Dive Insight:

A common infection, RSV results in only mild, cold-like symptoms in most people. Yet the disease causes 58,000 hospitalizations a year in children under 5 and 177,000 in the elderly. After decades of disappointment, vaccine makers may finally be close to successfully developing a preventive shot, while others have attempted to create a treatment akin to Roche’s Tamiflu for influenza.

Pfizer, one of the big names in RSV vaccine development, recently bought a private company called ReViral for its RSV antivirals in a deal that could amount to $525 million. One drug does exist already — Sobi’s Synagis — but is only cleared for some children younger than 2 years old.

Enanta is an experienced developer of antiviral medicines, having provided two different hepatitis C-fighting compounds as part of an AbbVie partnership, including one that is part of the dual-acting pill Mavyret. That partnership brought in $97 million in royalties in Enanta’s fiscal year that ended Sept. 30.

In RSV, Enanta has been studying EDP-938 in several patient groups in Phase 2 trials, including pediatric patients and people who have undergone stem cell transplants. The company just initiated a trial in elderly patients and those with respiratory and cardiovascular conditions that put them at high risk.

The trial that Enanta reported data from Wednesday involved otherwise healthy adults who were treated with EDP-938 within two days of becoming infected with RSV in the community. The study aimed to show the company’s drug could significantly reduce symptom scores and viral load 14 days after treatment when compared to a placebo, but missed on both measures. The company noted that viral load was significantly reduced after five days of treatment, although that measurement wasn’t part of the trial’s main analysis.

The data from the trial contrast with results from an earlier study that directly exposed patients to RSV and then gave them EDP-938 or a placebo, which showed the drug significantly reduced symptoms and viral load. Wall Street analysts said the latest results reflect real-world treatment, in which viral load would likely already be declining by the time patients took their first pill.

That same factor could lead to disappointing results in higher-risk patients, according to Brian Skorney, an analyst at Baird. “We are also more cautious on the probability of success as the kinetics of natural resolution of RSV are not very well understood,” he wrote in a May 18 note to clients.