After incidental exposure to natural varicella, up to 18% of vaccinees reported a breakthrough infection known as modified varicella-like syndrome (MVLS) over up to 10 years of postvaccination follow-up, compared with natural varicella occurring in similarly aged unvaccinated children at the rate of 9% per year. Children with MVLS are frequently asymptomatic, and their disease is characterized by having fewer lesions, less fever, and lasting fewer days than natural varicella. When a case of MVLS occurs there are few secondary cases, suggesting that it is infrequently transmitted. Sequelae such as secondary bacterial infection, cerebellar ataxia, encephalitis, and pneumonia occur infrequently.