Nirsevimab side effects: what 1,720 FDA adverse event reports show

Nirsevimab · Beyfortus · 24 reaction terms · 698 drug–reaction co-reports · data 2026-09-14

Drug class

Respiratory Syncytial Virus Anti-F Protein Monoclonal Antibody

FDA label (Established Pharmacologic Class)

Brand names

Beyfortus

openFDA label

Who reports

1,720 reports · 64% serious · 41 with death outcome · 43% female

openFDA FAERS counts, all years, deduplication not applied

Used for

BEYFORTUS is indicated for the prevention of Respiratory Syncytial Virus (RSV) lower respiratory tract disease in: Neonates and infants born during or entering their first RSV season. Children up to 24 months of age who remain vulnerable to severe RSV disease through their second RSV season.

FDA label, Indications and Usage

What it is

Nirsevimab-alip, a respiratory syncytial virus F protein-directed fusion inhibitor, is a human immunoglobulin G1 kappa (IgG1κ) monoclonal antibody produced in Chinese hamster ovary (CHO) cells by recombinant DNA technology.

FDA label, Description section

Boxed warning

None in the current label

How to read this page. FAERS is a spontaneous-reporting system: counts are not incidence rates and a signal is not proof of causation. Each reaction below is run through an automated screen — is the term in the FDA label (known), in the indications (the disease being treated), an administrative/coding term, or none of these (unlabeled)? That screen is mechanical text matching, not a causality assessment. The 30 hand-written Bradford Hill assessments are on the causality assessment page. Not medical advice.

Causality screen of the 19 strongest signals

Labeled adverse reaction 2Reporting / coding artifact 6Not in label — unassessed 11

What the badges mean

Labeled adverse reaction The exact term appears in the label's warnings or adverse-reactions sections: a known, expected effect. The FAERS signal confirms the label rather than discovering anything.
Reporting / coding artifact An administrative, monitoring or exposure term (product issue, off-label use, 'test normal', overdose...). Not a biological reaction; a high PRR here reflects how reports are coded.
Not in label — unassessed Not found in the label text, not an artifact pattern. This is a genuine candidate for a formal causality assessment (Bradford Hill / WHO-UMC); it has NOT had one here.

Strongest disproportionality signals

Flagged terms (PRR ≥ 2, χ² ≥ 4, n ≥ 10), ranked by PRR; infinite PRRs (no comparator reports) excluded. The EBGM column is a second, shrinkage-based estimate (MGPS) with its 5th percentile EB05 and the BCPNN IC025; a pair flagged by PRR alone is marked PRR-only and should be read as fragile. The strata column recomputes the PRR on serious reports only and flags pairs whose reports are unusually skewed by sex or come mostly from consumers rather than clinicians. 5 further terms are in the interactive explorer. Reaction links open the term's cross-drug page.

Show: hide coding/lab/outcome terms hide indication terms unlabeled only hide PRR-only

