London School of Hygiene & Tropical Medicine

A new study led by the London School of Hygiene & Tropical Medicine provides the most comprehensive review of respiratory syncytial virus (RSV) age distributions among children in low- and middle-income countries (LMICs).

Published in The Lancet Child & Adolescent Health, results show that very young infants bear the greatest burden of RSV deaths and intensive care unit admissions. These insights highlight the importance of recently approved passive immunisation strategies, including maternal vaccination and long-acting monoclonal antibodies for infants.

The authors were able to review 160 different datasets from 43 LMICs. From these data, the authors calculated RSV age distributions (by week of age) for a wide range of disease outcomes in children under five years old.

Deaths were found to most likely occur at three weeks old (modal age) for those outside the hospital, and four weeks inside the hospital. Up to 23% of total under-five deaths from RSV in a medical facility were in infants less than eight weeks old.

Admissions to ICU (intensive care) for RSV were, on average, most common in infants aged 7 weeks old, with 20% of under-five ICU admissions in infants younger than 8 weeks old. Emergency room visits for RSV were most common in infants at 10 weeks old.

The results, which broke down RSV cases at week-by-week age intervals, reaffirm the importance of immunising the youngest infants, who bear the greatest burden of severe RSV outcomes. The authors hope that their results can inform evidence-based public health policy.

Dr Kevin van Zandvoort, Assistant Professor at LSHTM, said: “The most severe RSV outcomes are especially concentrated in the youngest infants. Maternal vaccination and long-acting monoclonal antibodies can help protect children during this highly vulnerable stage of life and could prevent a substantial health burden.”

Maternal RSV vaccination is likely to become more affordable in the coming years and could provide protection from birth. In addition, long-acting monoclonal antibodies against RSV exist and can be given straight after birth with potentially higher coverage of infants.

While the review highlights the importance of protecting the youngest infants, the authors also found a substantial health and economic burden of RSV outcomes in older children, highlighting the need for other strategies to prevent the full burden of RSV disease in children younger than 5 years.

This review was conducted by researchers at LSHTM in partnership with the World Health Organization, University of Edinburgh, Nanjing Medical University and a large number of RSV collaborators from across the world. This study was funded by WHO and the Gates Foundation.

Mahmud S et al. Age distribution of respiratory syncytial virus disease in children younger than 5 years in low-income and middle-income countries: a systematic review and meta-analysis. The Lancet Child & Adolescent Health, 2026. DOI: https://doi.org/10.1016/S2352-4642(25)00349-9