Vaccine Efficacy Estimates During Institutional-Based Outbreaks, Indiana 2010

Background: With the licensure of the varicella vaccine in 1995, there was a substantial decrease in disease incidence and morbidity.  However, the decrease in incidence plateaued in 2004 and outbreaks continued to occur despite wide-spread coverage of the vaccine; leading to the 2007 ACIP recommendation for a vaccine  booster dose. 

Objectives:The purpose of this study is to analyze the impact of the 2-dose series on institutional and school-based outbreaks. 

Methods: From December 2009 through December 2010, 11 outbreaks were reported to ISDH (N > 147 cases).   All outbreaks were reported through passive surveillance, three outbreaks were laboratory confirmed. 

Results:  The median age of cases was 10.0 years (range: 1-37 years). Thirty-one percent of cases had no record of a valid vaccine dose prior to outbreak onset.  Sixty-four percent had received one valid dose of varicella vaccine and 5% of cases had received a 2-dose series of vaccine.  Seven cases had a previously reported history of chickenpox.  As expected, disease was more severe in cases with no vaccine dose than cases with 1 dose (p<.001) but there was no difference in disease severity between cases with 1 or 2 doses of vaccine (p=.13). Eighty percent of cases had mild or moderate disease (> 249 pox lesions).  The average size of an outbreak was 13.2 cases; (range: 5 to 36 cases).  Varicella vaccination rates were available for 36% (4/11) of outbreaks (65 cases).  Overall vaccination coverage was 77% (range per institution: 60.6% to 99.1%).  Vaccine efficacy for 5 outbreaks was estimated to be 64.3%; 90% efficacy with the 2-dose series.  Attack rates were much lower in individuals with at least 1 vaccine dose (1.7%) than individuals with no doses (5%) of vaccine.  

Conclusions: Further efforts to increase vaccination with the 2-dose series are indicated.