Reaction	Reports	PRR 95% CI · ROR · χ²/p	EBGM EB05 · IC025	Label screen	Serious-only & strata	Trial cross-check
Extra dose administered	82	25.1 20.3–31.0ROR 26.3 · χ² 1,872 / p <1e-300	24.9 EB05 20.7 · IC025 +4.1 3/3 methods agree	Not in label — unassessed	serious n=2	no trial data cached
Hypotonia	15	18.3 11.0–30.2ROR 18.4 · χ² 227 / p 1e-51	17.6 EB05 10.8 · IC025 +2.7 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 18.8	no trial data cached
Tachypnoea	19	17.1 10.9–26.8ROR 17.3 · χ² 272 / p 2e-61	16.6 EB05 10.9 · IC025 +2.8 3/3 methods agree	Not in label — unassessed	serious PRR 15.9	no trial data cached
Cyanosis (blue skin from low oxygen)	22	17.0 11.2–25.8ROR 17.2 · χ² 316 / p 6e-71	16.6 EB05 11.3 · IC025 +2.9 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 16.3	no trial data cached
Expired product administered	42	15.4 11.5–20.8ROR 15.8 · χ² 553 / p 1e-122	15.2 EB05 11.6 · IC025 +3.2 3/3 methods agree	Reporting / coding artifact	serious n=1 · 100% clinician	no trial data cached
Respiratory distress	33	14.6 10.4–20.4ROR 14.8 · χ² 404 / p 4e-90	14.3 EB05 10.4 · IC025 +3.0 3/3 methods agree	Not in label — unassessed	serious PRR 13.4 · 94% clinician	no trial data cached
Syringe issue	18	12.5 7.9–19.8ROR 12.6 · χ² 179 / p 3e-41	11.6 EB05 7.2 · IC025 +2.5 3/3 methods agree	Not in label — unassessed	–	no trial data cached
Incorrect dose administered	229	12.4 11.0–14.0ROR 14.2 · χ² 2,416 / p <1e-300	12.4 EB05 11.1 · IC025 +3.4 3/3 methods agree	Reporting / coding artifact	serious PRR 3.7	no trial data cached
Incorrect route of product administration	17	12.3 7.7–19.8ROR 12.4 · χ² 166 / p 3e-38	11.3 EB05 6.9 · IC025 +2.4 3/3 methods agree	Reporting / coding artifact	serious PRR 15.7	no trial data cached
Poor quality product administered	10	11.9 6.4–22.1ROR 12.0 · χ² 89 / p 2e-21	9.9 EB05 5.0 · IC025 +1.9 3/3 methods agree	Not in label — unassessed	–	no trial data cached
Petechiae	10	11.7 6.3–21.8ROR 11.8 · χ² 88 / p 4e-21	9.6 EB05 4.9 · IC025 +1.9 3/3 methods agree	Not in label — unassessed	serious PRR 11.5	no trial data cached
Wheezing	41	8.4 6.2–11.4ROR 8.6 · χ² 262 / p 3e-59	7.9 EB05 6.0 · IC025 +2.4 3/3 methods agree	Not in label — unassessed	serious PRR 10.3	no trial data cached
Product dispensing error	15	8.4 5.1–13.9ROR 8.5 · χ² 91 / p 8e-22	7.1 EB05 4.4 · IC025 +1.9 3/3 methods agree	Reporting / coding artifact	serious n=1	no trial data cached
Rhinorrhoea (runny nose)	31	5.9 4.1–8.3ROR 6.0 · χ² 121 / p 2e-28	5.4 EB05 4.0 · IC025 +1.8 3/3 methods agree	Not in label — unassessed	serious PRR 11.9	no trial data cached
Pallor	12	5.4 3.1–9.4ROR 5.4 · χ² 38 / p 3e-10	4.5 EB05 2.8 · IC025 +1.2 3/3 methods agree	Not in label — unassessed	serious PRR 6.3	no trial data cached
Product storage error	38	5.4 3.9–7.3ROR 5.5 · χ² 131 / p 1e-30	5.0 EB05 3.8 · IC025 +1.8 3/3 methods agree	Reporting / coding artifact	95% clinician	no trial data cached
Respiratory arrest	11	4.8 2.7–8.7ROR 4.8 · χ² 30 / p 3e-8	4.1 EB05 2.5 · IC025 +1.0 3/3 methods agree	Not in label — unassessed	serious PRR 4.4	no trial data cached
Crying	14	4.7 2.8–7.9ROR 4.7 · χ² 37 / p 6e-10	4.1 EB05 2.6 · IC025 +1.2 3/3 methods agree	Not in label — unassessed	serious PRR 5.7	no trial data cached
Accidental overdose	13	4.3 2.5–7.5ROR 4.4 · χ² 30 / p 2e-8	3.8 EB05 2.4 · IC025 +1.0 3/3 methods agree	Reporting / coding artifact	serious PRR 0.5	no trial data cached

Trial evidence (ClinicalTrials.gov)

Trial cross-check not yet run for this drug (ClinicalTrials.gov pull covers the 800 most-reported drugs first).

Most frequently reported reactions

By raw co-report count, regardless of disproportionality. Common terms here are often the illness itself or generic outcomes.

