Respiratory syncytial virus, or RSV, is the leading cause of bronchiolitis, a viral infection of the small airways that sends hundreds of thousands of infants to hospitals across Europe each winter. For decades, pediatricians had little to offer beyond supportive care, since a previous antibody product, palivizumab, was reserved for high-risk infants because of its high cost and the need for monthly injections. Nirsevimab changed that calculus. A single dose provides protection lasting at least five months, covering an entire RSV season, and clinical trials demonstrated strong efficacy against medically attended lower respiratory tract infection in healthy term and preterm infants alike.

When nirsevimab finally became available in Europe in late 2023, national and regional health authorities adopted it at different speeds and with different strategies. Some countries launched universal immunization programs targeting all infants entering their first RSV season, while others restricted initial campaigns to infants born during or just before the season, and a few had no program at all during the study window. This patchwork of implementation created an unintended natural experiment, and a large consortium of European pediatric emergency researchers seized the opportunity to measure what happened.

The research team, led by Léa Lenglart and Naim Ouldali with the EPISODES Study Group, assembled routinely collected data from 27 pediatric emergency departments spanning 12 European countries, covering the period from January 2018 through March 2024. They included every case of bronchiolitis diagnosed in infants younger than 12 months, yielding a total of 107,088 episodes. Regions were classified as intervention or control depending on whether nirsevimab had been implemented, and immunization coverage figures were drawn from official regional reports. Urinary tract infections in the same age group served as a control outcome, a standard technique to detect whether observed changes reflect a real intervention effect or some unrelated shift in emergency department attendance.

The analytical approach was a controlled interrupted time-series design, a method well suited to evaluating population-level interventions introduced at a known point in time. The models accounted for underlying seasonality and pre-existing temporal trends, allowing the researchers to estimate how much of the change in monthly bronchiolitis cases during the intervention period, from October 2023 through March 2024, could be attributed to the antibody program rather than to the ordinary rhythm of winter epidemics. Estimates were stratified by age group, separating infants aged 0 to 3 months, who face the highest risk of severe disease, from those aged 3 to 12 months.

The results were striking in both their magnitude and their heterogeneity. Reductions in bronchiolitis visits ranged from essentially no change, with one estimate of plus 0.6 percent, to a remarkable 61 percent decline, with a confidence interval stretching from minus 67.8 to minus 35.2 percent. Infants in the youngest age band generally showed the largest reductions, consistent with the biology of RSV, since the highest rates of hospitalization and intensive care admission occur in the first months of life. Meanwhile, bronchiolitis trends in control regions without nirsevimab programs remained stable, and urinary tract infection visits showed no comparable decline, strengthening the argument that the observed reductions were tied to the immunization campaigns rather than a general drop in emergency presentations.

Perhaps the most consequential finding was the strength of the statistical relationship between coverage and benefit. Across regions and age groups, the correlation between nirsevimab coverage and the percentage reduction in bronchiolitis visits was negative and pronounced, with a correlation coefficient of minus 0.86 and a p-value of 0.001. In practical terms, regions that immunized a larger share of their infant population saw proportionally larger drops in emergency department visits. The relationship held across both age strata, suggesting that the antibody’s protective effect translated into population benefit wherever it reached enough infants, whether they were immunized at birth during the season or through catch-up campaigns targeting older babies.

Why did coverage differ so much across Europe? The study points to implementation choices as the decisive variable. Countries and regions that embedded nirsevimab into routine newborn care, offering the dose in maternity wards before discharge, achieved far higher uptake than those relying on later appointments or opportunistic vaccination during other visits. Administrative hurdles, supply constraints, differences in eligibility criteria, and varying levels of parental and clinician acceptance all likely contributed. The authors emphasize that the antibody’s intrinsic efficacy, already demonstrated in randomized trials, was never in question; what varied was the fraction of the infant population that actually received it in time to be protected when the virus arrived.

The researchers are careful about the limits of their design. As an ecological study, it examines populations rather than individuals, so causality cannot be definitively inferred. Unmeasured regional differences, such as changes in testing practices, health-seeking behavior, viral circulation intensity, or concurrent public health measures, may have contributed to some of the observed variability. The interrupted time-series models adjust for seasonality and trend, and the stability of control outcomes argues against simple artifacts, but the authors acknowledge that residual confounding remains possible. Still, the consistency of the findings across 27 departments, multiple countries, and two age groups, together with the tight coverage-response relationship, makes a compelling case that immunization coverage was a key driver of the reductions.

