Editor,—Varicella, caused by primary infection with varicella zoster virus (VZV), is a common and highly contagious disease of childhood, and accounts for about one million cases per year in Japan. In 1994, the Welfare Ministry of Japan approved the use of oral aciclovir to treat varicella infections in otherwise healthy children. In spite of the recommendation that the treatment should not be usedroutinely for varicella in otherwise healthy children, the number of children treated with oral aciclovir has been increasing gradually in Japan.

It has rarely been reported that immunocompromised children with chronic VZV infection became resistant to aciclovir. However, no evidence that oral aciclovir treatment in otherwise healthy children with varicella leads to the appearance of resistant virus has been shown. To know the potential of antiviral resistance, we measured the susceptibilities to aciclovir in the paired …