Background: The efficacy of a therapeutic agent must be evaluated by objective criteria. However, in herpes zoster (HZ) studies there has been no generally accepted objective clinical criterion.

Objective: Our purpose was to establish a clinical method for determining objectively the point in time at which the eruptive phase of HZ is completed (no new vesicle formation). This point is said to be a clinical criterion for the end of viral replication in the skin and thus for measuring the efficacy of a virustatic agent.

Methods: Newly formed vesicles were marked with differently colored permanent marker pens each day. This method was evaluated by comparing the results of acyclovir therapy in two groups of patients with HZ. (Group A, no underlying malignancy; n = 9. Group B, underlying malignancy; 64% of these patients were undergoing cytostatic polychemotherapy or had immunodeficiency; n = 22).

Results: In both groups, acyclovir stopped the eruption of new vesicles within 1.8 and 2.8 days, respectively (not statistically significant). Group B showed a tendency toward more protracted hematogenous dissemination and a longer duration of therapy. The total duration of the eruptive phase depended solely on the length of the interval between the onset of the HZ and the beginning of therapy.

Conclusion: The method of marking new vesicles is independent of laboratory facilities, simple, and cost effective; in addition, this method is suitable for statistical evaluation. It is thus superior to other clinical methods for objective assessment of the progression of HZ.