Laboratory Diagnosis of Breakthrough Varicella in Children.

Breakthrough varicella (BV) develops in vaccinated persons as a result of infection by wild-type varicella zoster virus (VZV) more than 42 days after varicella vaccination. The clinical symptoms are atypical and clinical diagnosis can be difficult. We investigated laboratory-based diagnostic methods that are relatively simple and highly precise in order to conduct accurate surveillance. We enrolled 42 patients with suspected BV at two pediatric hospitals, and performed real-time polymerase chain reaction (PCR) on the skin lesions to confirm the BV diagnosis. We performed PCR on saliva and blood collected during the acute phase, as well as direct fluorescent antibody (DFA) imaging on lesions, and measured VZV immunoglobulin (Ig) G and IgM during the acute and convalescent phases. We confirmed the BV diagnosis in 31 of 42 enrolled patients. The sensitivity of DFA imaging of the lesion, and PCR of saliva and blood were 93.5%, 87.1%, and 61.3%, respectively. IgM was detected in 12.9% of patients during the acute phase and 65.5% during the convalescent phase. IgG increased more than four-fold in 86.2% of patients between the acute and convalescent phases. The sensitivity and specificity of the assay were 83.9% and 81.8%, respectively, when the diagnostic criteria for IgG were set to greater than 20 during the acute phase. The gold standard of laboratory-based diagnosis of BV has been PCR of samples taken from lesions. However, DFA of the lesion showed equivalent sensitivity when compared with PCR. PCR using saliva samples is an effective, non-invasive method of diagnosis. We found that high values of IgG during the acute phase can aid in the diagnosis of BV.