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No chickenpox by age 10 may mean child is immune

NEW YORK, Dec 17 (Reuters Health) - If a child is not known to have chickenpox by age 10, he or she is likely to be immune to the virus, according to Canadian investigators.

Their study of more than 2,200 fourth-graders showed that 92% had had chickenpox based on parents' reports. More than half had developed the illness before kindergarten.

And among those without a definite history of chickenpox, 63% had antibodies against varicella--the virus that causes chickenpox.

Children whose parents did not know if they ever had chickenpox were more likely to have these antibodies compared with children who were said to never have had the illness.

Dr. Bernard Duval, from Laval University in Quebec, and colleagues reported the findings in a recent issue of the Pediatric Infectious Disease Journal.

The investigators conducted the study to look into "catch-up" vaccinations against chickenpox. This refers to vaccines given to older children who were not vaccinated at a young age and have not yet had chickenpox. Usually, a person becomes immune to chickenpox after one bout with the illness. It is commonly assumed that children without a known history of chickenpox lack any immunity. But according to Duval's team, these finding do not support that perception.

The investigators found that if catch-up vaccines were based on parents' reports of a negative or unknown history of chickenpox, about 8% of the 10-year-olds would require vaccination. But based on the varicella antibody tests, two thirds of these children would be vaccinated "needlessly," the report indicates.

Prevaccination testing of such children, they note, could identify those who are immune. However, the researchers acknowledge, such testing could be difficult to implement and could end up reducing the number of children who get vaccinated.

Alternatively, Duval's team suggests, when parents report that their child has not had chickenpox or that they don't know, a follow-up telephone call may help confirm the report. In the study, children without a known history of chickenpox were less likely to show antibodies against the virus when parents were questioned by a nurse, as opposed to filling out a questionnaire.

SOURCE: The Pediatric Infectious Disease Journal 2001;20:1087-1088.

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