Information has been received from the CDC concerning sixteen patients at one daycare who were each vaccinated with one dose of varicella virus vaccine live. Between 01 DEC 2000 and 01 FEB 2001 the patients experienced breakthrough varicella. The first reported case had a rash onset date of 01 DEC 2000 and the last reported date of rash onset was 11 JAN 2001. In the building, there were 78 full time students with 74 of the children greater than 12 months of age. Of these 74 children, 58 had no history of chicken pox, and 16 had a history of chicken pox. Of the 58 children with no history of chicken pox, 42 had previously been vaccinated with varicella virus vaccine live, one was vaccinated with varicella virus vaccine live during the outbreak, and 15 had no vaccination with varicella virus vaccine live. Of the 42 vaccinated children, 16 developed breakthrough varicella. Fourteen of the cases were mild as defined as less than 50 lesions, and two cases were moderate as defined as 51-250 lesions. The remaining 16 vaccinated children did not experience a breakthrough rash. Of the 15 unvaccinated children, 8 developed natural varicella (two mild cases, five moderate cases, and one severe cases defined as greater than 500 lesions, and seven had no rash (all of the seven children were offered serologic testing of which four agreed and they were all seronegative. No complications were reported during the outbreak. The median age of the cases was four years (range 1-8), 76% were male. No cases occurred among infants <12 months of age (n=4) and none occurred in a child who had a prior history of varicella disease. The 24 cases of varicella at the daycare were assigned WAES01010074. One patient at the daycare developed herpes zoster during the outbreak (WAES01072736). Of the 74 children in the building, 46 attended preschool or Pre-K continuously. Of the 46, 31 had no history of chicken pox, and 15 had a history of chicken pox. Of the 31 children with no history of chicken pox, 24 were vaccinated and 7 were not vaccinated. Of the 24 vaccinated, 12 developed breakthrough varicella and 12 did not have any rash. Of the seven unvaccinated children, six developed varicella and one was without rash. One patient in the group of students was the index case (WAES01022046) who developed a mild case of chicken pox following exposure to his sister who had herpes zoster. A susceptible parent of two breakthrough cases developed a severe case of varicella (greater than 500 hemorrhagic lesions) (WAES01072737). Her rash lesions were swabbed for PCR analysis and were found to be positive for varicella. In addition to exposure to her children, she spent approximately 10-15 minutes twice daily in the building of the primary outbreak during the outbreak period, and may had been exposed to other cases of varicella during the outbreak. Follow up information from a published article indicated that "ninety-five (100%) parent and 94 (99%) provider questionnaires were returned. The median age of the 76 (80%) children who attended was 4.2 years (range: 6 months-8 years); 74% were male. Vaccination coverage was 76%. Of 23 cases of varicella, 15 (65%) occurred in vaccinated children and 8 (35%) occurred in unvaccinated children. The index case was a four year old vaccinee who developed BT on 01 DEC 2000. The median age of cases was 4.4 years (range 1-7). VZV was cultured in one case and identified by PCR in another. Vaccine effectiveness was 40.5% (95% CI: -7.2. 67) overall and 41.7% in the building where the outbreak was centered. No further information is available.  Reported on 04/11/2003: "Follow-up information from a published article as titled above indicated that in this outbreak vaccination provided poor protection against varicella, although there was good protection against moderate to severe disease.  A longer interval since vaccination was associated with an increased rish of vaccine failure.  Breakthrough infections in vaccinated, healthy persons can be as infectious as varicella in unvaccinated persons.  Follow-up information from a journal editorial as titled above indicated that the varicella vaccine has proved to be extremely well tolerated with no serious safety issues emerging.  The "breakthrough" varicella may develop in 10 to 15 percent of the vaccinated population.  No further information is available.  A copy of the published article is attached as further documentation of the patient's experience.