Changing Epidemiology of Varicella Outbreaks in the United States During the Varicella Vaccination Program, 1995-2019

We describe the changing epidemiology of varicella outbreaks informed by past and current active and passive surveillance in the United States by reviewing data published during 1995-2015 and analyzing new data from 2016 to 2019. Varicella outbreaks were defined as >= 5 varicella cases within 1 setting and >= 1 incubation period. During the 1-dose varicella vaccination program (1995-2006), the number of varicella outbreaks declined by 80% (2003-2006 vs 1995-1998) in 1 active surveillance area where vaccination coverage reached 90.5% in 2006. During the 2-dose program, in 7 states with consistent reporting to the Centers for Disease Control and Prevention, the number of outbreaks declined by 82% (2016-2019 vs 2005-2006). Over the entire program (1995-2019), outbreak size and duration declined from a median of 15 cases/outbreak and 45 days duration to 7 cases and 30 days duration. The proportion of outbreaks with <10 cases increased from 28% to 73%. During 2016-2019, most (79%) outbreak cases occurred among unvaccinated or partially vaccinated persons eligible for second-dose vaccination, highlighting the potential for further varicella control.

Automated summary

This article describes the changing epidemiology of varicella outbreaks in the United States, using data from 1995 to 2019. The introduction of one-dose and two-dose varicella vaccination programs significantly reduced the number, size, and duration of outbreaks. However, there are still cases of varicella outbreaks among unvaccinated or partially vaccinated individuals, highlighting the need for further control measures.