Every year, more than 3 million babies worldwide end up in the hospital because of a virus called RSV. It makes them wheeze, struggle to breathe, and sometimes land in intensive care. Yet doctors still have very few tools to help the sickest infants. Now, scientists in London say they have found important new clues that could lead to better treatments.

Researchers at University College London and Great Ormond Street Hospital built tiny working models of baby lungs right in their lab. They used real cells from infant airways, blood vessels, and a type of white blood cell called neutrophils that helps fight infection. They also built an adult airway model for comparison.

The team, led by Dr. Claire Smith, infected both models with RSV to see what happens inside a baby's lungs during infection. What they discovered surprised them. Baby airway cells attracted far more white blood cells than adult cells did. These cells flooded into babies' small airways and made it harder for them to breathe.

Even more interesting, the neutrophils that rushed to the lungs in the baby model were more activated and triggered a stronger inflammatory reaction than those in the adult model. This suggests it is not just the virus causing damage—it is the baby's own airway shaping a powerful immune response that can harm the lungs.

"This model allows us to watch early immune responses unfold and study them in a human setting that reflects the infant airway," Dr. Smith said. "That's something animal models often struggle to capture."

Next, the researchers tested two antiviral drugs already used against similar viruses: remdesivir and RSV604. Both stopped the virus from multiplying. But only RSV604 also calmed the overactive immune response, reducing levels of a key inflammatory protein linked to severe disease in babies. Remdesivir had no effect on the immune system.

First author Dr. Machaela Palor said the findings help explain why RSV hits babies so much harder than adults. "The pediatric airway actively shapes how immune cells behave during the infection," she explained.

The team hopes their lab model will help scientists test new treatments more quickly and safely. Smith said the model lets researchers ask two questions at once: Does the drug stop the virus? And does it help control the immune response in an infant's airway?

Carla Owen, CEO of Animal Free Research UK, praised the work, calling it "science at its best." The research used human cells instead of animals, which Owen said "brings hope to families."

With RSV season approaching each year, these tiny lab models may one day help protect millions of babies from serious illness.