• Courtesy of the University of Vermont
• Sean Diehl

The pervasive respiratory disease RSV, or respiratory syncytial virus, infects 97 percent of all children by the age of 2. Most make full recoveries, but severe RSV can cause pneumonia, bronchiolitis and permanent lung damage. It's the leading cause of hospitalizations in all infants and toddlers, according to the U.S. Centers for Disease Control and Prevention; globally, it's the second leading cause of death in children under age 1, behind only malaria.

Sean Diehl has been working to change that. He's a vaccine researcher and associate professor of microbiology and molecular genetics at the University of Vermont's Larner College of Medicine. Diehl's lab helped develop the RSV vaccine Beyfortus (nirsevimab), which was approved by the U.S. Food and Drug Administration on July 17.

It uses a monoclonal antibody, similar to the kind used to treat patients with COVID-19, and has been deemed nearly 80 percent effective in preventing children from being hospitalized with RSV; it can be safely administered to children as young as newborns.

Diehl has collaborated with UVM's Vaccine Testing Center since 2008 to develop immunizations for other infectious diseases, including rotavirus, dengue, Zika and COVID-19. He's been working on this particular type of immunization method for more than two decades. Called a "passive vaccine," it's the first of its kind to be approved for children of all ages, he said.

As Diehl explained in an interview, most immunizations are considered "active" vaccines, meaning they use a harmless piece of a virus or bacteria to teach the body's immune system how to recognize invading microbes, then build an army of cells to fight them. Among the main weapons in that arsenal are antibodies. Through trial and error, the immune system selects the best antibodies to fend off the infection, then stores that information for future reference.

A passive vaccine functions differently. It uses a naturally occurring antibody that was created by the immune system of someone who had multiple exposures to RSV. Researchers then grew that person's cells in a petri dish, which produced a variety of RSV antibodies. Diehl and his team screened thousands of those natural antibodies to select the ones that worked most effectively. The handpicked antibodies became the basis for the new vaccine.

As Diehl pointed out, naturally occurring antibodies don't replace a baby's own immune response, but they help it build its own defenses as it matures.

"The antibody vaccine acts alongside the baby's immune system, which is kind of cool," he said. "It's like a laser-guided missile that already knows, 'This is RSV. I need to destroy it.'"

An added benefit of the new vaccine, Diehl noted, is that it's long-acting and doesn't require multiple injections to provide coverage through a single RSV season, when the risks of complications and hospitalization are greatest.

Last winter, hospitals around the country saw an unprecedented surge in pediatric RSV cases. Experts are predicting that this new vaccine for kids, along with a different one recently approved for adults 60 and older, will dramatically reduce RSV cases this winter.