HealthDay News — The chicken pox vaccine is effective 14 years after immunization without any waning and may also reduce shingles risk, study results indicate.
Among 7,585 children immunized with varicella vaccine at age 2 years, vaccine effectiveness remained steady around 90% at 14-year follow-up, Roger Baxter, MD, of the Kaiser Permanente Vaccine Study Center in Oakland, Calif., and colleagues reported in Pediatrics.
In the same cohort of children, herpes zoster risk also appeared lower than the risk observed among a historical comparator group not vaccinated against varicella. Among those who received the vaccine, the relative risk of herpes zoster was 0.61 (95% CI: 0.43-0.89).
Since the varicella vaccine was first licensed in 1995, some in the healthcare community have expressed concerns that widespread vaccination against chicken pox could “decrease varicella virus zoster immune-boosting opportunities and lead to an increase in herpes zoster in adults,” according to background information in the study.
To fullfill postmarketing requirements for the Varivax varicella vaccine, Baxter and colleagues conducted a prospective cohort study among 7,858 children in the Kaiser Permanente Northern California healthcare system, who were vaccinated against chicken pox at age 2 years in 1995.
Preliminary study results suggested an 80% to 94% effectiveness against moderate-to-severe cases of chickenpox with a single dose of varicella vaccine, but breakthrough cases resulted in a 2006 recommendation to administer a second dose at age 4 to 6 years.
A total of 2,826 of study participants (38%) received the second vaccine dose from 2006 to 2009. Through 2009,a total of 1,505 chickenpox cases occurred, resulting in an average breakthrough incidence rate of 15.9 per 1,000 patient-years. This incident rate was still nine- to 10-fold lower than researchers expected for unvaccinated children based on estimates from a reference cohort.
At the end of the study period, vaccine effectiveness was 90%, with no evidence of waning over time. The majority of cases of varicella occurred soon after vaccination, and most cases were mild (74%). The rate dropped from 26 per 1,000 patient-years in the first 4 years after the vaccine was administered to just two cases per 1,000 in 2009. None of the children who received a second vaccine dose developed varicella.
Only 46 cases of herpes zoster were observed during the 14-year follow-up period, for an incidence of 0.45 per 1,000 patient-year versus the rate of 0.73 per 1,000 patient-years expected from historical rates after naturally-acquired varicella.
Study limitations included reliance on retrospectively-recalled disease severity and historical incidence rates, as well as the potential for over- and under-reporting.