Gene Therapy Safety: What Do Two Child Deaths Reveal?

What happened in the two pediatric gene therapy trials?

According to Nature's September 8 report, a six-year-old girl with Snijders Blok–Campeau syndrome, associated with changes in the CHD3 gene, died after receiving an experimental treatment. A boy died in a separate HuidaGene Therapeutics trial for Duchenne muscular dystrophy. Both treatments used adeno-associated virus, or AAV, to deliver genetic material. Nature reported that an investigation into the girl's trial and a related research paper had been announced. [Nature's reporting](https://www.nature.com/articles/d41586-026-02497-2).

The company attributed the boy's death to acute respiratory distress syndrome, a severe lung complication. These cases warrant investigation, but they cannot establish a mortality rate for gene therapy overall: the treatments, diseases and patients differed. The timing of public disclosure also matters because information about serious harm can affect other families' decisions. [Details of the two cases](https://www.nature.com/articles/d41586-026-02497-2).

Why can gene therapy cause serious complications?

Gene therapy describes several approaches to changing how cells function. Some treatments introduce a working gene; others modify existing genetic sequences. Some modify cells outside the body before returning them to the patient, while others deliver treatment directly into the body. These differences matter when interpreting safety findings because products grouped under the same broad label can have different biological effects. [FDA explanation of gene therapy](https://www.fda.gov/vaccines-blood-biologics/cellular-gene-therapy-products/what-gene-therapy).

A peer-reviewed report provides evidence of how delivery-related harm can occur. In 2023, Angela Lek and colleagues described a 27-year-old man with advanced Duchenne muscular dystrophy who died after receiving an experimental treatment delivered with a high dose of recombinant AAV9. Their findings implicated an innate immune reaction in acute respiratory distress syndrome. The authors also discussed how advanced disease could reduce a patient's ability to withstand severe complications. This individual case helps explain a possible mechanism; it does not establish the cause of either newly reported pediatric death. [New England Journal of Medicine case report](https://www.nejm.org/doi/full/10.1056/NEJMoa2307798).

Genome editing introduces additional concerns, including changes at unintended DNA locations and unexpected consequences at the intended target. FDA guidance recommends assessing both the editing process and the delivery method, including immune reactions and distribution to tissues outside the intended target. A treatment's ability to alter a gene therefore needs to be evaluated alongside evidence about its wider biological effects. [FDA genome-editing guidance](https://www.fda.gov/media/156894/download).

What safeguards matter before children receive experimental gene therapy?

FDA's 2024 genome-editing guidance describes special protections for pediatric research. For studies involving more than minimal risk under the cited pediatric provision, anticipated direct benefit must justify the risks. The guidance also recommends spacing treatment between participants to identify complications before additional patients receive treatment or doses increase. For families, these principles translate into practical questions about supporting evidence, dose selection and the findings that would trigger a study pause. [FDA recommendations for clinical studies](https://www.fda.gov/media/156894/download).

Follow-up also needs to extend beyond the initial treatment period. FDA explains that lasting biological changes can produce delayed adverse effects, so monitoring should reflect the specific product's risks. Its genome-editing guidance recommends follow-up for up to 15 years. Families should receive a clear explanation of the planned duration, assessments and arrangements for continued monitoring. An initially reassuring recovery cannot establish long-term safety. [FDA long-term follow-up guidance](https://www.fda.gov/regulatory-information/search-fda-guidance-documents/long-term-follow-after-administration-human-gene-therapy-products), [FDA genome-editing follow-up recommendations](https://www.fda.gov/media/156894/download).