Slim Fourati, Slim Fourati, Alawiya Reslan, Jérôme Bourret, Jean-Sébastien Casalegno, Yannis Rahou, Laurent Softic, Lionel Chollet, Pauline Trémeaux, Berthe-Marie Imbert-Marcille, Sylvie Pillet, Nicolas Veyrenche, Agathe Boudet, Luc Deroche, Sonia Burrel, Lina Mouna, Theophile Cocherie, Aurélie Schnuriger, Charlotte Pronier, Lynda Handala, Véronique Brodard, Floriane Gallais, Sylvie Larrat, Alexandre Regueme, Nefert Candace Dossou, Amandine Henry, Elyanne Gault, Cédric Hartard, Ségolène Brichler, Alice Moisan, Véronique Avettand-Fenoel, Caroline Lefeuvre, Ilka Engelmann, Sandrine Castelain, Audrey Mirand, Maud Salmona, Marie-Christine Jaffar-Bandjee, Mohamed Ader, Sarah Seng, Pierre Cappy, Christophe Rodriguez, Arnaud Ly, Georges Dos Santos, Olivier Schwartz, Chloé Petiot, Delphine Planas, Samar Berreira Ibraim, Kévin Da Silva, Amélie Recoing, Jean-Michel Pawlotsky, Frédéric Lemoine, Etienne Audureau, Marie-Anne Rameix-Welti, Marie-Anne Rameix-Welti, N. Zemali, Sonia Burrel, Pantxika Bellecave, Alice Moisan, Zakasoa-Mbololona Zavaoarisaina, Romain Legros, Boris Derman, Vincent Pargny, Hortense Petat, Jean‐Christophe Plantier, Véronique Avettand-Fenoel, Salim Ferrani, Jérôme Guinard, Clémence Guillaume, Gilbert Mchantaf, Victoria Marie, Laurent Bret, Fabien Lesne, Anthony de Oliveira, Alexandre Regueme, Kazali Alidjinou, Lionel Chollet, Vincent Gardan, Ségolène Brichler, Loic de Pontual, Camille Aupiais, Stéphane Marot, Aurélie Schnuriger, Marine Perrier, Pierre Jatteau, Djeneba Fofana, Théophile Cocherie, Elisa Teyssou, Cathia Soulié, Vincent Cálvez, Sylvie Larrat, Anne Faisant, Guillaume Mortamet, Caroline Tournegros, Mohamed Habib, Sylvie Pillet, Aymeric Cantais, Franck Zekre, Thomas Bourlet, Oulfa Boussetta-Charfi, Sara Chenafi-Adham
BACKGROUND: Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infection in infants. Nirsevimab, a long-acting monoclonal antibody targeting a conserved epitope on the prefusion F protein (site Φ), has shown high efficacy in clinical trials and early real-world studies. Although widespread resistance has not been reported, concerns remain about the emergence of escape variants, particularly among RSV-B viruses. During the 2024-25 RSV season in France, RSV-B predominated, providing a unique opportunity to examine breakthrough infections with RSV-B and resistance at a large scale. The study aimed to characterise RSV escape from nirsevimab using genotypic and phenotypic methods. METHODS: This POLYRES-2 project was a multicentre, national, observational study conducted in hospital settings (inpatients and outpatients) across France during the 2024-25 RSV season. We included infants aged 1 year or under with a RT-PCR-confirmed RSV infection in routine care, regardless of whether they had received nirsevimab. Infants were identified through hospital virology laboratory databases. Each participating centre was requested to include a balanced number of nirsevimab-exposed and non-exposed infected infants throughout the study period. Clinical data were retrieved from electronic medical records. We compared RSV susceptibility to nirsevimab in infants who received nirsevimab with that in nirsevimab-naive infants. Respiratory samples were sequenced for full-length RSV genomes. To ensure reliability, phylogenetic and mutational analyses were restricted to high-quality sequences with greater than or equal to 90% genome coverage and complete reads across the nirsevimab-binding site. Clinical RSV isolates were tested for neutralisation by nirsevimab. We analysed F candidate substitutions using a fusion inhibition assay. The primary outcomes were presence of resistance-associated substitutions (RASs) in the RSV F protein (site Φ) and phenotypic resistance to nirsevimab. FINDINGS: Among 1023 RSV-infected infants, 858 (83·9%) had full-length RSV genome sequences: 419 (48·8%) from nirsevimab-treated breakthrough infections (212 [50·6%] RSV-A, 207 [49·4%] RSV-B) and 439 (51·2%) from nirsevimab-naive infants (192 [43·7%] RSV-A, 247 [56·3%] RSV-B). RASs were identified in two of 195 RSV-A breakthrough infections (1·0%) and in 23 of 184 RSV-B breakthrough infections (12·5%). In RSV-A, the only RAS was F:K209E, conferring intermediate resistance. In RSV-B, resistance was more frequent and diverse than in RSV-A: 12 of 23 (52.2%) resistant viruses carried a substitution at residue 208 (F:N208D, F:N208I, F:N208K, F:N208S, or F:N208Y). Additional novel substitutions, including F:I64V/F:K65E, F:K68I, F:L204S, and F:P205S, also mediated resistance. Notably, a resistant RSV-B variant (F:N208S) was detected almost 1 year after prophylaxis. No resistant RSV was detected in nirsevimab-naive infants. INTERPRETATION: Resistance to nirsevimab in RSV-B can emerge in real-world settings, affecting around 12% of breakthrough infections and showing greater diversity than previously recognised, although the clinical impact remains constrained by available evidence. Detection of resistant variants long after prophylaxis highlights the need for extended genomic surveillance. Integration of clinical and virological data will be essential to sustain the long-term effectiveness of RSV monoclonal antibody programmes. FUNDING: This study was supported by a grant from the Agence Nationale de Recherche sur le Sida et les hépatites virales - Maladies Infectieuses Emergentes and the French Ministry of Health and Prevention.