Clinical Survey of Natural Varicella Compared With Breakthrough Varicella After Immunization With Live Attenuated Oka/Merck Varicella Vaccine

Abstract

To prospectively characterize varicella occurring in children previously immunized with a live attenuated varicella vaccine (breakthrough varicella) through daily observation by medical personnel and to compare it with natural varicella followed in the same manner. A blinded clinical survey. Four pediatric practices (two private; two hospital-based). Healthy 12-month-old through 17-year-old children with chickenpox were studied; 92 had natural varicella and 58 had breakthrough varicella. SELECTION PROCEDURES AND INTERVENTIONS: Study personnel, unaware of vaccination status, documented the clinical characteristics of each patient in the office or at the patient's home each day from enrollment until the day after the total number of lesions increased less than 10%. A standard form documenting number and description of lesions, temperature, duration of illness, and associated clinical complaints was completed each day by the same study personnel. Acute and convalescent sera were obtained on breakthrough cases. Antibody to varicella-zoster virus was measured by the glycoprotein-based enzyme-linked immunosorbent assay. Of those with sera available, 85% were serologically confirmed. Eighty-seven percent of enrollees had a known exposure to chickenpox, with at least two thirds of each group having a greater than 4-hour or a household exposure. The numbers of total and vesicular lesions were significantly higher in the natural varicella group, regardless of exposure status (P = .021 to < .001). The group with breakthrough varicella had a significantly lower incidence of fever (P < .001) and a significantly shorter duration of illness (P < .001). Other associated constitutional complaints and complications were not significantly different between groups. Varicella in vaccine recipients is clinically modified and significantly less severe than natural disease.