Impact of the US Two-dose Varicella Vaccination Program on the Epidemiology of Varicella Outbreaks Data from Nine States, 2005-2012

Background: A routine 2-dose varicella vaccination program was adopted in 2007 in the US to help further decrease varicella disease and prevent varicella outbreaks. We describe trends and characteristics of varicella outbreaks reported to the Centers for Disease Control and Prevention (CDC) during 2005-2012 from 9 states. Methods: Data on varicella outbreaks collected by 9 state health departments were submitted to CDC using the CDC outbreak reporting worksheet. Information was collected on dates of the outbreak, outbreak setting and number of cases by outbreak; aggregate data were provided on the numbers of outbreak-related cases by age group, vaccination status and laboratory confirmation. Results: Nine hundred and twenty-nine outbreaks were reported from the 6 states, which provided data for each year during 2005-2012. Based on data from these 6 states, the number of outbreaks declined by 78%, decreasing from 147 in 2005 to 33 outbreaks in 2012 (P = 0.0001). There were a total of 1015 varicella outbreaks involving 13,595 cases reported by the 9 states from 2005 to 2012. The size and duration of outbreaks declined significantly over time (P < 0.001). The median size of outbreaks was 12, 9 and 7 cases and median duration of outbreaks was 38, 35 and 26 days during 2005-2006, 2007-2009 and 2010-2012, respectively. Majority of outbreaks (95%) were reported from schools, declining from 97% in 2005-2006 to 89% in 2010-2012. Sixty-five percent of outbreak-related cases occurred among 5-year to 9-year olds, with the proportion declining from 76% in 2005-2006 to 45% during 2010-2012. Conclusions: The routine 2-dose varicella vaccination program appears to have significantly reduced the number, size and duration of varicella outbreaks in the US.