Most children with a negative or unknown varicella history are immune


NEW YORK (Reuters Health) – Contrary to widely held beliefs, most 10-year old children with negative or unknown chickenpox histories are actually immune to varicella, according to a report by Canadian investigators.

Dr. Bernard Duval, from Laval University in Quebec, and colleagues assessed the age-specific incidence of varicella among 2227 fourth grade students. A subset of children with negative or unknown chickenpox histories were tested for anti-varicella antibodies.

The study was performed to determine the proportion of children that would need to be vaccinated in a catch-up program, the researchers state in the November issue of The Pediatric Infectious Disease Journal.

The reported cumulative incidence of chickenpox at 10 years of age was 92%, the authors note. Furthermore, about half of the children developed chickenpox before entering kindergarten.

Of the children with negative or unknown varicella histories, 63% had antibodies against the virus. Children with an unknown history were significantly more likely than those with a negative history to harbor anti-varicella antibodies (p = 0.002). In addition, children whose history was obtained by self-administered questionnaire rather than by a study nurse were more likely to demonstrate such antibodies (p = 0.023).

If vaccination was based on the absence of a positive history of varicella, 8.4% of 10-year-old children would require vaccination, the researchers note. However, the current findings indicate that nearly two thirds of children without a positive history are actually immune.

Prevaccination testing could identify children who are immune, but such testing could be difficult to implement and might reduce vaccine coverage. Follow-up telephone interviews with parents who report negative or unknown histories for their children may help identify children who are actually immune.

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