The therapy, EDP-938, developed for treating respiratory syncytial virus (RSV) infection which currently has no available treatment, was compared with placebo, the company said.
RSV could lead to serious lung infections and even death in babies and the elderly who have a weaker immune system.
An average of 57,527 children younger than 5 years and 177,000 adults older than 65 years are hospitalized due to RSV infections every year, according to the Centers for Disease Control and Prevention.
Enanta said study results showed its therapy was generally safe and well tolerated and no drug discontinuations were observed.
Drugmakers Regeneron Pharmaceuticals Inc and Johnson & Johnson have abandoned certain programs to find a treatment for the condition, and an experimental vaccine developed by Novavax Inc had failed a late-stage trial, earlier this year.