ART4 and chickenpox: In addition to nonconverter (primary vaccine failure), mild breakthrough infections occurring among seroconverted vaccinees (commonly defined as those who had >5 glycoprotein enzyme-linked immunosorbent assay units/mL) may be a result of partial or “leaky” protection.4,21 Thus, second-dose varicella vaccination is important in completing the incomplete immune response following first-dose vaccination and maximizing the impact of varicella vaccination.