Risks Associated with Nirsevimab Injection
Nirsevimab has an excellent safety profile with minimal risks; the most common adverse reactions are mild rash (0.9%) and injection site reactions (0.3%), with no increased incidence of serious adverse events compared to placebo. 1
Common Adverse Reactions
The most frequently reported adverse effects are minor and self-limited:
- Rash occurring within 14 days of injection affects 0.9% of recipients (versus 0.6% with placebo) 1
- Injection site reactions (pain, swelling, or hardness) within 7 days occur in 0.3% of recipients (versus 0% with placebo) 1, 2
- Overall adverse events within 360 days were reported in only 1.2% of nirsevimab recipients, with 97% being mild to moderate in intensity 1
Serious Adverse Events
The clinical trial data demonstrate reassuring safety outcomes:
- The incidence of serious adverse events was not increased in the nirsevimab arm compared to placebo 1
- No deaths attributable to RSV or nirsevimab were reported in either the phase 2b or phase 3 trials 1
- No cases of anaphylaxis or immune complex disease were documented in the clinical trials 1
Specific Safety Concerns Evaluated
Two cases of thrombocytopenia were identified as adverse events of special interest, but neither was attributed to nirsevimab:
- One case was diagnosed as heparin-induced thrombocytopenia 1
- One case occurred in a patient with sepsis 1
- Neither event was assessed as being related to nirsevimab administration 1
Contraindications and Precautions
There is only one absolute contraindication:
- Nirsevimab is contraindicated in persons with a history of severe allergic reaction (e.g., anaphylaxis) to a previous dose or any product component 3, 2
Special precautions apply in specific clinical scenarios:
- Bleeding complications could theoretically occur in patients with bleeding disorders or those who bruise easily, as the intramuscular injection could cause problems in these infants 3, 2
- Caregivers should be advised to seek immediate medical attention if signs of hypersensitivity develop, including swelling of face/mouth/tongue, difficulty breathing, unresponsiveness, bluish discoloration, muscle weakness, or severe rash 2
Long-Term Safety Data
Extended follow-up from clinical trials provides additional reassurance:
- The incidence of medically attended RSV-associated LRTI 351–510 days after injection was not significantly different between nirsevimab (0.7%) and control (0.2%) groups, indicating no shift of RSV burden to the second year of life 1
- The incidence of new onset chronic disease was similar in nirsevimab (0.3%) and placebo (0.4%) arms 1
- Antidrug antibodies were detected in 6.1% of nirsevimab recipients versus 1.1% of placebo recipients through day 361, without apparent clinical significance 4
Real-World Safety Experience
Post-licensure surveillance continues to support the favorable safety profile established in clinical trials, with treatment-related adverse events occurring in only 2.1% of recipients in pragmatic real-world studies 5. The FDA and CDC recommend reporting any adverse events to FDA MedWatch or VAERS if co-administered with vaccines 3.