Announcement • Aug 22
Sanofi Shuts Down RSV Vaccine Clinical Trial For Infants And Toddlers After Infant Death
Sanofi shut down its RSV vaccine trial for infants and toddlers in 2024 after a baby died. It was later determined that the baby had an underlying congenital heart defect, which should have disqualified the infant from participating in the trial. A letter obtained through a Freedom of Information request reveals that Sanofi told the operators of its clinical trial sites that it paused the trial to re-educate all participating sites and investigators on the criteria for determining eligibility for participating in the trials. The letter indicates that it is a follow-up letter to previous Dear Investigator Letters issued in December 2024, January 2025 and March 2025. All of the letters referred to pausing the RSV trial for infants and toddlers. The infant's death constituted a serious adverse event that triggered the halt. Research published in the Pediatric Infectious Disease Journal indicates that most children who die from respiratory syncytial virus, or RSV, have comorbidities such as congenital heart or lung disease. Congenital disorders should have excluded babies from the study, and participants should have been assessed for preexisting conditions, according to the clinical trial's exclusion criteria. The FDA classifies failure to adhere to inclusion and exclusion criteria as an important protocol deviation -- and notes that violations must be investigated. Failures of this kind have triggered FDA warning letters to individual investigators in other trials, including cases in which subjects were enrolled even though their health histories should have disqualified them based on trial protocols designed to screen out certain health conditions. In May 2026, The Defender submitted a Freedom of Information Act request to the FDA seeking communications about the reported infant death in the Sanofi trial. The FOIA request specifically sought a Dear Investigator Letter, issued by Sanofi on December 13, 2024, that referenced a pause in the trial due to a Fatal SAE -- a fatal serious adverse event. The letter would have been sent to 130 investigators around the world conducting Sanofi's studies. Such letters are typically used to formally notify clinicians of important information that immediately affects a trial. The FDA has not acknowledged or responded to The Defender's FOIA request, and England's National Health Service has declined to provide Selley with more information in response to his requests. Beginning in 2023, Sanofi sponsored the Phase 3 PEARL trial to test its genetically modified intranasal RSV vaccine, RSVt, in healthy children ages 6-22 months. The Phase 3 trial followed a smaller Phase 2 trial that reported promising results with no safety concerns. The Phase 3 trial, which enrolled 6,300 children in the U.S. and globally, tested the vaccine against a placebo. Children were given two doses of the vaccine and monitored for safety for 24 months following the shots. Sanofi launched the study in 2023, according to the federal trials database. The trial was set to run through 2027. However, in October 2025, Sanofi told shareholders that it had pulled the plug on the trial. The public announcement came 10 months after Sanofi had paused the trial following the infant death. Sanofi never restarted the trial, according to a company spokesperson. The company didn't report any safety issues with the vaccine, stating instead that it ended the trial because the vaccine wasn't as effective as hoped. The safety profile was acceptable, and no signals of vaccine-associated enhanced respiratory disease were observed by the IDMC. Vaccine-associated enhanced respiratory disease, or VAERD, is a severe respiratory illness that develops when a vaccinated child who never had RSV is exposed to the virus and develops a more severe case of RSV than it would have if the child hadn't received the vaccine. Previous attempts to develop an RSV vaccine have been stymied by VAERD. Since the 1960s, RSV vaccine development has been riddled with serious safety concerns involving young children since the 1960s. An early National Institutes of Health experimental RSV vaccine was tested in children in the early 1960s, followed by a more concentrated formulation tested in low-income, Black infants -- without their families' knowledge or consent -- in 1965-66. Vaccinated children experienced unexpectedly severe RSV illness, and two infants later died after developing serious respiratory illnesses associated with VAERD. The episode stalled RSV vaccine development for decades. When RSV vaccine research accelerated again in recent years, safety questions persisted, particularly around pregnancy and young children. GSK halted a maternal RSV vaccine trial after a preterm-birth safety signal, while Pfizer's FDA-approved maternal vaccine has also been flagged for an association with preterm birth. The infant monoclonal antibody Beyfortus -- which is not a vaccine, but an immunization that provides infants with antibodies to temporarily protect against RSV -- was approved in 2023 -- 12 child deaths occurred during its clinical trials, although investigators said they were unrelated to the product. Adverse-event reporting of infant deaths has since drawn additional scrutiny. More recently, Moderna's experimental mRNA RSV vaccines for young children raised renewed concerns about the same phenomenon encountered in the 1960s. According to an FDA briefing document, data from Moderna's pediatric trial raised a potential VAERD safety signal after severe RSV illness occurred more frequently among vaccine recipients, prompting the FDA to pause enrollment in pediatric RSV vaccine studies involving RSV-naive children. Moderna halted its trial in 2024. Despite these red flags, research into RSV vaccines for infants and young children has continued, with some pediatric studies on the government clinical trials website listed as active or recruiting.