Varicella immunity: does it wane in children?

Journal of Family Practice, June, 2007

* Clinical question

Does immunity from the varicella vaccine decrease over time?

* Bottom line

Yes. Researchers found that children who had been vaccinated 5 or more years previously were twice as likely to have moderate to severe disease than those who had been vaccinated less than 5 years previously. These data are the basis for the recommendation that children who are between 4 and 6 years of age receive a second varicella vaccination.

Level of evidence

2b: individual cohort study or low-quality randomized controlled trials

Study design

Cohort (prospective)

Funding

Government

Setting

Population-based

Synopsis

Although varicella vaccination has generally been successful at reducing outbreaks, there have nonetheless been outbreaks in highly vaccinated school communities. Contributing factors may include increasing time from the primary vaccination and less exposure in the community to wild viruses that "boost" immunity.

The authors of this study used data from a surveillance project of 350,000 children in California. Any cases of varicella occurring more than 42 days after vaccination were identified by more than 300 reporting sites, which included medical clinics, public health clinics, and schools. The researchers developed regression models to study the effect of increasing time from vaccination on the risk of varicella infection.

Vaccination coverage in this group increased from approximately 15% in 1996 to more than 90% in 2003 and 2004. The total number of cases declined substantially during that time, from 2800 cases in 1995 to fewer than 500 in 2004. Of 11,356 cases of varicella during the 10-year study period, only 1080 subjects had been previously vaccinated. The vast majority of cases of varicella occurred in unvaccinated children in the 1990s. However, since 2003, while the absolute number of cases has been much smaller, the majority occurred in children who had been vaccinated.

Also, while most cases in the 1990s occurred in children who were between 3 and 6 years of age, most cases in 2004 occurred in children who were between 6 and 12 years of age. Disease in these older children was more likely to be moderate to severe than in younger children. In the multivariate analysis, the odds ratio for infection for children who had been vaccinated more than 5 years ago (compared with those vaccinated within the last 5 years) was 2.6 (95% confidence interval, 1.2-5.8).

FAST TRACK

These data are the basis for the recommendation that children between 4 and 6 years of age receive a second varicella vaccination

Chaves SS, Gargiullo P, Zhang JX, et al. Loss of vaccine-induced immunity to varicella over time. N Engl J Med 2007; 356:1121-1129.