RS virus admissions among infants fall sharply following vaccine rollout

A new study from the University of Copenhagen and Nordsjællands Hospital suggests that Denmark's new RS virus vaccination programme for pregnant women may have dramatically reduced the number of infants hospitalised with the virus.
The number of infants hospitalized with RS virus has decreased by 75% since pregnant women were offered the vaccine against the virus. Photo: Getty Images / GOLFX

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 Nordsjællands 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.

Findings mirror international experience

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.

Infants are particularly vulnerable

RSV can cause inflammation and mucus build-up in the smallest airways of the lungs. It is one of the most common causes of

About the study

  • The study is based on surveillance data from the Statens Serum Institut, combining hospital stays of more than 12 hours with a positive RSV test
  • It includes children aged 0 to 5 months
  • There were 296 RSV-related admissions among infants between 1 October 2025 and 29 April 2026
  • The average number of admissions during the previous three seasons was 1,128
  • The study was published in The Lancet Regional Health – Europe

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 Nordsjællands 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.

Bringing the virus under control

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.

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