Almost nine out of 10 eligible children were vaccinated against chickenpox in 2004. The chickenpox, or varicella, vaccine is recommended for all children aged 12 to 18 months and for older children and adults who have not had the disease.

The newly released report is one of the first to evaluate the impact of the vaccination program in medical terms. CDC researchers reported that hospitalizations due to chickenpox declined by a whopping 88% and chickenpox-related visits to physicians declined by 59% in the eight years following approval of the vaccine.

The findings are published in the Aug. 17 issue of The Journal of the American Medical Association.

"We were pleasantly surprised that the program seemed to impact all age groups," says Abigail Shefer, MD, of the CDC's National Immunization Program. "We have seen declines (in chickenpox rates) among infants who are too young to get the vaccine, as well as adolescents and adults."

Infants, Adults Protected, Too

There had been some concern that vaccinating all young children against the varicella virus would leave nonimmune adults and infants vulnerable to the infection. By vaccinating children against chickenpox, the infection could produce disease in adults.

Chickenpox is usually a benign infection but can cause severe illness. Chickenpox is much more likely to cause severe disease in adults than in children. They are 10 times more likely than children to be hospitalized for the severe consequences of chickenpox infection such as lung infections and inflammation of the brain.

Prior to the vaccine program there were about 100 chickenpox-related deaths in the U.S. each year, with most fatalities occurring among infants and adults.

The significant reduction in chickenpox-related hospitalizations among infants was particularly noteworthy, the researchers wrote, because it suggests a reduction in the virus at a population level. The vaccine is not given to children younger than age 1.

"There was some worry that when we decreased circulating virus among children we might see more disease in adults and infants, but that hasn't happened," Shefer tells WebMD.

Crunching the Numbers

According to earlier estimates from the CDC, the annual cost of vaccinating all children in the U.S. against chickenpox is approximately $144 million. The new report suggests that the vaccination program saves $63 million annually in direct medical costs. But medical costs represent only a small percentage of the total savings, experts say.

Government researchers are conducting a more comprehensive cost-benefit analysis of the chickenpox vaccination program. But pediatric public health researcher Matthew M. Davis, MD, says such an analysis may be impossible because there is no good way to measure crucial cost variables such as lost work time by parents caring for children with the disease.

In an editorial accompanying the study, Davis, an assistant professor of pediatrics and communicable diseases at the University of Michigan, writes that the CDC analysis cannot "conclusively confirm that childhood varicella vaccination is as cost-effective as originally anticipated."

"In reality, this vaccine is probably very cost-effective," he tells WebMD. "But we don't have enough information today to say that it is less, as, or more cost-effective than was projected 10 years ago."

In the past, Davis says, there has been a reluctance to consider cost as a major factor in making public health decisions about immunization against disease. But he adds that this could change as economic considerations increasingly drive health care choices.

"We need to have a better way to measure things like work time missed by parents so that we can better track the cost-effectiveness [of vaccine programs]," he says. "We need this information because when the rubber hits the road in terms of health care costs we may care very much about whether a vaccine is cost-effective or not in the future. We need better ways to evaluate this."

SOURCES: Zhou, F. The Journal of the American Medical Association, Aug. 17, 2005; vol 294: pp 797-802. Abigail Shefer, MD, medical researcher, National Immunization Program, CDC. Matthew M. Davis, MD, MAPP, assistant professor of pediatrics and communicable diseases, University of Michigan, Ann Arbor.