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BACKGROUND: In the past three years, Texas has not seen a varicella outbreak of greater than 10 cases. The Texas Department of Health Services, Health Service Region 8 epidemiologist investigated a varicella outbreak of 21 cases, which included three schools and a daycare center in one small Texas community from December 2014 to March 2015.
METHODS: Cases were identified by school nurses, a day care center, and a local provider. The case criterion includes an illness with acute onset of generalized maculopapulovesicular rash without other apparent cause.
RESULTS: Ages ranged from 6 months to 13 years. In the community outbreak, 38.1% (8/21) received no varicella vaccine doses, 23.8% (5/21) received one varicella vaccine dose, and 38.0% (8/21) received two varicella vaccine doses. In those that did not receive any varicella vaccine, five were under age for vaccination and three were exempt. In those with one varicella vaccine, three were under age to receive the 2nd dose, and two were in the 6th grade. The 6th grade class in Texas is on a catch-up schedule and is not required to have two doses of varicella for school entry. In this 6th grade class, 1 child was exempt from vaccination, 29 children had one varicella dose, and 59 children had two varicella doses. The attack rate of varicella in zero dose children was 100% (1/1), the attack rate in one dose children was 6.9% (2/29), and the attack rate in two dose children was 6.8% (4/59). There is no statistical difference between the attack rates of one dose children and two dose children (p > 0.99). The index 6th grader was exempt from vaccination and had a severe case of chickenpox. This case infected five classmates. One additional 6th grade case was infected by a two dose vaccinated person. The index case occurred in an unvaccinated infant whom was exposed to varicella from a parent with shingles.
CONCLUSIONS: Two doses of varicella vaccine do not prevent against breakthrough varicella; however, two doses of vaccine do decrease the severity of symptoms. The unvaccinated 6th grader was more infectious than those with one or two doses. However, varicella can still be transmitted from a person with two varicella vaccine doses.