Thousands of infants contract respiratory syncytial virus (RSV) every year. While most experience mild symptoms, the virus can cause severe breathing difficulties in newborns and young infants and may require hospital treatment. Since autumn 2025, pregnant women in Denmark have been offered an RSV vaccine to help protect their babies during the first months of life. New findings from the University of Copenhagen and Nordsjaellands Hospital now point to a substantial reduction in hospital admissions among the youngest children.
During the 2025/26 RSV season, the number of hospital admissions among infants aged 0 to 5 months fell by 75 per cent compared with the average of the previous three seasons. While admissions averaged 1,128 in those years, only 296 infants were admitted between October and April in the most recent season.
"That’s more than 800 infants who avoided hospital admission over the winter. For these very young children, RSV can be particularly serious and may cause severe breathing difficulties. It is a major strain on both children and their families, so this is a remarkable development," says Thea Kølsen Fischer, Clinical Professor at the Department of Public Health and co-author of the study.
The researchers emphasise that the study cannot establish a direct causal link between vaccination and the decline in admissions. Changes in transmission patterns, testing activity and other factors may also have influenced the trend.
Even so, the findings are consistent with experiences from countries including Belgium, Spain, the Netherlands, France and England, all of which have introduced RSV vaccination programmes.
"I work in a hospital every day, and colleagues from paediatric departments across the country tell me that the RSV season often place enormous pressure on their services. This year, they have not seen the usual surge in admissions," says Thea Kølsen Fischer.
RSV can cause inflammation and mucus build-up in the smallest airways of the lungs. It is one of the most common causes of respiratory infection during the winter months and affects both children and adults. For most people, symptoms are mild.
Infants and babies born prematurely are particularly vulnerable because their airways are small and their immune systems are still developing. As a result, inflammation and mucus can quickly lead to serious breathing problems.
RSV follows a clear seasonal pattern in Denmark, with most hospital admissions occurring between October and March. For that reason, the vaccine is offered to women due to give birth during or shortly before this period. In the first season, 72 per cent of eligible pregnant women accepted the offer of vaccination.
"It is encouraging that so many women chose to be vaccinated in the first season," says Thea Kølsen Fischer.
Thea Kølsen Fischer has studied RSV for many years, including before the first vaccines were approved in 2023. For her, the results reflect the impact of a long period of research. The findings were equally welcome for Caroline Klint Johannesen, a PhD student at Nordsjaellands Hospital and first author of the study.
"It is incredibly rewarding to see that what we have been researching does not simply work on paper, but can make a real difference in practice," she says.
Despite the sharp decline in admissions, RSV is unlikely ever to disappear completely because it is both widespread and highly infectious. However, researchers believe it may be possible to reduce the burden of severe disease to a much lower level than in previous years.
"I expect at least as many people to accept the vaccine next season, both because of these early results and because it is a very safe vaccine with very few side effects," says Thea Kølsen Fischer.
Older adults at increased risk, including people over 60 with conditions such as chronic obstructive pulmonary disease (COPD), can also receive financial support for vaccination. Thea Kølsen Fischer expects this to have a longer-term impact on the burden of disease.
"If fewer older people become seriously ill, I believe we will move much closer to controlling a virus that has traditionally caused a large number of sick days and hospital admissions," she says.
The Lancet Regional Health - Europe
Early impact of RSV prevention on infant hospitalisations in Denmark
22-Jun-2026
CKJ had no conflicts of interest to declare. AMEC declare support from the Independent Research Fund Denmark outside of this work. SL had no conflicts of interest to declare. ZBH received research grants from Greater Copenhagen Health Science Partners (CAG VAX - Clinical Academic Group Translational Vaccine Research in High-Risk Adults), Independent Research Fund Denmark (Inge Lehmanns grant number 3162-00031B), Helen Rudes Foundation, the Danish Cancer Society (Grant number KBVU-MS R327-A19137), Novo Nordisk Foundation (grant nr. NNF24SA0090556), and the Danish National Research Foundation (grant number DNRF170), all out of this work. ZBH is an ID consultant at Statens Serum Institut, pneumococcal laboratory. ZBH functions as Chair, Vaccine Study Group (EVASG), European Society of Microbiology and Infectious Diseases (ESCMID) and is a member of the Danish Vaccination Council (NITAG). TKF declared consultant fees from a consultant company, grants or contracts from Pfizer, payment for lectures, support for meetings and/or travel from Pfizer and GSK. All payments from GSK and Pfizer were made to a consultant company.