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