Moderna Halts Development of CMV mRNA Vaccine After Phase 3 Failure

October 23, 2025 — Moderna announced on Wednesday that its cytomegalovirus (CMV) vaccine candidate, mRNA-1647, failed to meet efficacy goals in a Phase 3 study and that the company will discontinue further development of the shot for most patients.

Low Efficacy Misses Target
The vaccine, which was being tested in women aged 16 to 40, demonstrated efficacy ranging from 6% to 23% depending on the case definition used. This result fell significantly short of the company’s target of at least 49% efficacy.

CMV is the most common infectious cause of birth defects in the US, with about one in five infected babies experiencing birth defects or long-term issues, most commonly hearing loss.

Moderna President Stephen Hoge expressed disappointment, stating the company had a “fair shot” at reaching the threshold and that the failure to meet an earlier interim analysis target had foreshadowed the final result.

Program Shutdown, Except for One Indication
The negative results effectively squelch one of Moderna’s longest-running programs, which began with a Phase 1 study eight years ago, predating the company’s major success in the COVID-19 vaccine market. Moderna will end development for congenital CMV. However, the company plans to continue evaluating mRNA-1647 in bone marrow transplant patients. Hoge noted that for this specific population—who are at high risk of CMV reactivation during immune suppression—the vaccine’s goal is entirely different: to boost the immune system against a major reactivation event. The vaccine was otherwise relatively well-tolerated, with the safety board raising no new concerns.