For a two-year-old, the fertility consult is theater. Before a child can receive the gene therapy the FDA cleared this month, a specialist is supposed to sit down with the family and walk through the options for preserving the child’s ability to one day have children of their own. For a seventeen-year-old, that meeting has a point: there are eggs to freeze, sperm to bank, a body that has already made the cells worth saving. A two-year-old has made none of them. There is nothing in the room to preserve, and the chemotherapy coming next is the reason the meeting had to happen at all.

That is the arithmetic underneath the FDA’s decision to expand Casgevy, the one-time gene-editing therapy from Vertex and CRISPR Therapeutics, down to children as young as two, for both sickle cell disease and transfusion-dependent beta thalassemia. The announcement arrived in the language these announcements always use: first, transformative, earlier access. What it does not say, and what the celebration around it mostly skips, is that curing a toddler’s sickle cell this way runs through several weeks of high-dose chemotherapy whose most reliable long-term effect, in a child that age, is infertility.

What was actually approved, and on what

Start with the therapy itself, because “CRISPR cure” does a lot of work in a headline and not much in a hospital. Casgevy does not edit a child in place. Doctors harvest the patient’s own blood stem cells, ship them to a lab where CRISPR switches fetal hemoglobin back on, and then, before the edited cells go back in, the child undergoes myeloablative conditioning: busulfan, a chemotherapy drug given at doses meant to destroy the existing bone marrow so the engineered cells have somewhere to take hold. The full journey, collection to recovery, runs close to a year, with a hospital stay for the chemo and infusion followed by weeks of isolation while a wiped-out immune system rebuilds.

The efficacy is genuinely striking, and I am not going to pretend otherwise. In the original single-arm trial that won approval for patients twelve and up, 29 of 31 evaluable patients, 93.5 percent, went at least twelve straight months without a severe vaso-occlusive crisis, the episodes of blocked blood flow that define the disease and, over a lifetime, shorten it. For a condition that spent decades near the back of the drug-development line, that is a result worth having.

Now look at what the pediatric expansion actually rests on. The label now reaches down to age 2. The trials behind it, CLIMB-141 and CLIMB-151, enrolled 11 children with sickle cell disease, the youngest of them 5. Eight had enough follow-up to evaluate, and all eight hit the crisis-free mark. On the thalassemia side, 15 children aged 5 to 12, of whom 9 were evaluable and 8 reached transfusion independence. No child under five appears anywhere in the efficacy data. The FDA extended the approval three years below the youngest patient anyone studied, on the reasonable-sounding but untested premise that a two-year-old’s marrow will behave like a five-year-old’s. That is a regulatory inference, not a trial finding, and the announcement presents it as neither.

The part the press release leaves for the fine print

Vertex, to its credit, did not hide the fertility problem during the trials. The company paid for trial participants to see reproductive specialists and to bank eggs, sperm, and tissue before conditioning. That is exactly the right thing to do for a teenager. It is also exactly why the expansion to toddlers deserves harder questions than it is getting.

Busulfan is toxic to the cells that make eggs and sperm. In adults and adolescents it can cause ovarian failure and halt sperm production. In a pre-pubertal child the same chemotherapy that clears the marrow also damages the immature germ cells sitting in the ovaries and testes, and there is nothing to freeze in advance because the body has not yet made a mature egg or sperm. An adolescent can bank sperm on a Tuesday and start conditioning on a Friday. A two-year-old has no such Tuesday. What exists for the very young, freezing ovarian tissue for a girl, is still limited, and for a pre-pubertal boy the options remain experimental. The consent form gets signed by a parent weighing a cure the child needs now against a fertility the child cannot yet want.

None of which makes the approval wrong, and that is the uncomfortable part. Sickle cell disease is brutal. It does its own cumulative damage to organs and brains starting in early childhood, and the case for treating sooner, before that damage accrues, is the one the investigators lead with. Dr. Haydar Frangoul, whose sites ran the studies, framed it as letting families act “before years of cumulative damage from these life-shortening diseases take hold.” That is a serious argument made by people who treat these children. It is also precisely why the fertility trade deserves adult-level honesty rather than the soft focus of a launch: the earlier you treat, the younger the child, and the younger the child, the less there is to save and the thinner the evidence that the treatment even works at that age.

The number that tells you how this is really going

Here is the figure the “first, transformative” framing tends to leave out. Casgevy has carried a U.S. list price of $2.2 million since it launched in December 2023. As of the end of September 2025, according to CRISPR Therapeutics’ own disclosures, 165 patients worldwide had started the first step, cell collection. Only 39 had actually been infused.

Read those two numbers next to each other. Nearly two years after launch, across dozens of authorized treatment centers on multiple continents, the entire global cohort that had completed treatment would not fill a school bus. A $2.2 million price, a year-long process, and weeks of chemotherapy isolation write the gap between the patients who begin and the patients who finish. A cure almost no one can reach is a remarkable scientific achievement and a marginal public-health one, and the distance between those two things does not close because the label now includes toddlers.

Expanding the approved age range costs Vertex nothing and enlarges the addressable market. Whether it enlarges the number of children actually cured is a different question, and it will be answered by the same forces, cost and complexity and access, that have kept the treated count in the dozens. The FDA approved a therapy for two-year-olds this month. Whether a two-year-old, or the health system standing around one, can realistically reach it is a question the agency did not have to answer, and did not.

Sources

  1. FDA – CASGEVY (approval scope, indication, pivotal efficacy)
  2. Healio – FDA expands Casgevy approval to children 2 years and older (2026-07-02)
  3. CRISPR Therapeutics – Q3 2025 disclosure (cell collections and infusions to date)
  4. STAT – Sickle cell gene therapies confront patients with an impossible choice: a cure or fertility (2023)
  5. Precision Medicine Online – Sickle cell gene therapy fertility risks spur calls for changes
  6. NCBI Bookshelf – Exagamglogene autotemcel: conditioning and gonadotoxicity