Why isn't there an RSV vaccine for kids?

Cases of RSV continue to surge in the U.S., but a vaccine to protect young children against the respiratory virus remains a long ways off.

RSV, or respiratory syncytial virus, is the most common cause of bronchiolitis and pneumonia in children under 1, causing thousands of pediatric hospitalizations and hundreds of deaths each year, according to the Centers for Disease Control and Prevention.

That there is no RSV vaccine for kids is not for lack of interest, experts say. But a trial gone wrong many years ago and a tricky target protein have made developing an RSV vaccine difficult.

Researchers’ attempts to develop an RSV vaccine go back decades, according to Dr. Ofer Levy, the director of the Precision Vaccines Program at Boston Children's Hospital.

In the 1960s, a pediatric hospital in Washington, D.C., developed a vaccine for the virus and conducted a clinical trial on it in young children. But things quickly went awry.

Instead of protecting against RSV, the experimental vaccine made the children more likely to develop more severe illnesses if they got infected. Many children in the trial were hospitalized, and two young children died.

“That was a disaster,” Levy said, “and it set the field back by 20 to 30 years.”

Researchers eventually returned to work on an RSV vaccine, but without much success, said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University Medical Center.

That's because the protein that the virus uses to infect human cells, called the F protein, is a wily target. It can rapidly change its structure, Schaffner said, making it difficult to develop a vaccine that can effectively target it.

This problem is not unique to RSV; it's similar to how the spike protein of the coronavirus rapidly changes, making it easier for the virus to dodge Covid vaccines, said Dr. Céline Gounder, a senior fellow at KFF, formerly known as Kaiser Family Foundation, and an infectious disease specialist.

Still, two vaccines — from drugmakers Pfizer and GlaxoSmithKline — have shown promise in late-stage clinical trials of older adults, a group that is also at risk for severe disease from the virus.

In August, Pfizer announced that its experimental RSV vaccine was nearly 86% effective in preventing severe illness in adults ages 60 and older. GSK said this month that its vaccine was about 83% effective in older adults.

Pfizer said it plans to expand its trial to younger age groups, but testing every age group down to 6 months could take years. GSK, however, said it abandoned its efforts last year to develop a pediatric RSV vaccine, because trials suggested it wasn't effective.

“We don’t have an Operation Warp Speed for RSV,” Gounder said. “Phase 3 clinical trials take time, lots of patients, and money. You can speed up the clock with money.”

Pfizer also has an ongoing RSV vaccine trial in pregnant women, and an interim analysis of the results suggested that pregnant women pass their protective antibodies to their babies in the womb.

One available option to protect at-risk infants from severe illness from RSV is a monoclonal antibody injection called Synagis. The injection, however, is only available to certain high-risk infants, like babies born prematurely or those with a low birth weight.

In the absence of a vaccine, experts stressed commonsense measures to stay healthy. That includes washing hands and avoiding close contact with other children, especially those who are coughing and sneezing, Schaffner said.

If your child is feeling unwell or has a fever, parents should call a pediatrician, Levy said. If a child is having difficulty breathing, they should go to the emergency room.

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