IE 11 is not supported. For an optimal experience visit our site on another browser.

Why is RSV so dangerous for some young children but not others?

Nearly every child gets RSV by age 2. Why do some fall extremely ill?

Nearly all children get respiratory syncytial virus by the time they turn 2, and for most parents, it’s barely a blip on the radar. So why does the common virus make some healthy children severely ill?

The answer is likely to frustrate parents: There is no obvious reason some healthy babies get so sick with RSV that they must be hospitalized, on oxygen, while others do not.

“Why do they end up in the hospital versus their 2-year-old counterpart from day care that just has a really bad cold?” asked Dr. Anita Patel, a pediatric critical care physician at Children’s National Hospital in Washington, D.C. “That’s the gray area of RSV.”

Dr. Kavita Patel, a primary care physician at Mary’s Center, a community health center in Maryland, said, "We can't predict who's going to get it worse, other than those with chronic conditions." Premature babies and those with asthma are among those at higher risk.

Anita Patel noted that the younger the child, the more vulnerable he or she is to the inflammation that accompanies an RSV infection.

It often comes down to the basics of anatomy, she said. Babies are born with the tiniest of airways. Viruses like RSV inflame those airways, making it difficult to breathe. The smaller the airway, the less inflammation it takes to close it off.

What’s more, viruses are usually more severe the first time they infect people. That means babies who have never been exposed to a virus like RSV may be hit harder than older children who've had previous RSV infections.

Babies younger than 6 months old have the highest rates of hospitalization for RSV compared to any other age group, according to the Centers for Disease Control and Prevention. As kids get older, hospitalization rates fall.

Even so, only a small percentage of the more than 2 million children under 5 who get RSV in the U.S. every year end up in the hospital: about 3%, according to the American Academy of Pediatrics.

A study published Thursday in The Lancet Respiratory Medicine found that in Europe, about 2% of healthy children under age 1 who get RSV require hospitalization.

“One in 56 children born healthy end up in the hospital in the first year of life” because of RSV, said the study’s author, Dr. Louis Bont, a pediatrician at Wilhelmina Children’s Hospital in the Netherlands.

The U.S. is in an unusual surge in respiratory viruses, including RSV, that have overwhelmed children's hospitals. As of Thursday, 77% of pediatric hospital beds were occupied, according to the Department of Health and Human Services.

The coronavirus pandemic may have played a role. Measures that protected young children from Covid also kept them safe from other viruses.

“We have a huge set of children who just weren’t as exposed to these common viruses,” Anita Patel said. “They’re experiencing their first RSV virus at the same time as other viruses,” including rhinoviruses that cause the common cold.

That can be a double whammy: Rhinoviruses can cause congestion and phlegm, she said. Viruses that clog up the nose make it even more difficult for small airways to get enough oxygen.

Indeed, doctors say they’ve seen young children testing positive for several respiratory viruses at once.

With no vaccine or antiviral drugs available for RSV, "there’s nothing you can do for them except support them," Kavita Patel said. "The vaccine can’t come soon enough."

Follow NBC HEALTH on Twitter & Facebook.