Vir Biotechnology reported on Nov. 8 that its monthly combination of elebsiran and tobevibart met the primary endpoint in the pivotal Phase 3 ECLIPSE 1 trial in chronic hepatitis delta. At Week 48, 44% of patients on the combination achieved undetectable HDV RNA plus normalization of liver enzymes, compared with 0% in the delayed-treatment arm. A key secondary endpoint was even more striking: 79% of treated patients had undetectable HDV RNA, again against 0% in the delayed arm.
The result arrives about six months after the FDA granted accelerated approval to Gilead's Hepcludex, the first therapy approved in the United States for hepatitis delta. That approval changed the question for the field. The question is no longer whether the disease can be treated. It is which treatment, for how long, and whether the virus can be pushed into a durable remission that outlasts the drug.
Vir's answer is a two-drug regimen that attacks the virus from two directions. Whether it can beat the first mover depends on data the company has not finished generating.
The disease
Hepatitis delta virus is a small, defective virus that cannot replicate on its own. It needs the surface antigen of hepatitis B, HBsAg, to assemble and infect new cells. People can acquire HDV at the same time as HBV, or later as a superinfection. The second route is more dangerous. As many as 90% of people with chronic HBV who acquire HDV may develop chronic infection, and the combination accelerates cirrhosis and liver cancer faster than HBV alone.
The virus is common enough to matter and rare enough to be neglected. Estimates of the global burden range from about 12 million to 20 million people, depending on the survey and the definition. In the United States, the estimate is 40,000 to 80,000 people, and testing is spotty: fewer than 11% of Americans with chronic HBV have ever been tested for HDV. The World Health Organization classified HDV as a Group 1 carcinogen in 2025.
For decades, the only option was off-label pegylated interferon, which many patients could not tolerate. Hepcludex changed that. The FDA approved it in May 2026 on an accelerated basis for adults without cirrhosis or with compensated cirrhosis. The approval was based on the MYR301 trial, where 48% of treated patients met a combined response endpoint at Week 48 versus 2% in the delayed arm. Undetectable HDV RNA rose from 20% at Week 48 to 36% at Week 96 and 50% at Week 144. The drug carries a boxed warning for severe flares after treatment stops.
The approval was a genuine milestone. As Wendy Carter, DO, put it at the time, it fills a critical gap in care. It also set a bar that any challenger now has to clear.
The trial
ECLIPSE 1 enrolled 124 patients across 39 sites in 10 countries. About 55% had compensated cirrhosis. Patients were randomized two to one to immediate treatment or delayed treatment starting at Week 12. The regimen is two separate subcutaneous injections once a month: tobevibart 300 mg and elebsiran 200 mg.
The primary endpoint was the proportion of patients with undetectable HDV RNA and normal ALT at Week 48. The combination met it: 44% versus 0% in the delayed arm, with a p-value below 0.0001. The key secondary endpoint, undetectable HDV RNA alone, was 79% versus 0%. The effect held across cirrhosis status: 85% of patients with cirrhosis and 73% without cirrhosis had undetectable virus. At Week 24, 100% of treated patients had a virologic response, defined as undetectable RNA or a decline of at least two logs. ALT normalization was 53% versus 5%, and 95% of treated patients had HBsAg below 10 IU/mL in an exploratory analysis.
The safety profile was relatively clean for a disease population that often has advanced liver disease. One patient had a Grade 3 treatment-related adverse event, a drop in neutrophils that resolved in 15 days. There were no treatment interruptions or discontinuations. The most common side effect was transient flu-like symptoms after the first dose.
The trial has limits. The delayed-treatment design is efficient, but it is not a classic placebo-controlled comparison for the full 48 weeks because the control arm starts treatment at Week 12. The sample is small, and the follow-up is short relative to a chronic liver disease that can take years to progress. The 44% combined response is a surrogate: it measures virus and liver enzymes, not cirrhosis, liver cancer, or death. Those outcomes take longer to accrue. That is why the FDA's accelerated approval pathway exists, and why confirmatory data matter.
How the combination works
Tobevibart is a broadly neutralizing monoclonal antibody against HBsAg. It blocks the virus from entering liver cells and helps clear viral particles from the blood. Its Fc domain is engineered for stronger immune engagement, and it has been modified for a longer half-life. Elebsiran is a small interfering RNA that targets HBV RNA transcripts, reducing the production of HBsAg. Because HDV needs HBsAg to assemble, lowering the supply of the protein deprives the virus of a building block.
The two drugs attack the same target from opposite ends: one reduces the supply of HBsAg, the other blocks what remains and helps clear it. Hepcludex works differently. It blocks the NTCP receptor that HBV and HDV use to enter liver cells. The two approaches are not mutually exclusive, but they ask different things of the patient. Hepcludex is a daily injection. Vir's regimen is monthly. Hepcludex is approved; Vir's combination is not.
