough incidence rate dropped steadily over time during the 14 years of follow up.
"(The increase in the vaccine's effectiveness over time) is likely the result of vaccine failure occurring early, while breakthroughs became rare due to high vaccine effectiveness both directly and through herd immunity," Roger Baxter, the lead author of the study, said.
There were 46 cases of herpes zoster, commonly known as shingles, among the children. The number of cases represented an approximate 40 percent decrease in the vaccinated children when compared to the pre-vaccine era.