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BACKGROUND: In 2006, as varicella outbreaks continued to occur in schools with high 1-dose varicella vaccination (VV) coverage, the Advisory Committee on Immunization Practices recommended implementation of a routine 2-dose VV program for children. In 2001, CA adopted single dose school entry requirements for kindergartners and children transferring from out of state schools at all grade levels. We report on further declines in varicella incidence and outbreaks 5 years after routine 2-dose implementation in 2006.
METHODS: We examined varicella incidence, age and VV status of case-patients, and number ofoutbreaks from 2006 through 2011 from a varicella active surveillance region, AV, CA. Varicella cases were verified to have had illness with acute onset of diffuse maculopapulovesicular rash without another known cause. On a biweekly basis, over 100 community-based sites submitted varicella case reports or documentation of no cases. Two HMOs and one emergency room reported electronically using diagnostic visit codes. VV coverage was estimated for children with ≥1 vaccination using an electronic vaccination registry of child residents enrolled in a HMO.
RESULTS: Two-dose VV coverage among 5 year olds in 2010 was 96% in AV. In 2011, overall varicella incidence was 0.2 cases/1000, an 82% decline since 2006 (1.1 cases/1000) and a 98% decline since vaccine introduction in 1995, 10.3/1000. Incidence declined in all age groups with 88% reductions among those aged 5-14 years during 2006-2011 from 4.9/1000 to 0.6/1000, in respective years. There was no significant change in the proportion of case-patients who were vaccinated with >1 doses of VV, 59.5% vs. 56% of case patients in 2006 and 2011, respectively. During 2006-2011, the median age of case-patients with no or unknown doses of varicella vaccine increased from 12 to 17 years, while the median age of vaccinated case-patients fluctuated between 8-9 years. Twenty-six varicella outbreaks were reported during 2006-2011 (none documented in 2011), compared to 78 during 2001-2005.
CONCLUSIONS: Varicella incidence has declined in the 5 years since adoption of a routine 2-dose VV program. Promotion of "catch-up" vaccination among older susceptible individuals should continue. National 2-dose VV coverage and improved case surveillance are needed to more fully understand the impact of routine 2 dose VV.