Each year, respiratory syncytial virus hospitalizes 58,000 to 80,000 children under age 5 in the U.S. The Food and Drug Administration recently approved an antibody injection for babies to protect them during the RSV season. There isn’t evidence the shots have killed any babies, contrary to social media claims.
Respiratory syncytial virus is a common respiratory virus that classically ramps up in the fall and peaks in winter. It causes a mild cold for most people, but the very young and old can experience severe illness, leading to hospitalization and even death. In the past several months, the U.S. Food and Drug Administration has approved various products to protect against lower respiratory tract disease from RSV in either young children or older adults.
One of these approvals was for nirsevimab, an injection that the Centers for Disease Control and Prevention has recommended for babies under 8 months going into their first RSV season. A small number of especially vulnerable children under 19 months are also recommended to get the shot going into their second RSV season.
Nirsevimab, which has the brand name Beyfortus, is a monoclonal antibody. It is a lab-produced antibody that recognizes and binds to a component of RSV, preventing the virus from entering cells. The nirsevimab injection is a form of passive immunization, which means that rather than training the immune system to recognize the virus and produce antibodies, as occurs with a vaccine, it provides the body with ready-made antibodies.
In clinical trials of babies entering their first RSV season, nirsevimab reduced the risk of RSV-associated illness requiring medical attention by 70% to 75% during the first five months after the injection. The drug also showed an “overall favorable safety profile,” according to an FDA briefing document, and deaths of children who received nirsevimab in clinical trials were not likely to be related to the monoclonal antibody. “The majority of deaths were clearly related to causes other than the study drug product,” the FDA document said. “Others were clearly complicated by underlying conditions.”
Yet widely shared social media posts implied without evidence that nirsevimab killed 12 babies and shared the unfounded claim that nirsevimab will kill more babies than RSV. The posts referenced an article from Robert F. Kennedy Jr.’s organization, Children’s Health Defense, which has an extensive history of spreading vaccine misinformation.
There isn’t evidence nirsevimab has killed any babies. In clinical trials, 12 out of 3,710 children who received nirsevimab and four out of 1,797 in the control groups died of various causes. The death rate in the nirsevimab group was “slightly” higher than in the control group, the FDA document said, but further scrutiny showed that “none of the deaths were likely related to the study drug.”
“The important fact is those deaths had no relationship to the administration of the monoclonal antibody Beyfortus,” Dr. H. Cody Meissner, a professor of pediatrics and medicine at Dartmouth Geisel School of Medicine, told us.
Dr. Sean T. O’Leary, a pediatric infectious disease physician at the University of Colorado Anschutz Medical Campus/Children’s Hospital Colorado, called the Children’s Health Defense article “fear-mongering.” He told us that by calling attention to the deaths, the article “is basically just trying to scare parents from getting the product, but there’s no basis in reality that this product is dangerous based on the data we have.”
Nirsevimab Not Linked to Clinical Trial Deaths
Early childhood is a relatively dangerous period of a person’s life, particularly when looking at mortality worldwide. In discussing the deaths in the nirsevimab trial, the FDA briefing document authors wrote, “For context, the global infant mortality rate in 2021 was 28 deaths per 1,000 live births (The World Bank 2021). Therefore, it is not surprising to have infant deaths occur during the global clinical development of nirsevimab.”
The FDA document reviewed five pediatric clinical trials of nirsevimab conducted in multiple countries. Over at least 360 days of follow-up, there were 3.2 deaths per 1,000 in the nirsevimab groups, versus 2.2 per 1,000 in the control groups.
This “imbalance” meant it was important to take a closer look at the deaths, the document said, including their causes and timing and the children’s medical conditions. This scrutiny is standard, experts explained.
“In any large randomized clinical trial there will be a certain number of deaths,” Meissner said. “Remember, the important thing is to look at why those deaths occurred.”
The deaths in the babies who received nirsevimab had diverse causes. Eight deaths had causes “clearly unrelated” to nirsevimab, the FDA document said, including deaths related to being hit by a car, cancer, COVID-19, heart diseases present at birth and gastroenteritis. (The deaths from gastroenteritis were in children in South Africa, where death from diarrheal disease is more common than in the U.S.)
The FDA said that two additional deaths from respiratory infections were “likely related to underlying conditions” the children had. The two remaining children died of unknown causes. One may have had an undiagnosed underlying condition, according to his health care provider, while the other death was “consistent with sudden infant death syndrome,” the document said.
“From the biology of the way these things work, there’s no reason to think that this product would have led to any of those [deaths],” said O’Leary, who is chairperson of the Committee on Infectious Diseases of the American Academy of Pediatrics.
A social media post falsely claimed, “FDA is doing their usual song & dance claiming every death in clinical trials had nothing to do with the shot. Same story, different day. These are babies we’re talking about.”
FDA scientists are just one expert group that scrutinizes deaths in clinical trials. Meissner explained that he is a member of a CDC work group that looked at the nirsevimab data. “There was no evidence that the monoclonal antibody played any role whatsoever in the death of those children,” he said.
