Context Limited data are available on the contagiousness of vaccinated varicella
Objectives To describe secondary attack rates within households according to disease
history and vaccination status of the primary case and household contacts
and to estimate varicella vaccine effectiveness.
Design, Setting, and Patients Population-based, active varicella surveillance project in a community
of approximately 320 000 in Los Angeles County, California, during 1997
and 2001. Varicella cases were reported by child care centers, private and
public schools, and health care clinicians and were investigated to collect
demographic, clinical, medical, and vaccination data. Information on household
contacts' age, varicella history, and vaccination status was collected.
Main Outcome Measures Varicella secondary attack rate among household contacts; vaccine effectiveness
using secondary attack rates in unvaccinated and vaccinated children and adolescents.
Results A total of 6316 varicella cases were reported. Among children and adolescents
aged 1 to 14 years, secondary attack rates varied according to age and by
disease and vaccination status of the primary case and exposed household contacts.
Among contacts aged 1 to 14 years exposed to unvaccinated cases, the secondary
attack rate was 71.5% if they were unvaccinated and 15.1% if they were vaccinated
(risk ratio [RR], 0.21; 95% confidence interval [CI], 0.15-0.30). Overall,
vaccinated cases were half as contagious as unvaccinated cases. However, vaccinated
cases with 50 lesions or more were similarly contagious as unvaccinated cases
whereas those with fewer than 50 lesions were only one third as contagious
(secondary attack rate, 23.4%; RR, 0.32 [95% CI, 0.19-0.53]). Vaccine effectiveness
for prevention of all disease was 78.9% (95% CI, 69.7%-85.3%); moderate disease,
92% (50-500 lesions) and 100% (clinician visit); and severe disease, 100%.
Conclusions Under conditions of intense exposure, varicella vaccine was highly effective
in preventing moderate and severe disease and about 80% effective in preventing
all disease. Breakthrough varicella cases in household settings were half
as contagious as unvaccinated persons with varicella, although contagiousness
varied with numbers of lesions.