Denmark’s nationwide vaccination policy for pregnant individuals, launched on October 1, 2025, has led to a sharp drop in infant hospitalizations for respiratory syncytial virus (RSV).
While the virus typically causes mild symptoms, it remains a leading cause of severe respiratory disease and death in vulnerable populations, including infants and older adults.
The strategy relies on maternal immunization, where antibodies induced by the vaccine cross the placenta to provide passive immunity to newborns. This protection is most critical during the first six months of life, a window when infants are at the highest risk of severe complications.
For premature infants or those whose mothers did not receive the vaccine, clinicians have utilized injections of monoclonal antibodies to provide immediate defense against the virus.
Caroline Klint Johannesen, a vaccine researcher at Nordsjællands Hospital in Hillerød, Denmark, said that the initial hospital data showed a surprising trend. “Something’s wrong with the data. There are too few people in hospitals,” she recalled a colleague remarking while analyzing health records from last winter. Johannesen confirmed that the immunization program is effectively reducing the number of infants requiring hospital care.
The World Health Organization (WHO) estimates that RSV is responsible for approximately 3.6 million hospitalizations and 100,000 deaths in children under 5 years old each year. The impact is disproportionately felt in low- and middle-income countries, where 97% of these fatalities occur.
RSV prevention options have changed rapidly since 2022, when European regulators approved nirsevimab, a long-acting monoclonal antibody. Following that milestone, three additional vaccines and a second antibody treatment have been developed for use in adults and pregnant individuals.
“It’s just remarkable,” said Buddy Creech, a paediatrician at Vanderbilt University in Nashville, Tennessee, to have gone “from no mechanism to prevent RSV to an embarrassment of riches.” Following these clinical advancements, immunization programs have been launched across Europe, Asia, and North and South America.
How long does maternal immunity last for a newborn?
Maternal antibodies transferred through the placenta typically provide protection for approximately the first six months of an infant’s life.
Why is RSV considered dangerous for older adults?
While often perceived as a childhood illness, RSV risk increases with age. Hospitalizations and mortality are significantly higher in individuals over 60, especially those managing chronic heart or lung diseases.
Vaccines stimulate the body’s own immune system to produce antibodies, whereas monoclonal antibodies—such as nirsevimab—provide direct, immediate protection against the virus without requiring the recipient’s immune system to mount a response.
Keep reading