During the previous epidemiological season, Spanish researchers detected an RSV variant with reduced susceptibility to nirsevimab in three paediatric infections. Two babies needed hospital care; one developed severe bronchiolitis and required intensive care and respiratory support. The cases were identified by the Centro Nacional de Microbiología of the Instituto de Salud Carlos III (ISCIII).
Genetic analysis found the N201T substitution in the F protein of RSV-B. The change is linked to reduced susceptibility to nirsevimab, the monoclonal antibody used in Spain to prevent severe respiratory syncytial virus disease in babies. The ISCIII said this is the first documented signal in Spain involving an RSV variant with this potential escape-associated change. N201T is located in antigenic site Ø of the RSV F protein, a basic target of nirsevimab, and researchers are investigating it as a possible mechanism of reduced antibody sensitivity.
The finding covers only three reported paediatric infections, but two of the children became seriously ill. One spent seven days in hospital. Another spent two days in a paediatric intensive care unit and received non-invasive BiPAP ventilation for severe bronchiolitis.
The third child was not hospitalised. That baby had received nirsevimab during the 2023/24 season, not during the season when the infection was identified. This limits the extent to which the case can be considered a direct breakthrough during active protection.
The results were published in Eurosurveillance, the journal associated with the European Centre for Disease Prevention and Control. María Iglesias Caballero, Vicente Mas and Inmaculada Casas led the study from the CNM-ISCIII Respiratory Virus Laboratory. Hospital Universitario de Badajoz and Hospital Universitario Virgen del Rocío de Sevilla also supported the work.
The mutation does not appear to be an isolated genetic event. Researchers found that it circulates naturally and has emerged more than once in different viral lineages. They said genomic surveillance must be paired with antigenic testing to identify and assess variants that can evade preventive treatments early.
RELECOV carries out that monitoring through a network linking microbiology laboratories in all of Spain's autonomous communities. Since the pandemic, the network has sequenced respiratory viruses under the coordination of the CNM-ISCIII. The finding adds another detail to the respiratory picture described in an earlier respiratory report about infections circulating in Spain.
Respiratory syncytial virus most often affects children during their first year of life and can cause bronchiolitis in severe cases. Estimates cited in the study indicate that the virus affects one in four children during their first 12 months. Around 14% need medical attention, nearly 2% require hospitalisation and 0.1% need intensive care. In Spain, outbreaks usually occur between November and January.
Nirsevimab has been used in Spain since 2023 and became part of the national prevention strategy for the 2023/24 season. Unlike a vaccine, it supplies antibodies directly and targets the virus's F protein, which the virus needs to enter cells. A single dose is designed to protect a child through one RSV season. Earlier ISCIII research found an 80% lower risk of RSV hospitalisation among immunised babies under one year old. Spanish regional campaigns have also reported large reductions in bronchiolitis- and RSV-related hospitalisations since the preventive antibody was introduced. In 2026, regional health services in Spain continued expanding RSV prevention campaigns using a single dose of nirsevimab for newborns and infants.
Protection remains large, but it is not absolute. The Eurosurveillance study describes the three infections as an early surveillance signal, not evidence of widespread prevention failure. It says the cases underline the need to monitor the antibody's performance as the virus changes. RELECOV and genomic analysis detected the escape-associated mutation while researchers could still study the cases in detail.