The implications for health policy are immediate. RSV prevention has entered a new era, with nirsevimab programs now expanding and maternal RSV vaccines adding another layer of protection for newborns. This study demonstrates that the public health return on these interventions depends not only on the biology of the product but on the machinery of delivery. Regions that achieved high coverage among both season-born and out-of-season infants reaped reductions in bronchiolitis visits of a scale rarely seen for a respiratory disease, easing pressure on emergency services during the busiest weeks of the winter. For health authorities weighing how to structure their own programs, the European experience of 2023–2024 offers a clear lesson: the fastest, broadest, and earliest possible reach into the infant population is what converts a highly effective antibody into a visible decline in disease. As nirsevimab programs mature and additional seasons of data accumulate, researchers will be watching to see whether the coverage-response relationship holds, whether protection extends to severe outcomes such as intensive care admission, and whether the substantial regional variability in implementation narrows as systems learn from one another.

Subject of Research: Association between nirsevimab immunization coverage and reductions in infant bronchiolitis emergency department visits across 12 European countries

Article Title: Association between nirsevimab coverage and pediatric emergency department visits for bronchiolitis in 12 European countries: An interrupted time-series analysis

Article References: Lenglart, L., Titomanlio, L., Alberti, I., Almeida, L., Aupiais, C., Akyüz Özkan, E., Barrett, M., Basmaci, R., Birbilen, A., Borensztajn, D., Schönenberger, C. B., Bressan, S., Buonsenso, D., Campos, T., Castanhinha, S., Chiaretti, A., Claret, G., De Zan, F., Durnin, S., … the EPISODES Study Group (2026). Association between nirsevimab coverage and pediatric emergency department visits for bronchiolitis in 12 European countries: An interrupted time-series analysis. PLOS Medicine, 23(9), e1005223. https://doi.org/10.1371/journal.pmed.1005223

Image Credits: AI Generated

DOI: 10.1371/journal.pmed.1005223

Keywords: nirsevimab, RSV, bronchiolitis, infants, pediatric emergency departments, immunization coverage, interrupted time-series, Europe, PLOS Medicine, monoclonal antibody, respiratory syncytial virus, public health

The study, led by Shaolong Ren and colleagues working with the World Health Organization, is the first to quantify how the uneven introduction of nirsevimab, a long-acting monoclonal antibody licensed in 2022, and RSVpreF, a maternal vaccine licensed in 2023, could reshape global inequalities in infant RSV disease. The researchers built a static deterministic model that combined comprehensive epidemiological data on RSV burden, seasonality, and hospitalisation risk by birth month and age in months with country-level information on licensing and uptake as of June 2025. They then compared two futures: a world in which no country had introduced the products, and the world as it actually stood in mid-2025, with implementation frozen at that moment.

The baseline picture for 2019 revealed stark regional contrasts. The WHO African Region carried the highest incidence of RSV-associated acute lower respiratory infection, at 117.4 episodes per 1000 person-years, and the highest attributable mortality, yet paradoxically recorded the lowest hospital admission rate, just 12.6 per 1000 person-years. The Eastern Mediterranean Region, by contrast, had the highest admission rate at 29.9 per 1000. The African Region also showed the highest in-hospital case fatality ratio at 1.54 percent, closely followed by the South-East Asia Region at 1.49 percent. Crucially, in-hospital deaths accounted for only about 21 percent of RSV-attributable deaths in the African Region, meaning roughly four in five infants who died of RSV did so without ever reaching a hospital bed. Together, the African, South-East Asia and Eastern Mediterranean regions accounted for 88 percent of all infant RSV deaths.

The authors argue that the low hospitalisation rates in the highest-burden regions do not reflect milder disease. Instead, they point to barriers of affordability, distance and time to reach care. Severe RSV illness is treatable with supportive measures such as supplemental oxygen and intravenous hydration, but children in resource-limited settings often cannot access these in time. A recent study in Bangladesh underscored the point: 18.4 percent of children requiring hospitalisation were denied admission because of insufficient bed capacity, and those turned away faced a hazard ratio for death of 1.56 compared with children who were admitted. This pattern also implies, the researchers note, that preventive interventions could deliver even greater absolute benefit in poorly resourced settings, by averting the out-of-hospital deaths that currently dominate the mortality toll.

