TUESDAY, Feb. 17, 2004 (HealthDayNews) -- The chicken pox vaccine still provides protection after eight years, although immunity to the disease is strongest during the first year, new research shows.
A study in the Feb. 18 issue of the Journal of the American Medical Association, by Yale University researchers, finds the chicken pox (varicella) vaccine was 97 percent effective during the first year after immunization. Overall, the vaccine was 87 percent effective. And in those cases in which the vaccine didn't provide complete immunity, the resulting chicken pox infections generally were mild and without complications.
"These data should be very reassuring for parents," says study co-author Dr. Marietta Vázquez, an associate research scientist in pediatric infectious disease at the Yale School of Medicine. "[The chicken pox vaccine] is safe, it works and so far it has a long effectiveness."
Vázquez recommends that researchers continue to study the long-term effectiveness of the varicella vaccine to determine if booster shots are needed by the time children become teenagers, or as adults.
The varicella vaccine was approved for use in 1995, and medical experts recommend that all healthy children be immunized between 12 and 18 months of age. Although chicken pox often isn't a serious disease, it can cause complications, such as pneumonia or even death.
Vázquez and her colleagues compared 339 children who had chicken pox with almost 700 children of the same ages who did not develop the disease. The children were selected from 20 different pediatric practices across Connecticut. Most of the children were white, and the group was about evenly divided between boys and girls. The average age of the children was 6, and most had been vaccinated when they were older than 15 months, the study reports.
Almost 125 of the children who had chicken pox had been vaccinated for the disease, whereas 470 of those without the disease had been vaccinated.
Effectiveness of the vaccine ranged from 97 percent immunity during the first year, to 86 percent effectiveness the second year and 81 percent by seven to eight years after vaccination, the study says.
Children who received the varicella vaccine before they were 15 months old had lower first-year immunity than children who were vaccinated after 15 months: 73 percent vs. 99 percent, respectively. However, after that time period, Vázquez says, the vaccine's decreased effectiveness for the younger group disappeared.
"Because this difference evens out, there doesn't appear to be enough data to suggest changing vaccination schedules," she says.
Dr. Dawn Sokol, a pediatric infectious disease specialist at Ochsner Clinic Foundation Hospital in New Orleans, says she agrees there is not enough data on the effectiveness of the vaccine in the younger group because only 35 children in the study who developed chicken pox had been vaccinated when they were less than 15 months old.
But Sokol says the study confirms what most doctors had suspected: "This vaccine works, but it wanes with time," she says.
The question for researchers now, she says, is whether the initial vaccination should be delayed until a child is older, or whether a booster shot should be given.
Both Vázquez and Sokol say the encouraging news in the study is that even when a vaccinated child develops chickenpox, it's usually a mild case with few complications.
More information
To learn more about the chicken pox vaccine, go to the American Academy of Pediatrics or to the U.S. Centers for Disease Control and Prevention.
