CHICAGO -- Children and adolescents who have received the varicella (chickenpox) vaccine and contract varicella are about half as contagious as those who have not been vaccinated, according to a study in the August 10 issue of JAMA, the Journal of the American Medical Association.
According to background information in the article, varicella is a highly infectious disease caused by the varicella zoster virus, which is spread by droplet or airborne transmission. Before the varicella vaccination program in the United States, approximately 4 million varicella cases occurred each year, resulting in 10,600 hospitalizations and 100 deaths. The majority of cases occurred in children, which reflects the highly contagious nature of the disease. Limited data are available on the contagiousness of vaccinated varicella cases.
Jane F. Seward, M.B., B.S., M.P.H., of the Centers for Disease Control and Prevention, Atlanta, and colleagues examined varicella transmission within households according to the varicella history and vaccination status of both the primary case and exposed household member(s) and estimated vaccine effectiveness from household secondary attack rates. The study consisted of a population-based, active varicella surveillance project in a community of approximately 320,000 in Los Angeles County 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.
During the study period, 6,316 cases of varicella were reported. The researchers found that 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 percent if they were unvaccinated and 15.1 percent if they were vaccinated. 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."
"As cases and the severe health burden from varicella further decline, monitoring age, severity, and vaccination status of the remaining varicella cases will become increasingly important. It is expected that with continuing increases in vaccine coverage, a higher propotion of cases will occur in vaccinated children. Further understanding of the severity and contagiousness of these cases in the community as well as in household settings will assist with future vaccine policy decisions," the authors conclude. (JAMA. 2004;292:704-708. Available post-embargo at jama.com)
Editor's Note: From 1997 through 2001, CDC funded the Los Angeles Department of Health Services under a co-operative agreement for Active Varicella Surveillance and Epidemiological Studies.
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