Against this backdrop, the implementation landscape as of June 2025 was strikingly skewed. Nirsevimab had been licensed in 54 countries and RSVpreF in 64, but only 16 and 12 countries respectively had incorporated them into their national immunisation programmes, and the overwhelming majority of those were high-income countries. The European Region accounted for 34 nirsevimab and 35 RSVpreF licences, and the Region of the Americas for 8 and 12, while the African Region had zero nirsevimab licences and just two for RSVpreF, and the South-East Asia Region only two of each. Of the countries that had licensed nirsevimab, 53 of 54 were high- or upper-middle-income; for RSVpreF the figure was 59 of 64. No low-income country had licensed either product.

The projections that followed were sobering. Under the June-2025 scenario, an estimated 4.64 million doses of nirsevimab and 1.96 million doses of RSVpreF would be administered annually, averting roughly 91,000 infant RSV hospital admissions and about 300 deaths worldwide. That translates to reductions of just 3.7 percent of global infant RSV admissions and 0.4 percent of deaths. The benefits were overwhelmingly concentrated in wealthy nations: high-income countries, which received about 90 percent of averted admissions and over 70 percent of averted deaths, saw reductions of 33.2 percent in hospitalisations and 29.1 percent in mortality. Middle-income countries saw relative reductions below 5 percent, and low-income countries, with no uptake at all, saw none.

The model’s assumptions were grounded in real-world evidence. Nirsevimab was assigned 83 percent effectiveness against RSV-associated hospital admission and 81 percent against attributable death, drawn from meta-analyses of real-world data; RSVpreF was assigned 71 and 77 percent respectively, informed by a test-negative study in Argentina. The model accounted for seasonal immunisation strategies with birth doses and catch-up doses for nirsevimab, year-round maternal vaccination in most countries, and mutual exclusivity between the two products to avoid double-counting protection. Uncertainty was propagated through 1000 simulation draws for each input parameter, and sensitivity analyses varying uptake, effectiveness, and alternative mortality data from the Global Burden of Disease Study 2023 all produced trends consistent with the main findings.

Most striking was what happened to measured inequality. The researchers ranked countries by RSV-attributable infant mortality and divided the global population into quintiles, then calculated the ratio of deaths between the highest- and lowest-risk fifths. Under no implementation, that ratio stood at 18.3, already a staggering expression of inequity. Under the June-2025 scenario it rose to 20.8, meaning the rollout of these lifesaving products is projected to make global RSV mortality inequality worse, not better. Within WHO regions, the increases were most pronounced where several countries had introduced the products: the ratio in the Region of the Americas climbed from 5.8 to 10.4, and in the European Region from 12.7 to 22.1. The authors warn that, drawing on the history of pneumococcal and rotavirus vaccines, coverage disparities between rich and poor countries can persist for years after initial introduction, so the true widening could exceed even these projections over the next five years.

There are signs that access may eventually improve. The WHO’s Strategic Advisory Group of Experts recommended infant RSV passive immunisation in late 2024, the WHO prequalified the maternal vaccine in March 2025, and Gavi’s board approved a funding window for RSV maternal immunisation programmes in July 2025. Yet the Gavi programme is not expected to introduce vaccines until 2028 and will initially reach only a handful of eligible countries. The study’s authors point to pooled procurement through UNICEF- and PAHO-led initiatives, the Gates Foundation-supported development of multi-dose vial formulations of the maternal vaccine, tiered pricing, technology transfer and stronger national immunisation advisory capacity as routes to accelerate adoption. At this juncture, they conclude, coordinated action by international agencies, governments, manufacturers and researchers is essential to ensure that the tools now transforming infant RSV outcomes in wealthy countries reach the infants who bear the greatest burden of the disease.

Subject of Research: Global regional inequality in infant RSV morbidity and mortality under expanded passive immunisation

Article Title: Regional inequality in infant RSV morbidity and mortality burden in the era of expanded RSV passive immunisation: a global projection modelling study

Article References: Ren, S., Cong, B., Zou, J., Guo, L., Nair, H., Sparrow, E., Feikin, D. R., & Li, Y. (2026). Regional inequality in infant RSV morbidity and mortality burden in the era of expanded RSV passive immunisation: a global projection modelling study. eClinicalMedicine, 100, Article 104219. https://doi.org/10.1016/j.eclinm.2026.104219

Image Credits: AI Generated

DOI: 10.1016/j.eclinm.2026.104219

Keywords: RSV, nirsevimab, RSVpreF, infant health, global health equity, modelling study, low- and middle-income countries, hospitalisation, maternal vaccination, monoclonal antibodies, WHO, vaccine access

