Two-dose varicella vaccine effectiveness and rash severity in outbreaks of varicella among public school students.

Varicella (chickenpox) is a highly contagious disease caused by the varicella zoster virus (VZV), which, in the absence of vaccination, is a universal infection acquired mainly in childhood. Prior to introduction of varicella vaccine in 1995, an estimated 4 million varicella cases, 11,000 hospitalizations and 100 deaths occurred each year in the United States.1 Since introduction of 1-dose varicella vaccination in the United States, dramatic declines were documented in the number of varicella cases,2 outbreaks,3 varicella-related hospitalizations and deaths.4,5 However, albeit smaller in size and shorter in duration, varicella outbreaks continued to occur in school and daycare settings with high 1-dose varicella vaccination coverage6-9; therefore, a routine 2-dose varicella vaccination was recommended in 2006 for school-aged children and a catch-up vaccination for children of all ages who lacked evidence of immunity.10 The 2-dose program appears to have been rapidly implemented in the United States in the first 5 years since it was recommended. For the 2012–2013 school-year, median 2-dose varicella vaccination coverage among the 36 states and District of Columbia requiring and reporting 2 doses was 93.8% (range: 84.6% in Colorado to ≥99.9% in Mississippi); 14 reported coverage ≥95%.11,12 There are limited data on the effectiveness of the 2-dose varicella vaccine regimen and very limited numbers of 2-dose breakthrough cases reported, thus more data are needed to describe 2-dose vaccine effectiveness (VE) and rash presentations of 2-dose breakthrough varicella. Therefore, a case control study was conducted in West Virginia public schools during 2010 and 2011 to evaluate 1-dose, 2-dose and incremental varicella VE and characterize 2-dose breakthrough varicella.