Long-term Effectiveness of Varicella Vaccine: A 14-Year, Prospective Cohort Study

ECDC comment

​Varicella vaccine was introduced in the United States in 1995 for individuals 12 months of age. A second dose was recommended in 2006. Varicella incidence and vaccine effectiveness were assessed in a 14-year prospective study conducted at Kaiser Permanente Northern California (a health care delivery system).

​Baxter R, Ray P, Tran TN, Black S, Shinefield HR, Coplan PM et al. Pediatrics 2013;131;e1389.

Description:

Varicella vaccine was introduced in the United States in 1995 for individuals 12 months of age. A second dose was recommended in 2006. Varicella incidence and vaccine effectiveness were assessed in a 14-year prospective study conducted at Kaiser Permanente Northern California (a health care delivery system). A total of 7585 children vaccinated with varicella vaccine in their second year of life in 1995 were followed up prospectively for breakthrough varicella and herpes zoster (HZ) through 2009. A total of 2826 of these children received a second dose in 2006–2009. Incidences of varicella and HZ were estimated and compared with prevaccine-era rates. In this cohort of vaccinated children, the average incidence of varicella was 15.9 per 1000 person-years, nine- to tenfold lower than in the prevaccine era. Vaccine effectiveness at the end of the study period was 90%, with no indication of waning over time. Most cases of varicella were mild and occurred early after vaccination. No child developed varicella after a second dose. HZ cases were mild, and rates were lower in the cohort of vaccinated children than in unvaccinated children during the prevaccine era.

Disclosure statement:

The financial disclosure of the authors can be found in the article. The article was sponsored by Merck Sharp & Dohme Corp.

ECDC comment:

This post licensure study assesses the long-term effectiveness of varicella vaccine and the impact on the epidemiology of varicella and HZ. The study confirms what is known: that varicella vaccine is highly effective and that a two-dose regimen is more efficacious.
 
Additionally, the study indicates the lasting effectiveness of the vaccine, with no waning noted over a 14-year period. The study also suggests that varicella vaccination may reduce the risks of HZ in vaccinated children when compared to one study performed during the prevaccine era(1).
 
A response to this article has been published by Goldman GS on a significant confounder(2). Goldman suggests that children in cohorts 1995-2004 were boosted not only via vaccination, but also through periodic exogenous exposures to children shedding varicella zoster virus (VZV). Therefore, the effectiveness over 14 years is confounded from the impact of the exogenous exposures, highly prevalent before the two-dose regimen was implemented.