BACKGROUND:
Since U.S. licensure, studies have shown the varicella vaccine's overall effectiveness to range from 44-100%, with substantial protection against moderate and severe varicella; however, breakthrough disease has been documented in up to 56% of vaccinated individuals.

OBJECTIVE:
To examine varicella vaccine effectiveness and the impact of breakthrough disease during an outbreak of varicella among elementary school students.

METHOD:
A varicella outbreak occurred in a Minnesota school with 319 students. Phone surveys were conducted with students' parents. Information was collected on students with recent varicella infections including onset date, rash characteristics, duration, and underlying medical conditions.

RESULT:
Fifty-four cases occurred following a primary, breakthrough case. Twenty-nine (53%) cases were vaccinated. Unvaccinated students had an increased risk of moderate varicella compared to vaccinated students (RR, 4.4; 95% CI, 2.2 to 9.1; p<0.001). The vaccine was 56% effective at preventing any varicella and 90% effective against moderate illness. Students vaccinated five or more years prior to the outbreak had a greater risk of breakthrough varicella compared to those vaccinated within four years (RR, 2.6; 95% CI, 1.3 to 5.4; p<0.01).

CONCLUSION:
Vaccinated students presented with milder varicella symptoms than unvaccinated students. Individuals with breakthrough disease can be highly infectious. Time since varicella vaccination was associated with illness. Despite 29 breakthrough cases, the varicella vaccine conferred a high degree of protection against moderate illness.

LEARNING OBJECTIVES:
Be able to identify the risk factors that are associated with varicella breakthrough disease