Clinical Question: How contagious is breakthrough varicella in vaccinated persons?

Setting: Population-based

Study Design: Cohort (prospective)

Synopsis: Compared with symptoms in unvaccinated patients, symptoms of breakthrough varicella in vaccinated patients usually are milder. Little is known, however, about the contagiousness of vaccinated varicella. From 1997 through 2001, child care centers, private and public schools, and health care physicians reported all cases of varicella to the health department of Los Angeles County, Calif. Using medical records and parental recall, the investigators collected follow-up information on the age, varicella history, and vaccination status of each household contact. Data were verified from written records in more than 80 percent of the cases.

In household contacts one to 14 years of age with no history of varicella who were exposed to unvaccinated cases, the secondary attack rate was 71.5 percent if they were unvaccinated and 15.1 percent if vaccinated. The secondary attack rate in similar household contacts exposed to vaccinated patients was 37.1 and 22.3 percent, respectively. However, vaccinated patients with 50 lesions or more were as contagious as unvaccinated patients, while those with fewer than 50 lesions were one third as contagious. Vaccination prevented 79 percent of all disease, 92 percent of moderate disease (50 to 500 lesions), 100 percent of physician visits for a complication, and 100 percent of severe disease.

Bottom Line: Patients who acquire varicella with 50 lesions or more despite vaccination are as contagious as unvaccinated patients with varicella. Patients with fewer than 50 lesions are one third as contagious. (Level of Evidence: 1b)