A century-old tuberculosis vaccine has shown an ability to meaningfully reduce insulin dependence in patients with type 1 and latent autoimmune diabetes, according to phase II trial results presented on June 5 at the annual meeting of the American Diabetes Association in New Orleans. The findings, led by researchers at Massachusetts General Hospital in Boston and published in a companion report in Nature, mark one of the more unexpected intersections between a preventive infectious disease vaccine and a chronic autoimmune metabolic disorder.
What the BCG Vaccine Did in the Trial
The Bacillus Calmette-Guérin vaccine — commonly known as BCG — has been administered since the 1920s to protect against tuberculosis and is one of the most widely deployed vaccines in medical history, with billions of doses given primarily in countries where TB remains endemic. The MGH trial enrolled two distinct patient groups: 34 adults with childhood-onset type 1 diabetes and 95 people with latent autoimmune diabetes in adults, a form of autoimmune diabetes often initially misdiagnosed as type 2. Participants in the active arm received six BCG injections over a five-year period; a matched control group received saline placebo injections on the same schedule.
In the type 1 diabetes cohort, the vaccinated group showed a statistically significant improvement in HbA1c — the standard measure of long-term blood sugar control — dropping from an average of 7.84 percent at baseline to 7.30 percent five years later. The placebo group showed no comparable improvement. More striking to the investigators was the sustained reduction in daily insulin requirements among BCG recipients, which the team attributed to a reprogramming of how immune T cells metabolize glucose at the cellular level.
The Mechanism: Rewiring T Cell Energy Use
The proposed mechanism does not operate the way most vaccines do. Standard vaccines train the immune system to recognize a specific pathogen. BCG appears to do something different in diabetics: it shifts the metabolic activity of autoimmune T cells — the same cells responsible for the immune attack on insulin-producing beta cells in the pancreas — away from oxidative phosphorylation and toward aerobic glycolysis, a lower-energy pathway that reduces the cells' destructive capacity.
"These T cells are essentially running hot on the wrong fuel," said a senior researcher at the MGH Faustman Lab who led the study. "BCG appears to cool them down by changing how they burn energy, and that translates into less immune assault on whatever beta cell function remains."
The team cautioned that the vaccine does not reverse type 1 diabetes or regenerate destroyed beta cells. Rather, it appears to slow the progression of immune damage and reduce the systemic burden of blood sugar dysregulation, particularly in patients who were diagnosed early in life and have retained at least partial beta cell function. Patients with complete beta cell loss showed smaller absolute benefits, though even in that group insulin requirements declined modestly over the five-year observation period.
Implications for the 1.6 Million Americans With Type 1 Diabetes
Type 1 diabetes affects an estimated 1.6 million Americans, according to the American Diabetes Association. Managing the disease requires daily insulin administration — either via injection or continuous infusion pump — and carries long-term risks including cardiovascular disease, kidney failure, and blindness. Current treatment paradigms manage the condition rather than modify its underlying biology, making the prospect of a widely available, low-cost intervention like BCG — the vaccine costs roughly $2 per dose in international markets — scientifically compelling, even if the effect sizes observed so far are modest rather than transformative.
Regulatory experts said the results, while encouraging, are not sufficient by themselves to support a Food and Drug Administration label change for BCG in the context of diabetes management. "You need phase III data, with larger cohorts and longer follow-up, before this goes into clinical guidelines," said an FDA regulatory science consultant who was not involved in the study. "But this is absolutely enough to justify the next trial."
A Phase III Trial Is Being Planned
The MGH team is finalizing a phase III protocol that would enroll approximately 400 patients across multiple U.S. sites and extend the follow-up period to 10 years. Faustman's group is in discussions with the National Institutes of Health and several private foundations about funding, with a target enrollment start date in late 2026 or early 2027. At the NIH's National Institute of Diabetes and Digestive and Kidney Diseases — which funded portions of the earlier phase II work — program officers are reviewing the application, according to two people familiar with the discussions.
If the larger trial confirms what the phase II data showed, BCG could become the first established vaccine repurposed as a disease-modifying therapy for an autoimmune metabolic condition — a category of treatment that currently does not exist in routine clinical practice. Endocrinologists attending the New Orleans conference said the presentation drew an unusually large audience, with researchers from multiple institutions expressing interest in joining the phase III effort as co-investigators. "Everyone is cautious," said one attending physician from a major diabetes clinic in Chicago. "But everyone is paying attention."