To determine the effectiveness of an antiviral to prevent herpes labialis during a brief, high-risk circumstance, 147 persons with a history of sun-induced recurrences were treated prophylactically with oral acyclovir or matching placebo and were observed during their ski holidays. Five (7%) of 75 acyclovir-treated subjects developed lesions compared with 19 (26%) of 72 persons in the placebo group.
Spruance SL, Hamill ML, Hoge WS, Davis LG, Mills J. Acyclovir Prevents Reactivation of Herpes Simplex Labialis in Skiers. JAMA. 1988;260(11):1597-1599. doi:10.1001/jama.1988.03410110105036