Varicella Disease Incidence and Clinical Presentation Post – Introduction of Two Dose Varicella Vaccine in An Active Surveillance Project, Los Angeles County Department of Public Health, Antelope Valley (AV), California

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.

Objectives:

• 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 Chi Square test was used to assess statistical assistance.  .

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 2^nd 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.