A large Canadian study suggests that nirsevimab, a long-acting monoclonal antibody used to prevent respiratory syncytial virus (RSV), is well tolerated in infants, with mostly mild, temporary symptoms following immunization.
The study, led by researchers at the University of British Columbia and published in the most recent issue of Human Vaccines & Immunotherapeutics, used parent-completed questionnaires to assess safety data from 1,559 infants during the 2024–25 RSV season. The questionnaires were completed in the week following immunization.
Mostly mild reactions, slightly higher with concurrent vaccines
Among the children in the study, 140 (9.0%) experienced local injection-site reactions, such as redness, swelling, or pain. Local reactions were more common when nirsevimab was given alongside other routine childhood vaccines: 17.4% of those who received co-administered vaccines experienced local reactions, compared with 5.5% when administered alone. Larger injection-site reactions (extending beyond the nearest joint) were rare, occurring in just 0.4% of cases.
The most frequently reported symptoms were runny nose (1.8%), cough (1.7%), eating changes (1.6%), fever (1.6%), and gastrointestinal symptoms (1.5%), and they were all generally mild and short-lived. Rash occurred in 0.6% of participants. Children who received nirsevimab alongside other routine vaccines were about twice as likely to have cough, congestion, diarrhea, and vomiting as those who received nirsevimab alone. Most reactions occurred within two to three days of immunization.
About 3.7% of participants reported a new or worsening condition that interfered with daily activities or required medical attention. New or worsening health events were similar whether nirsevimab was given alone (3.4%) or with other vaccines (4.2%).
Serious reactions uncommon
More-serious health events were rare. Ten children (0.6%) visited an emergency department. Of those, four had diagnosed or suspected viral infections. The other five infants didn’t receive a specific diagnosis, but their parents reported the symptoms that worried them most, including diarrhea, trouble breathing, fever, cough, and ongoing crying, which occurred between one and five days after vaccination.
The remaining infant, who received nirsevimab together with other routine childhood vaccines, was diagnosed with “post-vaccination allergic reaction.” Symptoms, including facial swelling, rash, red and swollen eyes, and respiratory symptoms, started an hour after vaccination and lasted for 10 hours. No systemic vaccine allergy was diagnosed.
“Serious health events, including emergency department visits and hospitalizations, were rare and appeared mostly unrelated to immunization,” write the authors.
Parent reports help fill safety data gaps
By drawing on parent-reported questionnaires and capturing symptoms that might not lead to medical visits, the findings reflect a wider range of symptoms than earlier studies and help give a more real-world picture of nisevimab’s safety.
“This participant-centered active surveillance study found that nirsevimab was well tolerated among Canadian infants, with a low incidence of parent-reported health events within seven days of immunization,” write the authors. “By capturing parent-reported acute outcomes, this study helps fill gaps in post-licensure safety data, support generalizability of safety events reported in the limited number of active surveillance studies reported today and offers valuable insights to guide clinical practice and vaccine communication strategies.”
