Children vaccinated against chickenpox who then contract chickenpox are half as contagious as those who have not been vaccinated

A study published in the current issue of the Journal of the American Medical Association shows that children and young adults who have had the chickenpox vaccine and gone on to contract chickenpox are half as contagious as those who have not been vaccinated.

Chickenpox (varicella) is a is a highly infectious disease caused by the varicella zoster virus and affects most children in the United States before their 10th birthday. Until recently it could not be prevented, only treated. Today, parents can have their children immunized against chickenpox. Chickenpox is one of the most common child-hood diseases. It is usually mild and not life-threatening to healthy children.

Before the vaccine became available, there were about 4 million cases of chickenpox in the United States each year. Anyone can get chickenpox at any age, but it occurs most frequently in children from ages 6 to 10.

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 proportion 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.

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