The comparison
The headlines invite a direct comparison: 44% combined response for Vir versus 48% for Hepcludex. That comparison is tempting and unreliable. The trials enrolled different populations, used different definitions of response, and ran in different eras of the disease. Cross-trial comparisons in hepatitis delta are especially fragile because the patient mix, the prevalence of cirrhosis, and the standard of care vary widely.
The more interesting difference is the depth of viral suppression. Vir's 79% undetectable HDV RNA at Week 48 is higher than Hepcludex's 20% at the same point in MYR301. Hepcludex's undetectable rate rose over time, reaching 50% at Week 144. Vir's regimen may simply suppress faster, or it may suppress more deeply. The only way to know is to follow both regimens over years, with the same assays and the same definitions.
The regulatory standard adds another layer. Hepcludex was approved on an accelerated basis, which means the FDA will require confirmatory evidence of clinical benefit. Vir's application will face the same standard. The agency has to decide whether a higher rate of undetectable HDV RNA is a meaningful improvement over the first drug, or whether the two regimens should be compared directly. A head-to-head trial is unlikely in a disease this rare, so the FDA may rely on cross-trial comparisons and mechanistic reasoning. That is exactly the kind of judgment that makes approvals uncertain.
For patients, the practical difference may be the injection schedule and the monitoring burden. Hepcludex is a daily subcutaneous injection. Vir's combination is monthly, but it is two injections and requires the same liver monitoring. The boxed warning on Hepcludex about post-treatment flares is a reminder that stopping either regimen can be dangerous. The field's goal is not just suppression but a finite course that leaves the virus undetectable.
That is where the functional-cure question comes in. Hepatitis delta is a chronic infection, and the current approved therapy is taken until response is lost, with an uncertain optimal duration. A regimen that can be stopped and leave the virus undetectable would be a different product. Vir's Phase 2 SOLSTICE data offered a hint: 28 patients who completed 96 weeks of treatment had undetectable virus, and 10 patients who stopped treatment remained undetectable for 12 to 48 weeks of follow-up, with no relapses reported. That is a small number of patients, and longer follow-up is needed. But it is the signal that matters most.
What comes next
Vir plans to report data from ECLIPSE 2 and ECLIPSE 3 in the first quarter of 2027 and to submit a global regulatory package to the FDA in mid-2027. The regimen has Breakthrough Therapy and Fast Track designations from the FDA and PRIME and orphan designations from the European Medicines Agency. Norgine holds commercialization rights in Europe, Australia, and New Zealand.
The FDA will have to weigh the same questions that applied to Hepcludex: the strength of the virologic response, the durability of the effect, the safety of long-term treatment, and the risk of flares after stopping. The accelerated approval pathway that allowed Hepcludex to reach patients early also requires confirmatory evidence. Vir's application will face a similar test, with the added complication that the first mover is already on the market.
The market reaction was immediate. Vir's shares rose more than 15% on the news, and coverage framed the result as a challenge to Gilead's franchise. The commercial calculus is real: the US population is small but the disease is severe, and a monthly regimen could be more attractive than a daily injection for patients who face years of treatment. The global burden is larger, but access and pricing will determine how much of it the product reaches.
What to watch
The durability data are the next milestone. ECLIPSE 2 and ECLIPSE 3 will add patients and follow-up. The SOLSTICE cohort will continue to report what happens after treatment stops. The FDA will review the package, and the label will define which patients are eligible. The cirrhosis subgroup matters: 55% of ECLIPSE 1 patients had compensated cirrhosis, and the response rate was high in that group.
The safety picture also needs longer follow-up. One Grade 3 event in 124 patients is encouraging, but hepatitis delta patients often have advanced liver disease, and the combination will be used for months or years. The boxed warning on Hepcludex is a reminder that stopping treatment can be dangerous. Vir's regimen will need a similar monitoring plan if it is approved.
The first approved drug changed the standard of care. The challenger has now shown a deeper suppression signal. The finish line is not the Week 48 snapshot. It is the point where a patient can stop treatment and stay well.
Primary sources
- Vir Biotechnology's Nov. 8, 2026 ECLIPSE 1 announcement for the trial design, primary and secondary endpoints, safety data, and regulatory timeline.
- Drug Topics for the May 22, 2026 Hepcludex approval, the MYR301 results, dosing, boxed warning, and US prevalence estimate.
- Springer Medicine for the mechanism of tobevibart and elebsiran and the SOLSTICE Phase 2 data.
- AJMC for the 96-week SOLSTICE follow-up and the off-treatment durability signal.
- Gastroenterology and Hepatology for the hepatitis delta disease burden and treatment review.
- Yahoo Finance for the share-price reaction to the ECLIPSE 1 results.