RSV, or respiratory syncytial virus, is an extremely common lung infection that affects nearly all children by age two. For most older kids and adults, it feels like an ordinary cold. But for babies, especially those under six months old, RSV can become serious because their airways are so small that even mild swelling and mucus buildup can make breathing difficult. Worldwide, RSV causes an estimated 3.6 million hospitalizations in children under five each year, and roughly half of RSV-related deaths in young children occur in infants under six months.
Why Babies Are Hit Harder
RSV targets the small breathing tubes (bronchioles) deep in the lungs. When the virus takes hold, it causes the lining of these tubes to swell and fill with mucus. In an adult or older child, those airways are wide enough to handle some swelling without much trouble. In an infant, the tubes are already tiny, so even a small amount of inflammation can partially or fully block airflow. That’s why a baby with RSV can go from a mild runny nose to visible breathing difficulty within just a few days.
The two main complications are bronchiolitis, where the small airways become inflamed and clogged with mucus, and pneumonia, where the infection spreads deeper into the lung tissue itself. The two can be hard to tell apart because they look very similar: fast breathing, wheezing, and difficulty feeding. Pneumonia sometimes brings decreased breath sounds or vomiting in addition to the typical bronchiolitis symptoms.
How Symptoms Progress
RSV typically starts mild and worsens over several days. The earliest signs look like any cold: a runny nose, a light cough, and slightly less interest in eating or drinking. Over the next two to four days, the cough often deepens and can progress to wheezing or labored breathing. This middle stretch, around days three through five of illness, is usually when symptoms peak.
Babies under six months may show less obvious cold symptoms and instead display more general warning signs: unusual irritability, decreased activity, reduced feeding, and apnea (pauses in breathing lasting more than 10 seconds). Apnea is particularly common in very young or premature infants and can sometimes be the first noticeable sign before any coughing or congestion appears.
Signs of Breathing Trouble to Watch For
Because RSV can escalate quickly, knowing what respiratory distress looks like in a baby is important. Three key signs to look for:
- Nasal flaring: The nostrils spread wide open with each breath, a sign your baby is working harder than normal to pull in air.
- Retractions: The skin pulls inward just below the neck, under the breastbone, or between the ribs each time your baby breathes in. You’ll see the chest appear to sink in rather than expand normally.
- Grunting: A short grunting sound with each exhale. This is the body’s way of trying to keep the lungs inflated and open.
Any of these signs, along with a bluish tint to the lips or fingernails, very fast breathing, or significant refusal to eat or drink, warrants immediate medical attention. Dehydration can develop quickly in a baby who isn’t feeding well.
How RSV Is Diagnosed
If your baby is brought in with breathing symptoms during RSV season (typically fall through early spring), a doctor will often test for the virus using a nasal swab. Rapid antigen tests give results quickly but can miss some infections. A more sensitive option called a PCR test (a type of molecular test) is better at detecting the virus, especially in cases where the rapid test comes back negative but RSV is still suspected. In practice, the diagnosis often comes from a combination of the test result, the time of year, and what the doctor sees and hears during the exam.
Treatment and What to Expect
There is no antiviral medication that treats RSV. Care is entirely supportive, meaning the goal is to keep your baby comfortable, hydrated, and breathing well while the virus runs its course. At home, that means frequent small feedings, gentle nasal suctioning to clear mucus, and monitoring breathing closely.
Most otherwise healthy infants recover without needing hospitalization. For those who do end up in the hospital, treatment typically involves supplemental oxygen if blood oxygen levels drop, IV fluids for babies too exhausted to feed, and in rare severe cases, a breathing machine to help the lungs. Most hospitalized babies improve within a few days and go home once they can breathe and eat comfortably on their own. Full recovery from RSV generally takes one to two weeks, though a lingering cough can hang around a bit longer.
Prevention: Immunization Options
Two relatively new options now exist to protect infants from severe RSV, and they work in different ways.
Nirsevimab for Babies
Nirsevimab (brand name Beyfortus) is a single-dose antibody injection given directly to infants, typically shortly after birth or before their first RSV season. It gives babies ready-made antibodies rather than asking their immune system to produce its own. Real-world data from the first U.S. RSV season after it was recommended showed it reduced emergency department visits for RSV by 77% and hospitalizations by 98%. That near-total reduction in hospitalizations makes it the most effective tool currently available for infant RSV protection.
Maternal Vaccine During Pregnancy
The other option is a vaccine (Abrysvo) given to the pregnant parent between 32 and 36 weeks of pregnancy. The parent’s immune system produces antibodies that cross the placenta and protect the newborn during those vulnerable first months of life. In clinical trials, this vaccine reduced RSV hospitalizations in newborns by 68% within the first three months after birth and by 57% within six months. It was especially effective at preventing the most dangerous outcomes: the risk of a baby needing intensive care, supplemental oxygen, or a breathing machine dropped by 82% in the first three months.
Babies generally receive one option or the other, not both. Your pediatrician can help determine which approach makes sense based on timing and availability.
Which Babies Face the Highest Risk
While RSV can affect any infant, certain babies are more likely to develop severe illness. Premature babies are at elevated risk because their lungs are less developed and their airways are even smaller. Infants with congenital heart defects or chronic lung conditions also face higher odds of complications. Babies with weakened immune systems and those under six months old during peak RSV season are in the highest-risk category. For these groups especially, prevention through immunization and basic hygiene measures like hand washing and avoiding contact with visibly sick people can make a meaningful difference.