"What is particularly interesting is that the rate of chickenpox is even decreasing in children who have not had the vaccine, presumably because there are fewer children with disease to catch it from," said lead author Dennis A. Clements, MD, PhD, an associate professor of pediatrics and pediatric infectious disease at Duke University in Durham, N.C. "If there are fewer kids transmitting the disease, there are fewer kids acquiring it."
At the 38th Interscience Conference on Antimicrobial Agents and Chemotherapy here, Clements noted that as the incidence of chickenpox decreases, "there will be less pressure for those that are unvaccinated, but who haven't had the disease, to receive the vaccine because they probably aren't going to be exposed to it anymore." As a result, there will first be a group of teenagers and then a group of adults who haven't had the vaccine or the disease. "We have to come to grips with that," said Clements, who predicts that the vaccine will eventually become mandatory. Otherwise, "these people will still be susceptible when they are older."
More than 50% of the approximately 3,000 children were younger than 4 years old. During the reported period, vaccine use increased from 4.28% to 43.32% in susceptible children. "It was slow going when the vaccine first became available [March 1995], but then it gradually picked up," said Clements. The greatest increase was in the 12-24 month age group. "Today, about 60% of 1-year-olds are receiving the vaccine," he said.
The cumulative varicella incidence decreased from 6% to 0% in 0-11 month-old children, 24% to 5% in 12-24 month old, 33% to 13% in 25-36 month old, and 33% to 28% in 37-48 month-old children, 24% to 5% in 12-24 month-olds, 33% to 13% in 25-36 month-olds and 33% to 28% in 37-48 month-olds.
"The vaccine works. It prevents disease," said Clements. "The vaccine appears to prevent about 85% of all varicella disease and 100% of significant disease."
The children attending the 11 day cares in the study were primarily middle and upper-middle class. "But we've given the vaccine to hundreds and hundreds of kids from all socioeconomic levels. It has proved to be very effective," he said. Furthermore, "in some ways, day care truly tests the vaccine because it crowds children together."
However, because the vaccine is not required, "there will be a group of people that will escape without the disease or vaccine. In 20 years, we need to be worried about that group," said Clements. "The only question is instead of waiting, should we do something about it now."
Clements noted that currently there are a handful of states who require varicella immunization to attend school, including Massachusetts. "All states should adopt a similar policy or they will have a bigger problem later," he said.
For your information:
- Clements DA, Walter EB, Bland C, et al. Epidemiology of varicella in day care after licensure of varicella vaccine. Presented at the 38th Interscience Conference on Antimicrobial Agents and Chemotherapy. Sept. 24-27. San Diego.