The number one cause of infants being hospitalised in the US and Europe is a virus you’ve probably never heard of: RSV. Most people experience it as a mild infection resembling a cold. But it can be very serious in babies and elderly people. The tell-tale symptoms are abnormally fast breathing, a caving-in of the chest between and under the ribs, and wheezing or crackles – worrying noises caused by the bronchial tubes being inflamed, or the small air sacs in the lungs filling with fluid. The virus makes it harder to breathe and feed, both of which are essential, but even more so for newborn babies. The gap between public awareness of RSV and the toll it takes is massive. Worldwide, it’s estimated that each year 64 million people have RSV, causing about 160,000 deaths. And it’s the most common cause of lower respiratory tract infections in young children worldwide, killing an estimated 13,000 infants under six months old and an estimated 101,000 children before they reach the age of five. In the UK, about 33,500 children under five are hospitalised with RSV each year, and it causes 20 to 30 deaths. While we tend to hear less about it, the burden on the NHS caring for RSV in children is higher than that for flu. Avoiding RSV completely is difficult given that it spreads easily via coughing, sneezing and infected surfaces. And it can affect any infant; the majority of hospitalisations, roughly 80%, are in otherwise healthy babies. For decades, paediatricians have had to rely on medical interventions to treat unwell babies, such as providing oxygen, rather than having a scientific tool to prevent them from becoming sick in the first place. But the past two years have seen two major scientific steps forward in reducing RSV-related illness and mortality. First, a new monoclonal antibody drug was approved in the UK and US that provides babies with temporary immunity to RSV. In clinical trials, nirsevimab was about 77% effective against both hospitalisations and cases of RSV requiring medical intervention. An independent international study found that babies who received a single dose of nirsevimab showed an 83% reduction in hospital admissions compared to babies who had standard care. This is an astonishing drop. Based on these findings, the US Centers for Disease Control now recommends that all infants who are under eight months old at the start of RSV (winter) season receive nirsevimab. Second, a vaccine against RSV given to pregnant women has been approved in the US and UK. A clinical trial found that, for mothers who were vaccinated between weeks 24 and 36 of their pregnancies, the shot was about 82% effective at preventing severe disease in infants in the first three months after birth. That dropped to 69% protection six months after birth. The vaccine works by containing a lab-made version of an RSV surface protein (F) that invades host cells. As a result, the vaccinated adult produces F-blocking antibodies that can prevent infection, which are passed via the placenta to the foetus during late-stage pregnancy. These maternal antibodies protect babies in the first few months of life while their own immune systems are developing. In Britain, the Joint Committee on Vaccination and Immunisation (JCVI), which advises the government, found both products suitable for a universal programme and noted that it didn’t have a preference for one intervention or the other. The JCVI recommended that “a RSV immunisation programme, that is cost-effective, should be developed for both infants and older adults”. But there are some big challenges to integrating these treatments into the NHS. Dr Ting Shi, a world-leading expert on RSV, told me that price is the main barrier: in the US nirsevimab is priced between $300 and $500 a dose, while the maternal vaccine is roughly $320 a dose. Tough negotiations with the pharmaceutical companies are required to bring down prices in order to make these remedies accessible to the British public. Galicia, in Spain, is the first place to add nirsevimab to its immunisation programme, meaning that all infants born during RSV season (between 25 September 2023 and 31 March 2024) will receive immunisation in the hospital within 24 hours of being born. France is planning a similar rollout, and Belgium, Italy and Luxembourg are recommending it, although they are also facing the cost challenge. If high-income countries are struggling to find the funds, the situation is even more difficult in low-income areas where the majority of child RSV deaths occur. These are logistical and political challenges that can be sorted out: the important news is that there are two new tools to stop babies from struggling to breathe and being admitted to hospital. RSV has also been a huge contributor to the NHS’s yearly winter crises, when rising infection rates for many diseases threaten to overwhelm the stretched service. If these medicines are rolled out quickly and effectively, RSV in young children could be all but removed from the equation. That’s good news and another win for science. Prof Devi Sridhar is chair of global public health at the University of Edinburgh