Researchers at the University of Veterinary Medicine Hannover set out to disentangle two factors that had previously been confounded in studies of RSV G protein heterogeneity. Earlier work had shown that G proteins made in primary human airway epithelial cultures, including air-liquid interface systems, reach an apparent molecular weight of approximately 170 kilodaltons and are more heavily glycosylated than the 55 to 90 kilodalton species typical of transformed lines such as HEp-2 or A549. Other experiments had shown that, within a single complete medium, different continuous cell lines produce G proteins differing in electrophoretic mobility and in reactivity with G-specific monoclonal antibodies. But in those comparisons either the cell line varied while the medium was held constant, or both cell identity and medium changed together. By varying the medium within a single cell background, the new study isolates the contribution of the medium itself from that of the cell.

The rationale for suspecting such an effect is grounded in glycobiology. Culture medium composition can influence the availability of nucleotide sugar donors, the activated sugars that glycosyltransferases in the Golgi apparatus attach to nascent glycoproteins, and can modulate the activity of glycosylation pathways more broadly. Because G protein glycans are likely to shape antigenic masking and functional behaviour of the virus, medium-dependent metabolic effects were a plausible driver of the heterogeneity reported across systems. The question also matters for antibody-based interventions: approved therapeutic monoclonal antibodies against RSV, including palivizumab, nirsevimab and clesrovimab, are all directed at the fusion F protein, yet virion composition and glycoprotein organisation may influence antibody binding and neutralising activity.

To test the principle directly, the team first used a recombinant expression system. A soluble form of the RSV G ectodomain, tagged for purification, was produced in HEK293T cells maintained in three different commercial media, designated A, B and C, which had been selected from an initial screen of nine formulations to represent the extremes and midpoint of the observed range. Purified proteins were analysed by SDS-PAGE and Western blot. The apparent molecular weight of the soluble G protein differed substantially depending on the medium: protein produced in medium A was the smallest at roughly 60 to 90 kilodaltons, medium B yielded an intermediate species of about 100 kilodaltons, and medium C produced the largest form at approximately 100 to 130 kilodaltons.

The critical control came from glycosylation-deficient cells. When the same soluble G protein was expressed in HEK293T cells knocked out for the GALE and GALK1 genes, which are deficient in galactose metabolism, the medium-dependent size differences vanished entirely. In these knockout cells the protein appeared as a lower molecular weight cluster of roughly 15 to 25 kilodaltons, consistent with incompletely processed, non-galactosylated glycans that the knockout does not eliminate. This result demonstrated that the differences in apparent size were caused by glycosylation and that medium composition, not cell-type-specific differences in the glycosylation machinery, defined the extent of that glycosylation. Importantly, the same hierarchy of medium-dependent molecular weights was reproduced during authentic infection: when A549 cells were infected with a reporter RSV strain in the three media and lysed 24 hours later, Western blots showed the same pattern, albeit with smaller absolute differences.

The team then asked whether the medium affects the biological properties of virus preparations. A549 cells were infected with an EGFP-expressing RSV at a multiplicity of one in each medium, with Opti-MEM, the reduced-serum medium B commonly used for RSV propagation, serving as the reference condition. Virus spread kinetics, quantified by integrated GFP fluorescence over 120 hours with a live-cell imaging system, revealed pronounced medium-dependent differences, with spread significantly higher in medium C than in media A and B. More strikingly, when virus stocks produced in each medium were used to infect fresh A549 cells at equal infectious doses and the inoculum was replaced with Opti-MEM after adsorption, the differences persisted: medium C virus spread most, medium B virus showed an intermediate phenotype, and medium A virus remained markedly attenuated. Because the production medium was no longer present during the infection period, and because a difference confined to the initial inoculum would be expected to produce comparable spread curves with a delay rather than a sustained gap, the result points to a stock-intrinsic property that also affects subsequent rounds of infection.

Analyses of the virus stocks themselves added a further layer. When stocks were adjusted to equal total protein concentrations and probed by Western blot, the relative G protein signal normalised to the viral N protein was lower for virus produced in media A and B than for virus produced in medium C, while relative F protein signals did not differ significantly between preparations. The G protein with the highest apparent molecular weight, consistent with the most extensive glycosylation, was thus associated with the highest relative G to N ratio, an association consistent with a link between G protein maturation and its relative abundance in stocks, though the authors are careful to note that this does not establish causality. A comparison of RT-qPCR signals with infectious titres proved revealing: virus produced in medium A yielded higher Cq values than medium C virus, yet its infectious titre was approximately 100-fold lower, indicating that the ratio of infectious to RT-qPCR-detectable viral material differed between preparations.

