Gilead halts RSV drug trials due to low infection rates

USA—Gilead Sciences has halted two mid-stage clinical trials of its antiviral drug obeldesivir, which were aimed at treating respiratory syncytial virus (RSV) infections in both adults and children.

The company attributed this decision to an unexpectedly low rate of RSV infections during the most recent season in the Northern Hemisphere.

This low incidence made it challenging to measure meaningful symptom reduction, the primary goal of the studies.

Specifically, Gilead was conducting separate Phase 2 trials: one involving nonhospitalized adults with acute RSV infection, and another focusing on children under five years old.

 Both trials were designed to assess the safety, tolerability, and efficacy of obeldesivir in shortening the duration and severity of RSV symptoms.

However, due to the insufficient number of RSV cases, the trials were terminated early in mid-2025.

Importantly, Gilead emphasized that the decision to stop these studies was not due to any safety or efficacy concerns regarding obeldesivir itself.

The company plans to analyze and share the data collected so far to guide potential future steps for the drug.

Nonetheless, since obeldesivir is currently Gilead’s only RSV candidate in development, the outcome of these trials will significantly influence whether the company continues to pursue this antiviral in the RSV market.

This is not the first time obeldesivir’s clinical progress has been disrupted by low infection rates.

In 2023, Gilead also terminated a Phase 3 trial of obeldesivir for COVID-19 treatment for similar reasons related to lower-than-expected COVID-19 cases and hospitalizations.

Meanwhile, Gilead is continuing to explore obeldesivir’s potential against other viral threats.

Notably, a clinical trial is ongoing to evaluate the drug’s efficacy against Marburg virus, a severe and often fatal disease.

Preclinical studies have suggested that obeldesivir could be effective against this virus, offering hope for future therapeutic options.

The RSV treatment landscape is currently dominated by vaccines, including Pfizer’s Abrysvo, GSK’s Arexvy, Moderna’s mRESVIA, and Sanofi/AstraZeneca’s prophylactic antibody Beyfortus.

Merck has also recently gained approval for its antibody therapy Enflonsia.

Despite this vaccine dominance, several companies have reported promising results with antiviral candidates, including Shionogi’s S-337395 and Enanta Pharmaceuticals’ zelicapavir.

Enanta has highlighted the ongoing need for antivirals, especially since vaccine protection can wane over time and adult vaccine uptake remains low.

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