Varicella vaccine was licensed in the United States in 1995 for individuals ≥12 months of age. A second dose was recommended in the United States in June 2006. Varicella incidence and vaccine effectiveness were assessed in a 14-year prospective study conducted at Kaiser Permanente Northern California.
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 (relative risk: 0.61 [95% confidence interval: 0.43–0.89]).
This study confirmed that varicella vaccine is effective at preventing chicken pox, with no waning noted over a 14-year period. One dose provided excellent protection against moderate to severe disease, and most cases occurred shortly after the cohort was vaccinated. The study data also suggest that varicella vaccination may reduce the risks of HZ in vaccinated children.
Comments
A Significant Confounder in the Long-term Effectiveness of Varicella Vaccine, A 14-year Prospective Cohort Study.
The children that received the varicella vaccine in 1995 were repeatedly exposed to epidemics of wild type virus in their respective communities through 2003/2004 in which exogenous exposures boosted cell- mediated immunity (CMI). Thus, during this "honeymoon" period of 1995- 2004, vaccinated children were boosted not only via vaccination, but through periodic exogenous exposures to children shedding varicella zoster virus (VZV)--resulting in a greater than 95% efficacy.
As CMI started to wane in years following 2003/2004, when exogenous exposures became rare, the children were boosted again via a second varicella vaccination in 2006-2009. Thus, the 90% effectiveness over 14 years is confounded and biased high from the impact of the exogenous exposures (boosts) that were prevalent through 2003/2004. In the course of another 4 to 5 years, the effectiveness in this same cohort will decline to below 70%. This pattern of declining efficacy was observed among household contacts after varicella incidence dramatically decreased in the varicella active surveillance project following moderate varicella vaccination coverage. [Goldman GS. Universal varicella vaccination: efficacy trends and effect on herpes zoster. Int J Toxicol 2005 Jul/Aug; 24(4):205-13]
Conflict of Interest:
None declared