The consequences extended to antibody recognition. Fluorescently labelled palivizumab, which binds both pre-fusion and post-fusion F, and the pre-F-specific antibodies nirsevimab and clesrovimab were used to stain infected cells cultured in the three media, with signals normalised to the GFP reporter. Binding of all three therapeutic antibodies was significantly lower in cells cultured in media B and C than in medium A, whereas a post-F-specific control antibody showed largely unchanged binding. Because relative F protein signals in the virus stocks did not differ, these binding differences were not accompanied by detectable differences in total F protein abundance, though the analysis did not distinguish F conformations. In neutralisation assays, palivizumab consistently showed the highest IC50 values at approximately 1.5 micrograms per millilitre, with nirsevimab and clesrovimab more potent, and no statistically significant differences in IC50 emerged between viruses produced in media B and C, indicating that the altered binding signals did not translate into measurable differences in neutralising potency under the conditions tested.

The authors conclude that the choice of culture medium is an important variable that should be considered whenever RSV preparations are produced for studies of the virus’s biological and immunological properties. Several caveats frame the findings: the precise compositions of the commercial media are proprietary, precluding identification of the responsible components; medium B is reduced-serum rather than serum-free, so its effects may partly reflect serum content and lower nutrient availability; the study used transformed cell lines and a single reporter virus strain, so whether similar effects occur in primary airway epithelial, air-liquid interface or organoid systems remains open. Notably, such advanced cultures are maintained in differentiation media that differ substantially from those used for continuous lines, raising the possibility that the heavier glycosylation of G reported in primary cultures reflects the medium as well as cell identity. Resolving the exact medium-dependent glycan patterns will require site-resolved glycoproteomics, and the F protein’s glycosylation and conformational distribution merit parallel investigation. For now, the message to the field is clear: two laboratories growing the same virus in the same cells but different media may not be studying the same virus at all.

Subject of Research: Effect of cell culture medium composition on respiratory syncytial virus G protein glycosylation and monoclonal antibody recognition

Article Title: Cell culture medium influences RSV G protein glycosylation and recognition by monoclonal antibodies

Article References: Engling, N., Knittler, M.-C., Tagore, R., Ludlow, M., Osterhaus, A. D., Rimmelzwaan, G. F., & Meineke, R. (2026). Cell culture medium influences RSV G protein glycosylation and recognition by monoclonal antibodies. Virology Journal, 23(1), Article 226. https://doi.org/10.1186/s12985-026-03318-8

Image Credits: AI Generated

DOI: 10.1186/s12985-026-03318-8

Keywords: respiratory syncytial virus, glycosylation, G glycoprotein, cell culture medium, monoclonal antibodies, palivizumab, nirsevimab, clesrovimab, glycobiology, viral infectivity, HEK293T cells, A549 cells

The research team collected RSV-positive upper respiratory tract samples from patients who tested positive between October 2023 and March 2025, spanning two consecutive seasons that straddled the deployment of nirsevimab. Whole viral genomes were recovered using an amplicon-based sequencing protocol carried out on Illumina platforms, with assemblies generated through a consensus-building pipeline known as vcf-consensus-builder. This approach allowed the investigators to capture near-complete genomes from a substantial fraction of positive samples, providing the resolution needed to place each virus within a fine-grained phylogenetic framework and to examine amino acid variation in the proteins most relevant to prevention and treatment.

In total, the study retrieved 302 RSV genome sequences, with 129 derived from the 2023–2024 season and 173 from the 2024–2025 season. The clinical composition of the cohort reflects the burden of RSV disease in the region: most infections occurred in infants aged one year or younger, who accounted for 56.0 percent of cases, and males made up 57.3 percent of the sampled population. Lower respiratory tract infections were documented in 72.7 percent of cases, 67.2 percent of patients required hospitalization, and 15.0 percent were admitted to an intensive care unit. Notably, the dataset included 17 nirsevimab breakthrough infections, cases in which infants who had received the monoclonal antibody nonetheless developed symptomatic RSV disease.

