Learning Objectives for this Presentation:
By the end of the presentation participants will be able to know the varicella vaccine effectiveness and possible cause/s of vaccine failure in this outbreak.

Background:
With a varicella vaccine coverage rate of 50% among elementary students, an outbreak occurred with more than half of the cases occurring among vaccinated students.


Objectives:
To identify the factor/s responsible for vaccine failure in this outbreak.

Methods:
During May 2004, the CCDPH vaccine-preventable diseases unit conducted an investigation of a varicella outbreak in 2 adjacent elementary schools in suburban Cook County, Illinois. We reviewed the school health record of each student enrolled during the outbreak period to determine demographic information, vaccination history, and history of varicella infection prior to the outbreak. Telephone interviews with case parents/guardian was performed to verify rash onset and severity of illness.

Results:
From March 6, 2004 through May 17, 2004, a total of 22 cases of varicella occurred among 623 students attending two elementary schools in suburban Cook County. Overall varicella vaccine coverage was 52% with vaccine effectiveness calculated greater than 96% against all forms of varicella. As noted in previous outbreaks students who were vaccinated at <15 months of age were at greater risk for vaccine failure however this association did not achieve statistical significance in this population (OR = 2.5, p = 0.11).

Conclusions:
This study shows the varicella vaccine to be highly effective in preventing illness and reducing the severity of breakthrough infections. Illinois ranks 42nd in the nation in varicella vaccine coverage (2004 Illinois vaccination rate is 57% compared to the national average of 76%), and this poor coverage was evident in our study population (67% vaccination rate). The resulting susceptible population combined with the reduced vaccine effectiveness in children vaccinated at <15 months created the opportunity for this varicella outbreak despite the available and highly effective vaccine.