Objective. To determine whether varicella can be prevented by administration of oral acyclovir (ACV) during the incubation period of the disease.

Subjects and methods. ACV (40 or 80 mg/kg daily in four divided doses) was given orally to 25 exposed infants and children for 7 days, starting 7 to 9 days after exposure from the index case in their families. Their clinical features were compared with those of 25 age-matched control subjects who had been exposed in their families but did not receive ACV. A fluorescent antibody to membrane antigen assay was used for determination of the antibody to varicella-zoster virus, and a nested polymerase chain reaction method was used for detection of viremia.

Results. Among the 25 who received ACV, 4 (16%) developed the disease and 1 (4%) had a fever. On the other hand, all of 25 control subjects developed the disease and 17 (68%) had a fever. The incidence of fever and the severity of skin rashes were significantly lower (P < .01) in the subjects who received oral ACV than in the control group. Seroconversion was observed in 84% of subjects who received ACV In some cases, varicella-zoster virus DNA was detected by polymerase chain reaction amplification in peripheral blood mononuclear cells from blood drawn approximately 14 days after exposure.

Conclusions. Varicella can be prevented or modified by administration of oral ACV late in the incubation period.

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