Varicella outbreak epidemiology in an active surveillance site, 1995-2005.

We describe trends and characteristics of varicella outbreaks identified in an active surveillance site from 1995 to 2005. Cases of varicella were reported to the active surveillance project, and outbreaks were defined retrospectively as > or =5 varicella cases epidemiologically linked to a common setting that occurred within 1 incubation period. Outbreaks were grouped by calendar year. From 1995-1998 to 2002-2005, varicella outbreaks significantly decreased in number, from 236 to 46 (P< .001); in size, from a median of 15 cases/outbreak to 9 cases/outbreak (P< .001); and in duration, from 44.5 days to 30 days (P< .001). The median age of case patients with outbreak-related varicella increased from 6 to 9 years (P< .001). The 1-dose varicella vaccination program has been successful in decreasing the number of outbreaks and cases; however, challenges remain with regard to controlling outbreaks among vaccinated persons and targeting vaccination efforts to susceptible persons in older age groups.