13-Year Study Finds Gene Therapy for Haemophilia B Safe, Effective, and Long-Lasting
London | June 13, 2025 — A breakthrough study published in The New England Journal of Medicine has reaffirmed the long-term effectiveness and safety of gene therapy for haemophilia B, a rare and life-threatening bleeding disorder. Conducted by researchers from St. Jude Children’s Research Hospital and University College London, the study is among the longest follow-ups of its kind and reveals sustained benefits 13 years after a one-time treatment.
What Is Haemophilia B?
Haemophilia B is a rare X-linked genetic disorder that affects about 1 in every 25,000 male births. It is caused by a deficiency of factor IX, a critical blood-clotting protein. Without sufficient levels, patients are prone to frequent spontaneous bleeding, including life-threatening internal hemorrhages.
Historically, treatment involves lifelong and costly supplementation with synthetic or donor-derived factor IX. However, gene therapy aims to address the root cause by delivering a working copy of the faulty gene directly into the patient’s body.
The Gene Therapy Approach
Between March 2010 and November 2012, 10 adult patients with severe haemophilia B received a single intravenous dose of an experimental gene therapy product. The treatment uses a viral vector to deliver the correct version of the factor IX gene to liver cells, where the clotting factor is naturally produced.
Initial results, published in 2014, showed a significant reduction in bleeding episodes and increased factor IX levels. The big question remained: would the treatment hold up over time?
13-Year Follow-Up: Results That Endure
Now, after 13 years of continuous monitoring, researchers report:
- Stable expression of factor IX in all 10 patients.
- A 10-fold reduction in the annualised bleeding rate.
- No late-onset side effects or toxic events.
- Sustained freedom from regular factor IX infusions.
“The key benefit is that gene therapy is a one-time, simple intravenous infusion that potentially has positive effects for a lifetime,” said Dr. Andrew Davidoff, Chair of the St. Jude Department of Surgery.
Long-Term Safety: A Crucial Breakthrough
Gene therapy, despite its promise, has faced scepticism over long-term safety, especially around liver toxicity and durability of expression. This study offers reassuring data:
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Over 90% of the therapy was absorbed by the liver.
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Mild liver inflammation occurred shortly after treatment but was managed with steroids and did not recur.
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There were no serious adverse effects over the 13-year span.
“We have not seen any side effects or toxic events in the long-term follow-up,” said Dr. Ulrike Reiss, Principal Investigator from St. Jude’s Department of Haematology.
A Paradigm Shift in Rare Disease Treatment
The study reinforces the transformative potential of gene therapy not just for haemophilia B but for other monogenic diseases. While current treatments rely on lifelong maintenance, gene therapy offers the possibility of a functional cure.
“For these 10 patients, the factor levels are stable and have been at the same level across these 13 years,” said Dr. Reiss.
The result is not just a scientific milestone, but a life-changing reality for patients who once faced a lifetime of hospital visits, bleeding risks, and economic burdens.
What This Means for the Future
Gene therapy is gaining global traction as a precision medicine tool, especially for inherited rare diseases. However, long-term efficacy and safety data have been lacking—until now.
This study provides critical evidence for regulators, insurers, and families who are weighing the risks and rewards of one-time genetic interventions.
With further trials ongoing in younger patients and across different demographics, the findings from this cohort may lay the foundation for wider approval and adoption.
This 13-year landmark study confirms that gene therapy can provide safe, stable, and effective long-term treatment for haemophilia B. As gene therapy technologies mature, such data offer hope for millions of patients worldwide living with chronic genetic diseases.