“Something’s wrong with the data. There are too few people in hospitals” with respiratory syncytial virus (RSV), epidemiologist Caroline Klint Johannesen recalls a colleague remarking while analysing Danish health records for last winter.

On 1 October 2025, Denmark introduced a nationwide policy of vaccinating pregnant people against RSV — a respiratory pathogen that usually causes mild symptoms but can lead to severe disease and even death in vulnerable people, including infants and older adults. Maternal antibodies induced by the vaccine cross the placenta to protect babies for around the first 6 months of their lives, when they are most at risk. Newborns at high risk of severe disease, including premature infants, whose mothers had not been vaccinated, were given injections of monoclonal antibodies.

RSV wave hammers hospitals — but vaccines and treatments are coming

Johannesen, a vaccine researcher at Nordsjællands Hospital in Hilleroed, Denmark, found¹ that infant hospital admissions plummeted. The immunizations are working, she says (see ‘Newborn protection’).

Global risks

The World Health Organization (WHO) estimates that every year, RSV hospitalizes roughly 3.6 million and kills about 100,000 children under 5 years old. Of these deaths, 97% occur in low- and middle-income countries (LMICs).

The virus is also dangerous to older adults — although those risks are widely underappreciated by both physicians and the public. Historically, testing has been sparse, and there are no global statistics for RSV-related adult deaths. But over the age of 60, hospitalizations and mortality increase with age, especially in people with other diseases such as chronic heart and lung conditions. In the United States, the Centers for Disease Control and Prevention (CDC) estimates that between around 10,000 and 25,000 people have died in each of the past two winters, especially in older age groups.

Since 2022, when European regulators approved nirsevimab — a long-acting monoclonal antibody against RSV — a second antibody and three vaccines have been developed for use in adults, one of which is suitable for pregnant people.

“It’s just remarkable,” says Buddy Creech, a paediatrician at Vanderbilt University in Nashville, Tennessee, to have gone “from no mechanism to prevent RSV to an embarrassment of riches”. Dozens of countries across Europe, Asia and North and South America have launched various immunization programmes.

Up-to-date immunizations