Moderna announced its decision to cease development of its vaccine mRNA-1647, which was being designed to treat birth defects such as cytomegalovirus (CMV), following its failure to reach the primary goal in its late-stage phase III clinical trial.¹

"We are clearly disappointed by the failure to prevent primary infection because it means there is still no vaccine for the prevention of congenital CMV despite the many decades of work by the field," said Moderna president Stephen Hoge, MD. “CMV does cause significant disease in other contexts, including reactivation of the latent virus in those undergoing bone marrow transplantation, and we will continue to explore the potential of mRNA-1647 to suppress disease associated with reactivation in those high-risk patients through our ongoing Phase II study.”

What was the primary goal of the phase III clinical trial?

According to a statement posted on Moderna’s website from CEO Stéphane Bancel, the study’s endpoint was preventing CMV infection in seronegative female participants of childbearing age (16-40 years).²

In the statement, Bancel explained, “Of course, this outcome is disappointing for families and healthcare professionals who have been eagerly awaiting a vaccine to prevent congenital CMV, a leading cause of infectious birth defects. The majority of the data from this trial is still forthcoming, and we will share our full results with the scientific community in hopes that our learnings can contribute to the continued pursuit of a congenital CMV vaccine.”

How will this decision affect Moderna moving forward?

As mentioned in Bancel’s statement, Moderna does not anticipate its decision to affect its 2025 financial guidance, or change its expectations to breakeven in 2028.²

Bancel followed up, saying, “We anticipated minimal initial revenue contribution from mRNA-1647 given necessary investments in market building and launch, and expected the product to be cash-flow negative in 2028.”

Moderna is expected to continue moving forward with executions on its research and development strategies, which includes its respiratory portfolio, aiming to deliver public health impact through its three approved treatments from this year, along with two additional products expected to be submitted for approval this year.² The company is also expected to focus on its cancer programs, which have displayed potential, including proof of concept from its mRNA-4157 candidate, which is currently awaiting its phase III trial results.

Why focus on cytomegalovirus?

As of October 2025, there are no approved vaccines preventing congenital CMV. However, nearly one in three children are infected with CMV by age five, along with one in 200 babies being born with the congenital form of CMV.³ Infection occurs through a common virus and stays in the body for life after infection.³

CMV spreads from person to person through body fluids, which includes blood, saliva, urine, semen, and breast milk. CMV infections can result in symptoms such as fever, fatigue and a sore throat, yet the virus rarely causes symptoms in healthy people.³ For patients who are pregnant, or have weak immune systems, CMV is cause for concern, as pregnant people with an active CMV infection can pass the virus to their babies. For people who have weakened immune systems, CMV infection can be fatal.

Sources

1. Moderna halts development of birth defect vaccine after trial setback Reuters October 22, 2025 https://www.reuters.com/business/healthcare-pharmaceuticals/moderna-ends-development-cmv-vaccine-after-trial-failure-2025-10-22/
2. Reflecting on Moderna’s Phase 3 CMV Vaccine Readout ModernaOctober 22, 2025 https://www.modernatx.com/en-US/media-center/all-media/blogs/phase-3-cmv-vaccine-readout-reflections
3. Cytomegalovirus (CMV) infection Mayo Clinic Access date October 23, 2025 https://www.mayoclinic.org/diseases-conditions/cmv/symptoms-causes/syc-20355358