Background: In June 2006, the Advisory Committee on Immunization Practices (ACIP) expanded its June 2005 recommendation for a second dose of varicella vaccine during outbreaks to a recommendation for routine school entry second dose varicella vaccination. In October 2006, the Arkansas Department of Health was notified of a varicella outbreak among students where some received a second dose during an outbreak-related vaccination campaign in February 2006.
Methods: The outbreak was investigated using a school-wide parental survey with a follow-up survey of identified case patients. Vaccination status was verified using state and local immunization records. Limited laboratory testing confirmed circulation of wild-type varicella, including varicella in 2-dose vaccine recipients.
Results: Vaccination information was available for 871 (99%) of the 880 children. Varicella vaccination coverage was 97% (2-dose, 39%; 1-dose, 58%). A review of the February vaccination clinic found no deficiencies; lot numbers did not differ between cases and noncases. Varicella was confirmed by PCR in 5 (42%) of 12 lesion specimens and by IgM in 1 (6%) of 16 serum specimens. Varicella was reported in 84 children, including 25 (30%) two-dose and 53 (63%) one-dose recipients. Attack rates among 2-dose recipients (10.4%) and 1-dose recipients (14.6%) were not significantly different (RR: 0.72, 95% CI: 0.44-1.15). All 2-dose recipients and 80% of 1-dose recipients reported having 50 or fewer skin lesions.
Conclusion: This outbreak is the first to document varicella in both 1- and 2-dose vaccine recipients; both groups had mild disease. The vaccine effectiveness of 1 and 2 doses were similar.