Phylogenetic analysis using maximum-likelihood methods implemented in IQ-TREE, with a GTR plus F, invariant sites, and gamma model of substitution, assigned the circulating viruses to established clades within the two major RSV subgroups. The most frequently detected clades were A.D.3, representing 26.8 percent of sequences, and B.D.E.1, representing 20.2 percent, followed by A.D.1.2 at 17.5 percent and A.D.5.2 at 14.2 percent. This distribution confirms that multiple lineages of both RSV-A and RSV-B co-circulated in Lombardy throughout the study period, with no single clade achieving complete dominance across the two seasons.

Beyond clade assignment, the researchers applied Bayesian coalescent methods under a constant population size prior and a strict molecular clock to define what they termed large phylogenetic clusters, or LPCs. These were groups of more than ten sequences supported by a posterior probability of one, indicating tightly linked transmission chains or rapidly expanding lineages. Four such clusters were identified, and two of them proved particularly striking: a cluster of 56 A.D.3 strains and another of 17 A.D.1.2 strains were detected exclusively in the 2024–2025 season, the first season of routine nirsevimab use. The emergence of season-specific clusters suggests that new viral lineages established themselves in the population after the prophylaxis program began.

A central concern of the study was the monitoring of mutations associated with resistance to the preventive and therapeutic agents now deployed against RSV. The fusion protein, or F, is the target of the monoclonal antibodies palivizumab, nirsevimab, and clesrovimab, while the nucleoprotein, or N, is the target of the small-molecule inhibitor zelicapavir. Resistance-associated mutations were identified in two samples from the 2023–2024 season and two from the 2024–2025 season. These included K272E in an A.D.5.3 strain and N262Y in an A.D.3 strain, both associated with reduced susceptibility to palivizumab, and N208D in a B.D.E.1 strain as well as the combination of K68I and L204S in a B.D.E.1 nirsevimab breakthrough infection, mutations linked to nirsevimab resistance.

Among the 17 nirsevimab breakthrough infections, the investigators also detected the S213R substitution at antigenic site Ø of the F protein in an A.D.1.2 sample. Site Ø is the principal binding site for nirsevimab, and changes at this region can diminish antibody binding. By contrast, no mutations conferring resistance to clesrovimab, a newer monoclonal antibody that binds a different region of F, were detected during the study. However, four substitutions were observed at the clesrovimab binding site: K433R in a B.D.4.1.1 strain, C439R in an A.D.1.2 strain, N437S in an A.D.5.2 strain, and N444S in an A.D.3 strain, with three of the four appearing during the 2024–2025 season. These variants were not classified as resistance mutations, but their presence at a pharmacologically critical epitope warrants continued attention.

The nucleoprotein analysis likewise revealed signals relevant to antiviral therapy. The I129M substitution in the N protein, which is associated with resistance to zelicapavir, was detected in a B.D.E.1 sample from the 2024–2025 season at an intrapatient relative abundance of 0.05, indicating that the variant existed as a minor species within the quasispecies of that infection rather than as the dominant viral population. Such low-frequency variants are typically invisible to conventional diagnostic assays but can be resolved by deep sequencing, and their detection underscores the value of high-resolution genomic surveillance in identifying resistance determinants before they become established at the population level.

Multivariable analysis comparing the two seasons revealed several statistically significant differences associated with the 2024–2025 period. That season was characterized by an increased number of sequences belonging to large phylogenetic clusters, greater amino acid variability at antigenic site Ø of the F protein, and higher Ct values on diagnostic testing, which correspond to lower viral loads in respiratory samples. Hospital intensive care unit admissions were also fewer in the second season. While the observational design of the study precludes drawing direct causal links between nirsevimab deployment and these shifts, the concurrence of prophylaxis introduction with reduced disease severity, altered viral population structure, and expanded F protein variability provides a compelling rationale for sustained monitoring.

The overall message from the Lombardy surveillance program is one of cautious reassurance combined with vigilance. Nirsevimab breakthrough infections remained rare relative to the scale of the prophylaxis campaign, and bona fide resistance mutations were detected in only a handful of samples across two seasons. At the same time, the virus is demonstrably diversifying, with season-specific phylogenetic clusters and accumulating variation at antibody binding sites on the F protein. As monoclonal antibodies and vaccines become mainstays of RSV prevention worldwide, the Lombardy experience illustrates how whole-genome sequencing, phylogenetic clustering, and mutation-level analysis can be woven into routine public health practice, ensuring that shifts in viral evolution are detected early enough to inform and adjust prophylaxis strategies before they compromise protection of the most vulnerable patients.

