About Respiratory Syncytial Virus
RSV is the most common cause of bronchiolitis (inflammation of the small airways in the lung) and pneumonia in children under one year of age in the United States and a significant cause of respiratory illness in older adults and immunocompromised individuals.1,2 According to the Centers for Disease Control and Prevention, virtually all children in the United States get an RSV infection by the time they are two years old.3 RSV represents a significant health threat for adults older than 50 years of age, with up to 180,000 hospitalizations associated with RSV infections annually in the United States.2 Overall, in the United States, RSV accounts for up to 6.8 million outpatient visits and approximately 370,000 hospitalizations, with 24,000 deaths.4
In the United States RSV accounts for approximately 370,000 hospitalizations for children and older adults
= 17,500 hospitalizations
In the United States RSV accounts for up to 6.8 million outpatient visits for children and older adults
Enanta’s Approach to Treating RSV
We are advancing two clinical stage oral, once-daily antiviral treatments for RSV – zelicapavir and EDP-323. Our lead candidate, zelicapavir, is a potent inhibitor of the RSV nucleoprotein (N-protein), while our second candidate, EDP-323, targets the RSV polymerase (L-protein). Both candidates inhibit viral replication and the production of new virions They differ from fusion inhibitors, which act only at viral entry. Together these candidates represent complimentary approaches to disrupting RSV replication. Both zelicapavir and EDP-323 have Fast Track designation from the U.S. Food and Drug Administration (FDA).
We believe that antiviral treatments for RSV, such as zelicapavir and EDP-323, have the greatest potential to show optimal efficacy in high-risk populations, as these patients have reduced RSV immunity or other comorbidities, which manifest in a higher and longer duration of viral replication and greater disease severity, allowing a bigger window to realize the full potential of the treatments. Our development program is focused on evaluating zelicapavir in high-risk populations, including high-risk adults and pediatric patients, all of which have significant unmet need.
Zelicapavir was initially evaluated in a Phase 2a randomized, double-blind, placebo-controlled, human challenge study in healthy adult subjects inoculated with RSV which showed that zelicapavir resulted in a statistically significant (p<0.001) reduction in viral load and resolution of clinical symptoms compared to placebo. The results were published in The New England Journal of Medicine.
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Most recently, in September 2025, we announced positive topline results from RSVHR, a Phase 2b randomized, double-blind, placebo-controlled study evaluating zelicapavir in high-risk adults with RSV, including those who are older than 65 years of age and those who have asthma, chronic obstructive pulmonary disease (COPD), or congestive heart failure (CHF). In the study, a clinically meaningful improvement in time to complete resolution of all 13 RSV symptoms was observed for zelicapavir compared to placebo, with a benefit of 2.2 days for the overall efficacy population and 6.7 days for patients with CHF, COPD, or age 75 or older, termed the HR3 population, which comprised the majority (81%) of the efficacy population. Zelicapavir also showed an improvement in time to complete resolution on the 29-parameter total RiiQ™ symptom scale of 3.6 days for the efficacy population and 7.2 days for the HR3 population compared to placebo. Additionally, there was a 3.0-day faster time to complete resolution of lower respiratory tract disease (LRTD) symptoms in the HR3 population. However, no effect was observed on the time to resolution of the LRTD subset of four symptoms to mild, which was the primary endpoint. The study met the secondary endpoint of time to improvement in the Patient Global Impression of Severity (PGI-S) score, with a statistically significant 2-day faster resolution with zelicapavir compared to placebo. Importantly, a lower hospitalization rate was observed for patients treated with zelicapavir compared to placebo. The study met key secondary virology endpoints showing a robust antiviral effect. The study also showed that zelicapavir demonstrated a favorable safety profile and was well-tolerated.
In December 2024, we announced results from a Phase 2 randomized, double-blind, placebo-controlled study in hospitalized and non-hospitalized pediatric RSV patients aged 28 days to 3 years old. In the study of 96 patients, an antiviral effect was observed for the primary and secondary virology endpoints in the overall pooled efficacy population. The primary endpoint in Part 2 of the study, which focused on virology, showed a pronounced antiviral effect with a 1.4 log decline in viral load at Day 5 compared to placebo. Additionally, a rapid and robust virologic effect was observed in a prespecified subset of patients who were randomized within 3 days of symptom onset, with a 1.2 log decline in viral load at Day 5 compared to placebo. The study also showed that zelicapavir demonstrated a favorable safety profile and was well-tolerated in this pediatric population. In a post-hoc analysis, treatment with zelicapavir was associated with shortened time to complete resolution of RSV symptoms, defined as absent and discharged from hospital. Specifically, time to complete resolution of symptoms showed an estimated Kaplan-Meier median of 6.99 days for zelicapavir versus 8.60 days for placebo. Similarly, an analysis of sustained resolution (defined as absent and remaining absent at all subsequent time points and discharged from hospital) resulted in 6.99 days for zelicapavir versus 10.68 days for placebo.
Overall, zelicapavir has been dosed in more than 700 people to date and continues to be well-tolerated with a favorable safety profile.
Our second clinical RSV candidate, EDP-323, has sub-nanomolar potency in vitro and protected mice from RSV infection in a dose-dependent manner as demonstrated by both virological and pathological endpoints. EDP-323 is not expected to have cross-resistance to other classes of inhibitors and has the potential to be used alone, or in combination with other RSV mechanisms, to broaden the treatment window or addressable patient populations.
In September 2024, we announced positive topline results for EDP-323 in a Phase 2a challenge study of healthy adults infected with RSV, which demonstrated statistically significant reductions in RSV viral load and symptoms. In addition, a post-exposure prophylaxis analysis was performed in subjects who were not infected by Day 5 after RSV exposure. The data showed the potential for EDP-323 to be effective in preventing RSV infection when initiated up to five days after RSV exposure.
With positive results from the zelicapavir pediatric and high-risk adult studies, as well as the EDP-323 challenge study, we continue to have the leading portfolio of RSV therapeutics with broad optionality including potential first- and best-in-disease compounds. These distinct mechanisms have the potential to be developed as once-daily single agents or in combination for specific populations. This optionality provides us with multiple ways to address different patient populations.
- Centers for Disease Control & Prevention – About RSV. Last accessed: March 2026.
- Centers for Disease Control & Prevention – RSV In Adults. Last accessed: March 2026.
- Centers for Disease Control & Prevention – RSV in Infants and Young Children. Last accessed: March 2026.
- CDC Preliminary Estimates of RSV Burdenfor 2024-2025; for period: 10/1/24-9/27/25. Last accessed March 2026.