The study findings, published in the journal Pediatrics, were based on a 14-year study consisting of 7,585 children ranging in age from 12 to 23 months.
The researchers aimed to examine the long-term effectiveness of the vaccine and its influence on the epidemiology of varicella (chickenpox) and herpes zoster (shingles). Additionally, they looked at the results of a second dose of varicella vaccine, which became available in 2006.
Varicella Vaccine Proves Effective Over TimeThe varicella vaccine became licensed in the United States in 1995 and was then suggested for regular use to children by the Advisory Committee on Immunization Practices. Before this, chickenpox was prevalent worldwide - with over 90 percent of adolescents becoming infected before the age of 20.
Along the length of the follow-up period, the prevalence rate of chickenpox in this cohort was 9 to 10 times less than the corresponding rates in kids of the same age who were not vaccinated before the vaccine came onto the market.
This means the varicella vaccine has a total effectiveness rate of nearly 90 percent.
Randy Bergen, MD, chief of outpatient pediatrics at Kaiser Permanente's Walnut Creek Medical Center and a pediatric infectious disease consultant, said:
"Clearly, the vaccine is a very effective tool in preventing or limiting the severity of chicken pox in young people. 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."
Within the study cohort, in the 14 years after the varicella vaccination, there were a total of 1,505 breakthrough cases of chickenpox reported. "Breakthrough cases" are labeled this way because they happen even though a child has gotten the varicella vaccine.
Chickenpox cases were categorized into:
- mild - less than 50 lesions
- moderate - 51 to 300 lesions
- severe - over 300 lesions
Second Dose of Vaccine Even More EffectiveOnly a few cases were severe, however, prior to the vaccination era, the majority of kids experienced severe symptoms. Prevention of moderate to severe chicken pox was successfully reached after one dose of varicella vaccine - no cases were documented after the second dose.
These vaccines help protect small infants when they come into contact with vaccinated kids, according to previous research by the CDC in 2011.
The prevalence of breakthrough varicella dropped over time, and no rise was seen during the 14 year follow-up period.
The obvious increase in the vaccine's success over time, according to lead author Roger Baxter, MD, co-director of the Kaiser Permanente Vaccine Study Center, "is likely the result of vaccine failure occurring early, while breakthroughs became rare due to high vaccine effectiveness both directly and through herd immunity."
The steady decrease in breakthrough rates seen in 2008 and 2009 could have been due to the administration of the second dose in 2006, the researchers suggested. The second dose of varicella is normally administered between ages 4 to 6 years.
However, the researchers point out that if given earlier after the first dose, it could be more beneficial - by giving more protection.
A separate study carried out by Yale University researchers in 2011 showed that double doses of the chickenpox vaccine were found to be more effective than one.
The risk of herpes zoster, frequently known as shingles, was not elevated in vaccinated children and was seen to be reduced in vaccinated children compared with the pre-vaccine era. Among the cohort, there were 46 cases of shingles - showing a nearly 40 percent drop in incidence of herpes zoster in vaccinated children.
In July of 2011, a study from the National Center for Immunization and Respiratory Diseases reported that chickenpox may soon be eradicated in the U.S. Their study showed that deaths from the disease since the vaccine began to circulate have been slim to none.
Written by Kelly Fitzgerald