Subject of Research: Genomic evolution and monoclonal antibody resistance of respiratory syncytial virus in Lombardy before and after nirsevimab introduction

Article Title: Genomic characterization of circulating respiratory syncytial virus strains in Lombardy before and after the introduction of nirsevimab prophylaxis

Article References: Uceda Renteria, S., Galli, C., Ferrari, G., Cossu, M. V., Lombardi, A., Tagliabue, C., Galli, G., Parisi, A., Pellegrinelli, L., Romano, G., Bono, P., Pitrolo, A. M. G., Salpini, R., Salvini, F., Svicher, V., Bosis, S., Callegaro, A., Gori, A., Baldanti, F., … Alteri, C. (2026). Genomic characterization of circulating respiratory syncytial virus strains in Lombardy before and after the introduction of nirsevimab prophylaxis. Genome Medicine. https://doi.org/10.1186/s13073-026-01768-x

Image Credits: AI Generated

DOI: 10.1186/s13073-026-01768-x

Keywords: respiratory syncytial virus, nirsevimab, genomic surveillance, viral evolution, monoclonal antibodies, fusion protein, phylogenetics, whole-genome sequencing, antiviral resistance, breakthrough infections, Lombardy, public health

The vaccine in question is built on the prefusion F protein, the conformation of the viral surface glycoprotein that RSV displays when it is about to fuse with a host cell. This prefusion form exposes the key antigenic sites that elicit the most potent neutralizing antibodies, which is why it has become the preferred scaffold for both RSV vaccines and antibody therapeutics. When administered during pregnancy, typically between 24 and 36 weeks of gestation, the vaccine stimulates the mother to produce antibodies that cross the placenta, arming the newborn with passive protection during the most vulnerable first months of life. The strategy has proven effective, but it has also been shadowed by safety concerns that have shaped the entire regulatory debate around maternal RSV immunization.

Those concerns are not trivial. In earlier clinical testing, some trials of maternal RSV vaccines were halted after interim data suggested an excess of preterm deliveries among vaccinated mothers. A previous rapid review pooling all tested maternal RSV vaccines, including candidates that never reached the market, reported an association with preterm birth. Because prematurity remains the leading cause of death in children under five worldwide, and because survivors face elevated odds of lifelong complications ranging from respiratory disease to neurodevelopmental impairment, even a modest safety signal demands rigorous scrutiny. At the same time, RSV itself kills, and the alternative preventive option, the monoclonal antibody nirsevimab, is expensive enough that maternal vaccination remains the primary strategy in much of the world. Policymakers therefore need a clear answer about where the true risk lies.

To provide that answer, a Finnish research team conducted a systematic review and meta-analysis following the PRISMA reporting guidelines. The authors searched PubMed, Scopus, and Web of Science on August 1, 2025, with no language or date restrictions, and screened the resulting records in Covidence. Their inclusion criteria were deliberately narrow: they considered only studies of the market-approved prefusion F vaccines, RSVPreF and RSVPreF3, given to pregnant individuals, and only studies reporting preterm birth, defined as any delivery before 37 weeks and zero days of gestation. They excluded trials of the non-approved vaccine formulations precisely because those candidates had already been withdrawn over preterm birth signals and could confound the safety picture of the products actually in clinical use.

The screening process winnowed 404 identified studies down to just seven that met all criteria. Three were randomized controlled trials, together encompassing 12,833 births, and all were multinational, conducted across as many as 24 countries. Four were observational studies, retrospective or prospective, covering 4,245 births, and all four came from the United States. Vaccination typically occurred between gestational weeks 24 and 36. Where birthweight was reported, in four of the seven studies, the means were comparable between vaccine and control groups, ranging from 3.2 to 3.34 kilograms in vaccinated pregnancies against 3.15 to 3.42 kilograms in controls. Risk of bias was low in two of the randomized trials and raised some concerns in one, while the observational studies were rated moderate in three cases and serious in one, assessed with the Cochrane RoB 2.0 and ROBINS-I tools respectively.

