In October 2024, pharmaceutical giant Sanofi shut down its Phase 3 clinical trial for a respiratory syncytial virus (RSV) vaccine targeting infants and toddlers. The closure went largely unreported, earning only a brief mention in trade publication Fierce Biotech. What the company disclosed publicly was narrow: the vaccine had not performed as effectively as hoped. What documents obtained through a Freedom of Information Act (FOIA) request later revealed was something the company had not volunteered — a child died during the trial.

What Sanofi Said — and What It Didn’t

When Sanofi ended the trial, the company stated that the vaccine’s “safety profile was acceptable” and that no signals of vaccine-associated enhanced respiratory disease (VAERD) were observed by the Independent Data Monitoring Committee (IDMC). The official reason given for terminating the trial was lack of sufficient efficacy, not safety concerns.

It was only after The Defender, a publication of Children’s Health Defense, submitted a FOIA request to the U.S. Food and Drug Administration in May 2026 that the infant death during the trial was documented and reported. A Sanofi spokesperson subsequently confirmed the death, stating that the child had an underlying congenital heart disease and that the company’s analysis, supported by the IDMC, concluded that “the vaccine candidate was not the underlying cause of the event.”

The spokesperson also indicated that Sanofi plans to publish trial results in a peer-reviewed journal, but was unable to provide a timeline for when that publication would be finalized or available to the public.

A Critical Detail: Exclusion Criteria

The circumstances surrounding the infant death raise a procedural question that has gone largely unaddressed. According to the clinical trial’s own exclusion criteria, congenital disorders should have disqualified babies from participating in the study. Participants were to be assessed for preexisting conditions before enrollment.

If the child who died had a congenital heart disease — the condition cited by Sanofi as the underlying cause of death — questions arise about whether that child should have been enrolled in the trial at all, and if so, under what assessment protocol their eligibility was determined.

Research published in the Pediatric Infectious Disease Journal has established that most children who die from RSV do have comorbidities, including congenital heart or lung disease. That context is relevant both to understanding RSV mortality broadly and to evaluating what happened within this specific trial.

What Is VAERD, and Why Does It Matter?

Sanofi’s public statement specifically noted that no signals of vaccine-associated enhanced respiratory disease were observed. VAERD is a condition in which a vaccinated child who has not previously been infected with RSV is later exposed to the virus and develops a more severe form of the disease than they would have experienced without vaccination. It is a known risk consideration in RSV vaccine development for infants and has previously prompted other manufacturers to halt their own trials.

The absence of VAERD signals, as confirmed by the IDMC, is relevant context. However, the broader question of whether the trial’s safety data — including the infant death — was adequately disclosed to the public in real time remains unanswered. The death was not part of the company’s initial public communications when the trial was terminated.

The RSV Landscape for Infants

RSV is the most common cause of lower respiratory tract infections in infants and is a leading cause of infant hospitalization worldwide. Nearly all children experience RSV by the age of two. While RSV disease is typically mild, it can become severe, particularly in very young infants and those with underlying health conditions.

Current CDC guidance for RSV prevention in infants does not include a directly administered RSV vaccine for babies. Instead, the recommended approaches are maternal RSV vaccination during pregnancy using Pfizer’s Abrysvo, or the administration of long-acting RSV monoclonal antibodies — nirsevimab or clesrovimab — directly to the infant. Monoclonal antibodies are not vaccines; they do not activate the immune system but instead provide ready-made antibodies to protect against the disease.

Sanofi’s terminated trial represented one of several industry efforts to develop an active RSV vaccine that could be administered directly to infants and toddlers — a distinct and more complex immunological challenge than maternal vaccination or passive antibody administration.

Transparency, Timing, and the Public’s Right to Know

The sequence of events here is worth examining carefully. A clinical trial involving vulnerable infant subjects ended in October 2024. The company’s public explanation focused on efficacy. The death of a trial participant was not disclosed in the company’s public statements at that time. It became known only after an independent FOIA request was submitted — nearly two years later — and the information was pursued by a non-mainstream health publication.

Clinical trial transparency is a longstanding issue in pharmaceutical research. Regulatory frameworks in the United States require that serious adverse events, including deaths, be reported to the FDA. Whether those reports are then disclosed to the public in a timely and accessible manner is a separate question — one this case illustrates with particular clarity.

The company has confirmed it intends to publish full trial results in a peer-reviewed journal at some unspecified future date. Until that publication is available, the full scope of the trial’s safety and efficacy data remains inaccessible to independent researchers, clinicians, and the public.

An Industry Pattern Worth Noting

Sanofi is not the only RSV vaccine developer to have encountered difficulties in infant trials. Moderna previously halted its own mRNA-based RSV vaccine trial in babies following adverse side effects — a development that was similarly noted for how quietly it was handled before independent reporting brought it wider attention.

The pattern across multiple manufacturers — trial terminations, understated public disclosures, and safety signals emerging through investigative channels rather than proactive company communications — reflects a systemic issue in how clinical trial outcomes, particularly negative ones, reach the public record.

What Remains Unanswered

Several questions remain open in the Sanofi case. How was a child with congenital heart disease enrolled in a trial that listed congenital disorders as an exclusion criterion? What was the full scope of adverse events observed during the trial beyond the reported death? When will the peer-reviewed results be published, and will they include complete safety data? And why was the infant death not included in the company’s public disclosures when it ended the trial in October 2024?

Sanofi’s position — supported by the IDMC — is that the vaccine was not the underlying cause of the child’s death. That determination may ultimately prove correct. But the manner in which the information came to light, through a FOIA request filed nearly two years after the trial ended, does not reflect the kind of proactive transparency that clinical research involving infants demands.

The peer-reviewed publication, when it arrives, will be the appropriate venue for independent scientific assessment of those claims. Until then, the public record contains a terminated trial, a company statement citing poor efficacy, and a child’s death that was not part of the original announcement.

This article draws on reporting from Activist Post / The Defender, Children’s Health Defense, data from ClinicalTrials.gov (NCT04491877), guidance from the CDC, and research published in Vaccines (PMC).