In the first U.S.-based analysis of a new monoclonal antibody against respiratory syncytial virus (RSV) in infants, nirsevimab worked well to prevent RSV-associated hospitalizations. Now, demand for it is outstripping supply.
“It’s a one-time shot that can last for up to six months, and it’s recommended for all infants, whether they are full-term or preterm and with or without an underlying condition, said coauthor Natasha Halasa, M.D., MPH, Craig Weaver Professor of Pediatric Infectious Diseases at Vanderbilt University Medical Center and Director of the Vanderbilt Infection Surveillance and Prevention Research Program, and the coauthor of a study in the Morbidity and Mortality Weekly Report.
“It replaces palivizumab, which had to be given monthly for five months and was restricted to only high-risk infants. Nirsevimab is a very effective, safe vaccine that can protect kids for up to six months. Given the risks and benefits, I highly recommend this. If I had my own young infant, I would give it to them.”
90 Percent Effective
Phase 3 clinical trials in Western Europe had shown nirsevimab’s effectiveness against RSV-associated hospitalizations to be 81 percent. In the U.S.-based study, nirsevimab effectiveness was 90 percent against RSV-associated hospitalizations with a median time from receipt to symptom onset of 45 days.
The U.S. analysis used data collected by the New Vaccine Surveillance Network. It’s a population-based research platform for acute respiratory illnesses in children younger than 18 years. The network collects demographic, clinical and immunization data related to pediatric respiratory disease treated at Vanderbilt and six other academic medical centers.
“It’s a one-time shot that can last for up to six months, and it’s recommended for all infants, whether they are full-term or preterm and with or without an underlying condition.”
“Among 699 infants hospitalized with acute respiratory illness, 59 (8 percent) received nirsevimab seven or more days before symptom onset. Nirsevimab effectiveness was 90 percent against RSV-associated hospitalization,” the authors wrote.
They concluded that infants should be protected by either maternal RSV vaccination or by receiving a dose of nirsevimab early in infancy, often during a visit early in life to their own community pediatrician.
Reason for Hospitalization
Administering the nirsevimab vaccine to all young children, including those who have no underlying condition, is very important.
“The majority of infants hospitalized with RSV do not have an underlying medical condition. While there is not high mortality with it, there is high morbidity,” Halasa explained.
In fact, RSV is the number one cause of hospitalizations among infants in the U.S., resulting in 50,000 to 80,000 hospitalization each year in children under the age of 5, the authors wrote. Among those admitted to ICUs, Halasa’s own research shows nearly a quarter require mechanical ventilation.
“RSV impacts so many children that most people know somebody whose baby has had it or themselves have had a child who became very sick in the outpatient setting or was hospitalized,” said Kristina Betters, M.D., an associate professor of pediatrics and critical care medicine at Vanderbilt University Medical Center. A child can be sick for weeks with the illness, she said.
Demand Outpacing Supply
“We saw a lot of vaccine hesitancy in the pediatrics population after the pandemic,” Betters said. “There was concern that caregivers and parents weren’t going to want to give this one to their children. We saw the opposite.”
She ascribes unexpectedly high parental demand for the monoclonal antibody to both the potential seriousness of the illness and its ubiquity.
In the US, not enough of the monoclonal antibody was manufactured. Betters said she gave birth in September 2023 and had difficulty obtaining it for her own child.
“If a pregnant woman gets vaccinated during RSV season, she can pass the immunity to the baby. And she may not be able to get nirsevimab for the baby later on.”
“People really wanted this vaccine and there wasn’t enough, so it was very restricted,” she said. “Community pediatricians couldn’t keep it in stock. They were running out of it in one or two days.”
As a result, large numbers of children that should have received it did not receive it.
“The community underestimated the demand. They’d based it on rollouts of prior new pediatric vaccines, with not everybody wanting to get it. The situation was quite the contrary,” Halasa said.
Assessing New Therapeutics
The New Vaccine Surveillance Network which published the U.S.-based analysis is an active prospective survey system. It started in 2000, with Vanderbilt as one of the original sites led by Kathryn M. Edwards, M.D., Sarah H. Sell and Cornelius Vanderbilt Professor of Pediatrics.
“It’s population-based, so it gives rates. The goal is to see the impact of new therapeutics,” Halasa said. The network prospectively enrolls children with fever or respiratory symptoms and collects a sample that is then tested for multiple viruses including RSV, she explained.
Researchers within the network approach families whose children live in specific counties and invite them to enroll their children. After informed written consent, a member of the research team interviews the parents or guardians about their child’s illness and also conducts a medical chart review. That researcher documents if the child received an RSV monoclonal antibody or if their mother received a vaccine while pregnant, which imparts immunity to her infant.
Patients Should Accept Either
Some pregnant women have asked Betters which is preferable, the maternal vaccination or nirsevimab.
“We don’t know yet. In light of the shortages, we advise patients to take what they can get,” Betters said.
“If a pregnant woman gets vaccinated during RSV season, she can pass the immunity to the baby. And she may not be able to get nirsevimab for the baby later on.”
