Dec. 11, 2002 -- A chickenpox outbreak at a New Hampshire day-care center raises troubling questions about the effectiveness of the vaccine now given to prevent the disease.
A CDC investigation revealed that the source of the December 2000 outbreak was a four-year-old boy who had been vaccinated against chickenpox three years earlier. The boy infected more than half of his day-care classmates; 17 of the 25 children who became ill had also been vaccinated. The report is published in the Dec. 12 issue of The New England Journal of Medicine.
CDC investigators concluded that in this case vaccination provided poor protection against infection by the varicella virus, which causes chickenpox. But it did protect the vaccinated children who became ill from developing severe symptoms. Symptoms were characterized as mild in 15 of the 17 vaccinated children, while six of the eight unvaccinated children had moderate to severe illness.
"This particular vaccine is extremely good at protecting children and adults from dying of chickenpox or from developing severe disease," says lead investigator Karin Galil, MD, MPH. "But it did not do a very good job in this population of preventing infection."
First licensed for use in the United States in the mid-1990s, the varicella vaccine is now routinely given as a single dose to children who are 12 to 18 months old. The CDC estimates that roughly 75% of eligible children are now vaccinated against chickenpox, and studies conducted over the past few years suggest that the vaccine's effectiveness in protecting against the disease ranges from 71% to 100%.
But in the New Hampshire outbreak, effectiveness was just 44% -- far lower than has been seen in any previous investigation. Galil tells WebMD that it is unlikely the protection from the vaccine is really that low. But the investigation indicates the vaccine might be slightly less protective than has been believed.
"It is something we really need to keep an eye on and continue to study," she says. "But people should not lose sight of the fact that vaccination is a very effective strategy for protecting people against this illness."
Galil points out that before the vaccine was licensed there were 11,000 hospitalizations and 100 deaths caused by chickenpox every year. CDC surveillance studies in two communities found that chickenpox and chickenpox-related hospitalizations declined by 80% after routine vaccination began.
In an editorial accompanying the CDC report, varicella virus expert Anne A. Gershon, MD, of Columbia University, suggests that giving children two chickenpox immunizations instead of one might be warranted to increase protection. In the CDC investigation, children vaccinated three or more years before the start of the outbreak were found to be at twice the risk for chickenpox as those vaccinated within three years of the event, suggesting that immunity might have waned over time.
"A second dose of varicella vaccine, given routinely, should decrease the number of children who have primary vaccine failure and might also prevent waning immunity, if it does indeed currently occur," Gershon noted.
"It is noteworthy that it took the routine administration of two doses of measles vaccine to children to control measles in the United States. The time for exploring the possibility of routinely administering two doses of varicella vaccine to children seems to have arrived."