Clinical trials also may have a Data and Safety Monitoring Board, a group of outside experts that monitors trial outcomes as the trial progresses. If they see a concerning pattern of adverse events or deaths, they can stop the clinical trial. And the investigators in charge of the clinical trial and each trial site review deaths, O’Leary explained.
RSV Causes Significant Harm
In the U.S., RSV is the leading cause of hospitalization in babies under age 1. Meissner explained that it can cause bronchiolitis, or “swelling in the tiny airways” of a baby.
However, the Children’s Health Defense article said that “medical experts” called the CDC’s recommendation of nirsevimab “unnecessary.”
RSV is mild for many children, but this is not always the case. Many otherwise healthy infants can be hospitalized. “Hospitalization is a big deal for a child, and there are also often long-term effects after hospitalizations for children,” O’Leary said. “It’s tremendously stressful for a family to have their child in the hospital. Often, these children end up in the ICU for several days, if not weeks.”
“Even though there are very few children who die of RSV bronchiolitis, that is correct, it still typically results in two or three days of hospitalization to give the baby supplemental oxygen and sometimes mechanical ventilation to make sure they get over their illness,” Meissner said.
O’Leary called this claim “absolute nonsense.”
“I think whoever says something like that hasn’t really looked at the facts, because if you look at the facts, nirsevimab hasn’t killed any babies,” Meissner said.
In contrast, RSV does kill some children, although estimates vary on how many. The CDC estimates there are 100 to 300 deaths per year in the U.S. from RSV in children under 5 years of age.
“I think people should understand that this is really a big breakthrough,” Meissner said. “RSV was first described in 1956, 75 years ago. People have been working on finding a way to prevent severe disease due to RSV for a long time.”
Nirsevimab Isn’t a Vaccine, But Plays a Similar Role
One area of confusion has been around the classification of nirsevimab and whether it is a vaccine. It is not a vaccine, but it is fulfilling a similar purpose.
In the context of infectious disease, the word vaccine is generally understood to describe a weakened or partial version of a microbe, or genetic instructions for making part of this microbe, given to someone to induce an immune response. This teaches a person’s immune system to mount a defense the next time it encounters the threat.
Rather than teaching the body to make antibodies to RSV, as a vaccine would, nirsevimab provides the child with the antibodies directly.
Meissner explained that nirsevimab provides a child with antibodies similar to those they would make if they were infected with RSV. “This just prevents the child from getting a severe infection and offers antibodies that we know are very safe,” he said.
Nirsevimab is not the first monoclonal antibody to be FDA approved for prevention of RSV in infants. Palivizumab was approved in 1998 with the brand name Synagis and is recommended as a monthly injection for some high-risk children during RSV season.
Nirsevimab will be available for all infants and will also replace palivizumab for high-risk children, Meissner said. Nirsevimab has been modified to persist longer in the body than a typical antibody, he said, allowing a single dose to protect a child through an entire RSV season. Palivizumab, in contrast, has to be given as many as five times in a single RSV season, and also doesn’t work as well at reducing RSV hospitalizations as nirsevimab.
Despite this history, social media posts highlight the novelty of nirsevimab. “Another experiment. This time on NEWBORNS…,” one post reads.
But as we have explained, nirsevimab has been tested in multiple clinical trials and is not an entirely novel type of product. “From the people that work in the field, we don’t have any undue concern that there will be some kind of a safety surprise here, because we do have a lot of experience with other monoclonal antibody products, and this particular product has a very specific target on the RSV virus,” O’Leary said.
The Children’s Health Defense article also referenced a post by anti-vaccine internist Dr. Meryl Nass, who said that the CDC is “playing fast and loose with the definition of vaccine” in its treatment of nirsevimab.
Nirsevimab will indeed be treated similarly to a vaccine in some contexts, to ensure access for a broader group of children. “CDC has determined that nirsevimab is eligible for inclusion in the childhood immunization schedule and Vaccines for Children program,” according to an Aug. 3 presentation from the CDC’s Dr. Georgina Peacock.
The VFC program provides recommended vaccines at no cost to children whose families might not be able to afford them. This inclusion is possible due to the lack of a “statutory” definition of vaccine in relevant laws, according to Peacock.
While nirsevimab is not a vaccine, Meissner said, it makes sense to ensure its coverage. “We want to be sure everybody, regardless of their insurance, is able to get this protection,” he said, adding that nirsevimab will be the first monoclonal antibody covered under the VFC program.
In other contexts, nirsevimab will be considered a therapeutic, potentially causing challenges, Peacock’s presentation explained. For example, it will be coded as a drug/therapeutic, potentially leading to issues with insurance policies not paying for counseling by a health care provider associated with the shot. Adverse events will be reported to different government monitoring systems depending on whether nirsevimab is given alongside vaccines or on its own.
The FDA will also soon decide whether to approve an RSV vaccine to be given during pregnancy. This vaccine would cause the mother to produce RSV antibodies, which would then be passed along to the fetus. O’Leary said that if the maternal vaccine is approved, the recommendation will be for healthy infants to get protection either from the maternal vaccine or nirsevimab, but not both.
Update, Aug. 22: The FDA approved Pfizer’s maternal vaccine, Abrysvo, on Aug. 21 for pregnant individuals at 32 to 36 weeks of gestation.
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