Infants treated for RSV with the monoclonal antibody nirsevimab have significantly higher mortality rates than those treated with a different monoclonal antibody or with a placebo, according to an analysis by the Japanese journal Med Check of three major randomized control trials.
The report reanalyzed clinical trial data showing that nirsevimab — marketed by Sanofi and AstraZeneca as Beyfortus — reduced RSV-associated lower respiratory tract infection and hospitalizations in both high-risk and healthy infants.
However, babies treated with the drug had a higher mortality rate likely linked to adverse effects — including thrombosis (blood clotting) — from the drug itself, the study found.
“Due to the increased mortality, nirsevimab should not be used in any infants,” the study concluded. “Do not use nirsevimab for universal immunization.”
Shortly after its approval by the European Medicines Association in October 2022, and fast-track approval by the U.S. Food and Drug Administration in July 2023, the U.S., France, Spain and Luxembourg launched universal Beyfortus infant immunization campaigns for the 2023-24 RSV season.
Med Check published its appraisal of the clinical trials shortly after France’s National Agency for the Safety of Medicines and Health Products (ANSM) published its first pharmacovigilance data on the drug, compiled during the 2023-24 season.
The ANSM report tracked adverse events related to Beyfortus in France between Sept. 11, 2023, and April 30, 2024. Of 244,495 doses delivered, 198 adverse events were reported to the pharmacovigilance system, of which 153 were considered serious.
The report identified safety signals for stroke, respiratory conditions and hypotonic-hyporesponsive episodes — when an infant suddenly loses muscle strength and becomes “floppy.” Hypotonic-hyporesponsive episodes also are associated with the diphtheria-tetanus-pertussis, or Tdap, vaccine and other vaccines.
There were also three reports of sudden infant death syndrome (SIDS), although researchers said at least one of them was likely not linked to the drug.
The ANSM said each of the signals would be closely monitored in the future, but that a causal link between Beyfortus and those adverse events had not yet been established. The agency concluded the results confirm that the benefits of using the drug to treat bronchiolitis, an RSV infection, outweigh the risks.
The ANSM continues to recommend all newborn babies receive the Beyfortus shot this RSV season.
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