A two-dose varicella vaccination program, begun in 2006, has been shown to be more effective in promoting population immunity than the single-dose regimen introduced more than a decade earlier, in addition to further reducing disease severity and incidence.
Although the single-dose regimen, implemented in 1995, was associated with dramatic declines in varicella-related illness and deaths of 90% and 88%, respectively (Pediatrics 2011;128:214-20), a new study, published online Oct. 7 in Pediatrics showed that implementation of the two-dose scheme not only slashed cases further, but conferred protection even among unvaccinated infants and adults (Pediatrics 2013;132:1-7 [doi: 10.1542/peds.2013-0863]).
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A two-dose varicella vaccination program has been shown to be more effective in promoting population immunity than the single-dose regimen introduced more than a decade earlier, in addition to further reducing disease severity and incidence.
For their research, Dr. Stephanie R. Bialek of the National Center for Immunization and Respiratory Diseases, Atlanta, and her colleagues analyzed incidence rates and disease characteristics in two metropolitan centers totaling 650,000 in population between 1995 and 2010; one was a suburb of Los Angeles and the other was an inner-city area of Philadelphia. The study period covered the rollout of the single-dose vaccine and the two-dose scheme.
In 2010, the California surveillance area showed an incidence of 0.3 cases per 1,000 population, a decline of 76% since 2006 and a 97% decline from 1995. The Pennsylvania site, with 0.1 cases per 1,000, saw a 67% decline since 2006 and a 97% decline since 1995. From 2006 to 2010, 61.7% of case patients in both surveillance areas had been vaccinated with a single dose and 7.5% with two doses. Hospitalizations declined by half in both areas between 2006 and 2010.
Approximately 15%-20% of children do not adequately respond to a single dose of vaccine, Dr. Bialek and her colleagues noted, and the two surveillance areas continued to see outbreaks even after the single-dose scheme was in effect. About 65% of outbreak cases after 2007 in California had received one dose, and a larger proportion had milder disease (50 lesions or less) than in outbreaks earlier in the study period. During the two-dose period, the California surveillance area saw a fourfold decrease in outbreaks while the Pennsylvania area reported no outbreaks at all.
"The substantial declines in varicella incidence and outbreaks we report on from these two active surveillance areas during the first 5 years of the two-dose varicella vaccination program provide additional evidence of the program’s sustained impact," Dr. Bialek and her colleagues wrote in their analysis. "With full implementation of the two-dose varicella vaccination program, it may be possible to eliminate the most severe outcomes of varicella."
Dr. Bialek and her colleagues noted as limitations to their study the fact that not all cases reported in the study were laboratory confirmed, allowing for potential overreporting of cases and an underestimation of declines. They also acknowledged that some varicella cases may not have been reported, leading to overestimation of declines, and that their data sources for estimating two-dose coverage levels were limited.
The research was publicly funded, and none of the investigators reported conflicts of interest.