Nirsevimab Cuts Hospitalizations for RSV in Spanish Babies by 80%

The results, recently published in the journal Eurosurveillance, indicate the treatment’s effectiveness was comparable to its performance during the 2023-2024 season, the first time the medication was available.

Researchers emphasize that nirsevimab differs from traditional vaccines. Instead of prompting the infant’s immune system to create antibodies to fight the virus, it provides temporary protection against infection.

The study also measured how the effectiveness of nirsevimab diminishes over time. While the protective effect decreases, it remains sufficiently high to offer substantial benefit.

The study authors suggest that, rather than initiating a systematic vaccination program at the start of the respiratory virus season, the timing should be determined dynamically based on the prevalence of RSV.

Nirsevimab received approval in Europe in late 2022. Spain was among the first countries to implement a strategy of immunizing infants to protect them from RSV, the leading cause of bronchiolitis, a common lung infection in children under two years old.

Approximately 26% of infants under one year of age become infected with the virus each winter, and around 2% require hospitalization.

Worldwide, the World Health Organization estimates that RSV causes more than 100,000 deaths annually.

The success of the immunization strategy with nirsevimab was first observed during its initial availability in the 2023-2024 season.

A Model for European Implementation

The research published in Eurosurveillance confirms these initial findings and extends them to other European countries, including Portugal and Belgium.

The data comes from centers within the Vaccine Effectiveness, Burden and Impact Network (Vebis) hospital network.

The goal is to provide data to support the expansion of the immunization program to other European nations.

In Spain, the current recommendation includes children under six months during the winter season – those born between April 1st of the start of the season and March 31st of the following year.

Premature infants (born before week 35 of pregnancy) under 12 months, and children under two years with a high risk of severe illness if infected, are also included.

The current study included children up to 24 months old, a total of 2,201 infants. The first cases of infection occurred in week 37 of 2024, with a peak between November and December of last year.

The effectiveness against hospitalization was high, at 79%, but varied with timing. It was 85% within the first 30 days of immunization, decreased to 78% after three months, and reached 69% by the end of the season (day 215 post-immunization).

In infants up to 6 months old, the overall effectiveness was 80%, starting at 85% and decreasing to 78% after three months, and then to 53% at the end of the season.

For infants between 7 and 23 months, the overall effectiveness was slightly lower, at 74%. Researchers do not have sufficient data to establish effectiveness for different time periods within this age group.

The researchers concluded that, between 0 and 6 months, the effectiveness of nirsevimab is consistent with results from clinical trials. They also noted that reinfections are common due to short-lived or incomplete protection.

The authors also suggest the possibility of “optimizing the start of immunization with monoclonal antibodies of long duration,” which would be important “during RSV seasons that are delayed.”

In response to inquiries, researchers noted that “the risk of hospitalization for RSV decreases with increasing age of children, which compensates for the possible effect of decreasing antibody levels as infants grow.”

Pedro Jesús Alcalá Minagorre, a pediatrician at Dr. Balmis Hospital in Alicante, has seen many infants hospitalized with bronchiolitis. The arrival of nirsevimab brought about a dramatic change.

“Now we are seeing bronchiolitis [caused] by other viruses, but it’s not the epidemic peak we used to see with RSV,” he said.

Regarding the duration of effectiveness, he doesn’t anticipate problems unless there is a shift in the virus season, as occurred with COVID-19.

The SARS-CoV-2 infection did not leave room for other respiratory viruses until the spring of 2021. At that time, cases of RSV began to increase, and an unusual peak occurred in the summer.

“I don’t think there will be a new displacement due to nirsevimab,” Alcalá Minagorre said, “but we need to be vigilant for new virus species that could take prominence.”

The only drawback he sees with the monoclonal antibody is that, “unlike vaccines, the benefit is individual, because babies are usually recipients of the virus, not transmitters. The vehicle of transmission can still be, for example, their older sibling.”

Despite the high coverage achieved in Spain (over 90% in the two seasons completed), he believes that efficacy can be further improved through vaccination of the mother, which is still under investigation.

Manuel Sánchez Luna, Head of Neonatology at Gregorio Marañón Hospital and Professor of Pediatrics at Complutense University of Madrid, participated in a review of the impact of RSV in Spain.

“After analyzing more than 1.5 million hospital discharge records in children under one year of age, we found that bronchiolitis, primarily due to RSV, was the most frequent cause of pediatric hospitalization.”

Before the arrival of nirsevimab, another monoclonal antibody existed, but it had a shorter duration — requiring monthly administration — and was “very expensive,” so it was only recommended for infants at high risk of severe illness.

However, 98% of children under one year of age hospitalized for respiratory infection were previously healthy infants.

“I was fortunate to collaborate with the prophylaxis group at the Ministry of Health,” Sánchez Luna said, who also serves as a spokesperson for the Spanish Society of Neonatology (Seneo).

“We were the first and only country to make an official recommendation throughout the national territory and have been a model for universal prevention of all babies.”

In the first season, more than 277,000 doses of the drug were administered, and coverage reached 92% in newborns and 87% in infants under six months.

“There was a decrease of around 45% in visits to primary care among children under five years of age that year,” he recalled with pride. “And hospitalization in infants under one year of age was reduced by 75%.”

For the following season, however, there were still doubts. “We were afraid that children who received the prophylaxis the year before might have a more serious process if they became infected.” But the success was repeated, and those fears were quickly dispelled.

Therefore, the pediatrician believes that “the strategy should not change.” It will only change “when we have a vaccine for the pediatric age group, but the data from those under investigation are not so positive at the moment.”

Meanwhile, despite the success of nirsevimab, the study authors remind us of the importance of washing hands “when picking up, feeding, or touching the baby,” as well as cleaning objects that the infant comes into contact with.

They also point out that people with respiratory illnesses “should avoid approaching the baby.” And they note that breastfeeding “transfers protection against respiratory syncytial virus.”