A “substantial proportion” of infants immunized against the RSV virus with the monoclonal antibody shot nirsevimab — marketed by Sanofi and AstraZeneca as Beyfortus — developed a nirsevimab-resistant strain of the illness, according to a French study.
Over 12.5% of infants with breakthrough cases of RSV following immunization had variants of the illness with “intermediate to high-level resistance” to nirsevimab, including new strains of the virus not seen previously.
“The emergence of RSV-resistance described by this paper is simply incredible,” Children’s Health Defense Senior Scientist Karl Jablonowski said. He added:
“Most studies that shed light on the emergence of pathogens resistant to prophylaxis are population-wide and after many years of their use. Examples include the pertussis vaccine, the pneumococcal vaccine and the meningitis vaccine.
“This paper appears to show, with a high degree of certainty, the genesis of resistance.”
The study, designed to monitor escape variants of the virus and published on The Lancet’s preprint server, was conducted by a team of researchers from hospitals and research institutions across France.
The preprint follows a peer-reviewed paper published in The Lancet earlier this year by some of the same authors showing escape variants among a smaller number of infants. However, that study concluded the emergence of mutated variants was rarer.
The RSV monoclonal antibody shots were approved by the European Union and the U.K. in November 2022 and by the U.S. Food and Drug Administration in July 2023.
In September 2023, France was one of the first countries to launch a national immunization campaign with nirsevimab for newborns.
Many countries, including the U.S., followed France’s lead. The Centers for Disease Control and Prevention (CDC) added the shots to the childhood immunization schedule, recommending that either pregnant mothers get vaccinated or newborns get the monoclonal antibody shot.
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