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Chickenpox Vaccine Less Effective Over Time

But Protection Overall Is Still High, Study Shows
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WebMD Health News

Feb. 17, 2004 -- The chickenpox vaccine becomes less protective over time, but it remains highly effective for preventing more serious forms of the virus, a new study shows.

Researchers followed a group of children who had and had not been immunized to assess the long-term effectiveness of the varicella virus, or chickenpox, vaccine. They found that while the vaccine remained highly protective during the eight-year observation period, protection was greatest for most children in the first year following immunization. The vaccine's overall effectiveness in preventing disease was 97% in that first year compared with 84% during subsequent years.

The vaccine is typically given to children between the ages of 12 and 18 months. The researchers also found protection to be compromised during the first year following vaccination with varicella in children immunized prior to age 15 months, but the difference in effectiveness disappeared over time.

The findings are reported in the Feb. 17 issue of the Journal of the American Medical Association.

No Need for Booster, Yet

Chickenpox is a highly contagious childhood illness, characterized by itchy blisters that cover the face, head, and trunk. Yet children in the study who got chickenpox despite vaccination tended to have very mild cases of the disease, with far fewer blisters than are typically seen and an illness duration that was shorter than the average 10 to 14 days.

For this reason, the researchers concluded that the overall effectiveness of the vaccine is good. In an interview with WebMD, researcher Marietta Vazquez, MD, of the Yale School of Medicine, said the evidence to date does not favor increasing the age at which immunizations are given or giving booster doses of the vaccine.

"Even though protection was not as great for children vaccinated earlier than age 15 months during that first year, they were just as protected as the older children a year out," she says. "I think that all we can say at this point is there does not seem to be compelling reason to change vaccination times. And there is not a strong reason to support a second dose of the vaccine."

"When everyone gets the vaccine and there is very little chickenpox out there, we don't know if that one dose that children receive at 12 months will last until they are in their 40s or 50s," she says. "That is why it is very important to continue to monitor the effectiveness of this vaccine."

But Maybe in the Future

Pediatric infectious disease specialist Cody Meissner, MD, agrees that there is currently no reason to change the dosing schedule for the varicella vaccine or to add a booster dose to help improve the effectiveness of the vaccine. Meissner, a member of the American Academy of Pediatrics' committee on infectious diseases, also says that the group is studying the one-dose policy which could change in the future. He says two doses might be better, even though kids who develop so-called "breakthrough" chickenpox following vaccination tend to have mild cases of the disease.

"There are no big surprises in this study," he tells WebMD. "But the additional data could lead us to think more about whether we might want to consider giving two doses of the chickenpox vaccine instead of one in the future. That booster dose might reduce the incidence of breakthrough disease to even lower levels."

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