TUESDAY, Aug. 10, 2004 (HealthDayNews) -- Like all vaccines, the one for chicken pox isn't foolproof, but a new study finds that when vaccinated children still get the disease, they are only half as contagious as unvaccinated kids.
Their cases are also typically much less severe, said study author Dr. Jean F. Seward, of the U.S. Centers for Disease Control and Prevention.
"In the average case, there are 200 to 400 lesions," she said, referring to the blister-like rash that occurs with the disease. "A bad case is up to 1,000 lesions. These 'breakthrough' cases get typically less than 50 lesions, but quite commonly 20 or less."
The study appears in the Aug. 11 issue of the Journal of the American Medical Association.
Seward's team tracked more than 6,000 cases of chicken pox among children aged 1 to 14 years. They called the household where chicken pox had occurred, asking about infection among other household members, the number of lesions, and the vaccination status of household members. They also received vaccination status information from doctors, schools, or immunization records, besides asking the parents.
"A household contact study represents the most intense test of a vaccine performance," explained Seward, chief of the viral vaccine preventable diseases branch of the CDC's National Immunization Program. "The varicella [chicken pox] vaccine provides about an 80 percent protection from developing any kind of chicken pox, and is 92 to 100 percent effective in preventing moderate to severe disease."
The spread of the disease within a household varied by age, severity of the disease, and vaccination status of the children, she found. The so-called secondary attack rate was 71 percent if the children exposed to chicken pox by another household member were not vaccinated, but just 15 percent if they were inoculated.
Overall, the children who were vaccinated were half as contagious as those who didn't get the immunization. But those vaccinated children who came down with chicken pox and had 50 or more lesions were nearly as contagious as unvaccinated children, Seward found. Those with fewer than 50 lesions were only one-third as contagious as unvaccinated children.
Chicken pox, caused by the varicella zoster virus, is spread by droplet or airborne transmission. It is marked by fever and the development of the blister-like rash. Most children recover without further problems, but some get complications such as skin infections or pneumonia, said Seward, adding, "There's no way to predict in healthy children which may have a severe course."
The chicken pox vaccine was licensed in the United States in 1995 and began to be used routinely in 1996, Seward said. It is recommended at age 12 to 18 months and for older children who are susceptible, she said.
Before the vaccine was available, about 4 million children a year got chicken pox, with more than 10,000 hospitalizations and 100 deaths. Now, the number of cases has declined by about 85 percent, Seward said.
The study "shows at very intense exposure that the vaccine is pretty effective," said Dr. Robert Frenck, a member of the American Academy of Pediatrics' Committee on Infectious Diseases and a professor of pediatrics at the UCLA David Geffen School of Medicine. "It basically gave a very strict test to the vaccine."
Two strengths of the study, added Frenck, are the large number of people involved and the fact that they interviewed household contacts, an environment where transmission of the disease is very likely.
To learn more about the chicken pox vaccine, visit the U.S. Centers for Disease Control and Prevention.