Moderna’s experimental RSV vaccines face fresh safety questions over infant trial

Fresh concerns have emerged over the safety of Moderna’s experimental respiratory syncytial virus (RSV) vaccines for infants following the disclosure of 13 suspected serious adverse drug reactions recorded during a clinical trial in the United Kingdom.

The latest information, obtained from the UK Medicines and Healthcare products Regulatory Agency (MHRA), has raised questions about the safety monitoring and reporting of the trial, which was suspended after vaccinated infants developed severe RSV infections at higher rates than those who received a placebo.

The trial, known as the Rhyme Trial, evaluated two experimental vaccines: an RSV vaccine, already licensed for some adults as mRESVIA, and a combined RSV and human metapneumovirus (hMPV) vaccine.

Moderna paused the trial in July 2024 after severe RSV cases among vaccinated infants reached a predetermined safety threshold. The US Food and Drug Administration (FDA) subsequently placed the study on formal clinical hold in December 2024.

According to the newly obtained UK records, ongoing monitoring of participants generated 13 reports of Suspected Unexpected Serious Adverse Drug Reactions (SUSARs).

SUSARs refer to serious and unexpected reactions suspected to be associated with a drug or vaccine, potentially resulting in death, life-threatening conditions, disability or prolonged hospitalisation.

Dr. Peter Selley, a physician who reviewed the findings alongside Karl Jablonowski, a senior research scientist at Children’s Health Defense, questioned why the nature of the 13 reported reactions had not been publicly explained.

“It is remarkable that MHRA states that none of the 13 SUSARs were due to Enhanced Respiratory Disease. Perhaps they are classifying it differently. Moderna should come clean as to what these SUSARs are,” Selley said.

The researchers also raised concerns about an animal study Moderna submitted to support its application for human trials.

The study, published in 2025, examined the vaccines in mice and cotton rats. According to Selley and Jablonowski, their independent review of the cotton rat data identified signs of lung inflammation among vaccinated animals subsequently exposed to RSV.

Lung inflammation is among the symptoms associated with vaccine-associated enhanced respiratory disease (VAERD), a condition in which vaccinated individuals experience more severe respiratory illness following exposure to the actual virus.

However, Moderna’s researchers had concluded that the vaccinated rats did not develop enhanced lung inflammation after exposure to RSV.

READ ALSO; Experts urge suspension of Moderna’s mRNA Flu vaccine over reported adverse reactions

Selley and Jablonowski disputed that interpretation, arguing that the statistical methods used in the study may have concealed important differences between vaccinated and control animals.

They particularly questioned the use of median scores and a percentage conversion system to assess inflammation, saying the approach could obscure severe reactions in individual animals.

Selley said his recalculation of the available figures showed more signs of inflammation in several lung tissues among vaccinated animals than in placebo groups.

Jablonowski similarly reported what he described as a mild but consistent inflammation signal that was not highlighted in the published study.

The concerns come against the background of Moderna’s early trial results, which showed that five of 40 vaccinated infants aged five to seven months developed severe RSV illness, compared with one of 20 infants who received a placebo.

Documents obtained from the MHRA reportedly indicate that additional severe RSV cases and hospitalisations were recorded during subsequent monitoring in 2025.

However, Moderna has not publicly released the updated findings, despite previously indicating that trial data would be shared as they became available.

The FDA’s December 2024 decision to halt the study cited an unreasonable and significant risk of illness or injury to participants and insufficient information in Moderna’s investigational new drug application to adequately assess the risks.

The development has renewed scrutiny of the history of RSV vaccine research, particularly following earlier trials involving severe respiratory complications in vaccinated children.

In the 1960s, an experimental RSV vaccine was associated with severe illness and two deaths among vaccinated toddlers. More recently, GSK discontinued a maternal RSV vaccine trial in 2023 following concerns about increased preterm births, while Sanofi halted a paediatric RSV vaccine trial in 2024.

Meanwhile, questions remain about the 13 SUSAR reports, the interpretation of Moderna’s animal study and the complete safety outcomes of the suspended infant trial.