Background: The Varicella Active Surveillance Project has conducted population-based active surveillance for varicella disease in the AV since 1995. From 1995 to 2005 varicella incidence rates (VIR) declined by 90% and single dose vaccine coverage was 90% for LAC children <24 months. In 2006, the Advisory Committee on Immunization Practices endorsed a second dose of varicella vaccine for children 4-6 years due to increasing varicella cases after primary vaccination.
- Compare VIR and proportion of breakthrough (BT) disease during the end of single dose varicella vaccination era (2005-2006) and the initiation of two dose era (2007-2008).
- Describe and compare the clinical presentation of varicella disease during these two periods.
Methods: Varicella cases had illness with acute onset of diffuse maculopapulovesicular rash without other cause. Verified cases had complete case reports and resided within AV. BT varicella cases had illness >42 days after varicella vaccination. VIR were calculated using 2005-08 US AV census data. The Results: Conclusions:
Results:From 2005-2008, 1270 verified varicella cases were documented. VIR among all age-groups declined significantly from 2.6 (2005-2006) to 1.7 cases per 1000 (2007-2008) (P<0.05). The proportion of BT cases declined in all age groups from 56% (2005-2006) to 43% (2007-2008) (P>0.05). Cases reporting <50 lesions increased from 55% (2005-2006) to 62% (2007-2008) (p=0.02). Fewer cases reported 50-250 lesions, 35% to 30% (P>0.05), and >250 lesions, 9% to 6% (P>0.05). The proportion reporting mostly macular/papular rash increased 62% to 70% (P=0.01) and those reporting vesicular rash decreased 37% to 23% (P<0.01).
Conclusions:VIR and BT disease have declined significantly since the adoption of the 2nd varicella dose regimen. Increases in clinically milder cases may underestimate actual varicella cases. Two dose vaccination coverage needs to be addressed to understand the impact of the recommended 2-dose regimen.