The pooled results tell a story of two evidence streams in tension. In the randomized trials alone, the meta-analysis found a statistically significant increase in preterm birth risk among vaccinated mothers, with a relative risk of 1.26 and a 95 percent confidence interval of 1.08 to 1.46, and notably zero heterogeneity across trials. Yet when the researchers turned to the observational data, the signal vanished and even reversed direction: the relative risk was 0.78, with a confidence interval spanning 0.46 to 1.32 and substantial heterogeneity of 63 percent. Combining all seven studies yielded no overall association between maternal RSV vaccination and preterm birth, with a relative risk of 0.96 and a wide confidence interval of 0.68 to 1.38, accompanied by 80 percent heterogeneity. The certainty of this evidence was rated as low using the GRADE framework, downgraded for risk of bias and imprecision, since the interval includes both meaningful benefit and meaningful harm.

The authors attribute the discordance between trial and observational findings primarily to differences in the lower gestational age threshold for vaccination. The randomized trials enrolled women from 24 or 28 weeks of pregnancy, whereas every observational study administered the vaccine only from 32 weeks onward. Because the risk of spontaneous preterm delivery naturally concentrates in the window shortly after vaccination in earlier-gestation cohorts, a lower enrollment threshold is more likely to capture deliveries that occur soon after immunization, a pattern that can be misread as vaccine-caused when the two events are merely close in time. The geographic restriction of the observational data to the United States, compared with the multinational scope of the trials, adds a second layer of difference in populations, healthcare systems, and coding practices that may further explain the divergence.

On the secondary outcome, the analysis rested on only two studies reporting neonatal intensive care unit admissions, covering 3,620 births. The pooled estimate actually favored vaccination, with a relative risk of 0.74, meaning roughly 24 fewer NICU admissions per 1,000 infants, but the confidence interval of 0.34 to 1.61 was so wide that no firm conclusion is possible. The certainty of evidence here was rated very low, reflecting the observational origin of the data, small sample, high heterogeneity of 92 percent, and risk of bias. Birthweight and other neonatal morbidities were prespecified as additional outcomes, but inconsistent reporting across the included studies prevented formal meta-analysis, underscoring how early the evidence base for neonatal safety outcomes beyond preterm birth remains.

These findings arrive at a consequential regulatory moment. The Global Advisory Committee on Vaccine Safety has concluded that, despite the preterm birth concerns, the benefits of maternal RSV vaccination outweighed the risks in 98 percent of simulations when the vaccine is given between 27 and 36 weeks of gestation. On that basis, the WHO Strategic Advisory Group of Experts issued recommendations in September 2024 that were subsequently endorsed by the WHO itself. Real-world effectiveness data from the 2024-25 season in England and Scotland have already shown highly favorable outcomes, confirming that the vaccine works when deployed at population scale. The authors note, however, that hybrid immunization strategies deserve continued consideration, because nirsevimab offers superior protection, and the preterm birth signal observed within the randomized trials has not been conclusively resolved.

The review’s authors are candid about its limits. With only seven studies, publication bias could not be properly assessed; ongoing trial registries were not searched; and rare vaccine-related adverse events cannot be detected without large-scale nationwide surveillance. The subgroup signal in randomized trials, a 26 percent relative increase in preterm birth with tight confidence intervals, stands as the single most important caveat in an otherwise reassuring picture, and the low-to-very-low certainty ratings mean the true effect could plausibly fall on either side of harm or benefit. What the analysis establishes is that the totality of current evidence on the approved prefusion F vaccines does not confirm an overall association with preterm birth, and that NICU admissions were numerically lower among vaccinated infants. As maternal RSV vaccination scales up globally, the authors argue, continued pharmacovigilance focused on gestational outcomes, alongside studies of broader neonatal morbidity, is not optional but essential. The vaccine’s promise for protecting newborns from one of childhood’s deadliest pathogens is real; so is the responsibility to keep watching.

Subject of Research: Safety of maternal RSV prefusion F vaccination regarding preterm birth and neonatal intensive care admission

Article Title: RSV‐Pre‐F Vaccination During Pregnancy and Neonatal Outcomes—A Systematic Review and Meta‐Analysis

Article References: Leskinen, N., Haapanen, M., & Kuitunen, I. (2026). RSV‐Pre‐F Vaccination During Pregnancy and Neonatal Outcomes—A Systematic Review and Meta‐Analysis. Immunity, Inflammation and Disease, 14(9), Article e70510. https://doi.org/10.1002/iid3.70510

Image Credits: AI Generated

DOI: 10.1002/iid3.70510

Keywords: RSV, maternal vaccination, preterm birth, prefusion F vaccine, meta-analysis, systematic review, neonatal outcomes, NICU admission, nirsevimab, vaccine safety, randomized controlled trials, public health