Viral sensitivity following the introduction of acyclovir.

Extensive use of acyclovir for the treatment or prevention of herpes simplex and varicella zoster infections in immunocompetent patients has not resulted in significant changes in virus sensitivity. Some minor reductions have been observed, but these have not affected clinical responses. In severely immunosuppressed patients, virus with reduced acyclovir sensitivity has very occasionally been isolated and this has sometimes been associated with clinical resistance. Resistance has, in all but one instance, resulted from the inability of the virus to express thymidine kinase. Follow-up where possible to subsequent recurrences has in all cases revealed sensitive virus. Therefore, resistance to acyclovir in the clinical setting seems to be transient and its significance slight.