Reaction	Reports	PRR 95% CI · ROR · χ²/p	EBGM EB05 · IC025	Label screen	Serious-only & strata	Trial cross-check
Incorrect dose administered	229	12.4 11.0–14.0ROR 14.2 · χ² 2,416 / p <1e-300	12.4 EB05 11.1 · IC025 +3.4 3/3 methods agree	Reporting / coding artifact	serious PRR 3.7	no trial data cached
Extra dose administered	82	25.1 20.3–31.0ROR 26.3 · χ² 1,872 / p <1e-300	24.9 EB05 20.7 · IC025 +4.1 3/3 methods agree	Not in label — unassessed	serious n=2	no trial data cached
Expired product administered	42	15.4 11.5–20.8ROR 15.8 · χ² 553 / p 1e-122	15.2 EB05 11.6 · IC025 +3.2 3/3 methods agree	Reporting / coding artifact	serious n=1 · 100% clinician	no trial data cached
Wheezing	41	8.4 6.2–11.4ROR 8.6 · χ² 262 / p 3e-59	7.9 EB05 6.0 · IC025 +2.4 3/3 methods agree	Not in label — unassessed	serious PRR 10.3	no trial data cached
Product storage error	38	5.4 3.9–7.3ROR 5.5 · χ² 131 / p 1e-30	5.0 EB05 3.8 · IC025 +1.8 3/3 methods agree	Reporting / coding artifact	95% clinician	no trial data cached
Respiratory distress	33	14.6 10.4–20.4ROR 14.8 · χ² 404 / p 4e-90	14.3 EB05 10.4 · IC025 +3.0 3/3 methods agree	Not in label — unassessed	serious PRR 13.4 · 94% clinician	no trial data cached
Rhinorrhoea (runny nose)	31	5.9 4.1–8.3ROR 6.0 · χ² 121 / p 2e-28	5.4 EB05 4.0 · IC025 +1.8 3/3 methods agree	Not in label — unassessed	serious PRR 11.9	no trial data cached
Cyanosis (blue skin from low oxygen)	22	17.0 11.2–25.8ROR 17.2 · χ² 316 / p 6e-71	16.6 EB05 11.3 · IC025 +2.9 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 16.3	no trial data cached
Tachypnoea	19	17.1 10.9–26.8ROR 17.3 · χ² 272 / p 2e-61	16.6 EB05 10.9 · IC025 +2.8 3/3 methods agree	Not in label — unassessed	serious PRR 15.9	no trial data cached
Syringe issue	18	12.5 7.9–19.8ROR 12.6 · χ² 179 / p 3e-41	11.6 EB05 7.2 · IC025 +2.5 3/3 methods agree	Not in label — unassessed	–	no trial data cached
Incorrect route of product administration	17	12.3 7.7–19.8ROR 12.4 · χ² 166 / p 3e-38	11.3 EB05 6.9 · IC025 +2.4 3/3 methods agree	Reporting / coding artifact	serious PRR 15.7	no trial data cached
Product dispensing error	15	8.4 5.1–13.9ROR 8.5 · χ² 91 / p 8e-22	7.1 EB05 4.4 · IC025 +1.9 3/3 methods agree	Reporting / coding artifact	serious n=1	no trial data cached
Hypotonia	15	18.3 11.0–30.2ROR 18.4 · χ² 227 / p 1e-51	17.6 EB05 10.8 · IC025 +2.7 3/3 methods agree	Labeled adverse reaction exact term	serious PRR 18.8	no trial data cached
Crying	14	4.7 2.8–7.9ROR 4.7 · χ² 37 / p 6e-10	4.1 EB05 2.6 · IC025 +1.2 3/3 methods agree	Not in label — unassessed	serious PRR 5.7	no trial data cached
Accidental overdose	13	4.3 2.5–7.5ROR 4.4 · χ² 30 / p 2e-8	3.8 EB05 2.4 · IC025 +1.0 3/3 methods agree	Reporting / coding artifact	serious PRR 0.5	no trial data cached

Unlabeled candidate signals

These flagged terms were not found in the FDA label text and do not match an artifact pattern. They are the pairs that would merit a structured assessment (temporality, dose-response, biological plausibility, consistency across databases, trial or cohort evidence). The screen has not done that; string-matching misses label wording, and a strong PRR can still be stimulated reporting, co-medication, or the disease itself.

Extra dose administered — n=82, PRR 25.1
Respiratory distress — n=33, PRR 14.6
Wheezing — n=41, PRR 8.4
Rhinorrhoea — n=31, PRR 5.9

Want to assess one of these properly? The runbook describes the Bradford Hill / trial-evidence workflow used on the assessment page, and handoff_top1000_signals.csv lists the strongest unassessed pairs across all drugs.

Common questions

What are the most reported side effects of Nirsevimab?

In FDA FAERS, the most frequently co-reported reaction terms for Nirsevimab are extra dose administered, wheezing, respiratory distress, rhinorrhoea, cyanosis, tachypnoea. These are report counts, not rates: FAERS has no denominator, reporting is voluntary and uneven, and a term can be reported because of the illness being treated rather than the drug.

Does Nirsevimab have an FDA boxed warning?

No. The FDA label retrieved for Nirsevimab has no boxed warning; see the label's Warnings and Precautions for the recognised risks.

Do FAERS signals prove that Nirsevimab causes these side effects?

No. A disproportionality signal (PRR, ROR) means a reaction is reported more often with this drug than with others in the database. That can reflect a real effect, but also the disease being treated, stimulated reporting after news or litigation, co-medications, or how reports are coded. Causality needs temporality, dose-response, biological plausibility, consistency across data sources and, ideally, trial or cohort evidence. This page screens each term against the FDA label and ClinicalTrials.gov results; it does not establish causation.

Sources & methods

FDA label via openFDA (set ID 2f08fa60-f674-432d-801b-1f9514bd9b39, effective 20260713; matched on openfda.generic_name.exact:"NIRSEVIMAB")
Bayesian shrinkage measures computed offline over all 1,317,300 cached pairs: BCPNN information component (IC, with Norén closed-form IC025) and MGPS empirical-Bayes geometric mean (EBGM, EB05) with a two-component gamma prior fitted by maximum marginal likelihood (α₁=0.57, β₁=0.02, α₂=1.42, β₂=0.70, w=0.17); unstratified
FAERS pair statistics from the OpenPV build of 2026-09-14 (openFDA count= aggregation, field patient.drug.openfda.generic_name.exact, 24 reaction terms, min 3 co-reports); PRR/ROR with 95% CI, chi-square with Yates correction
Serious-only and stratified counts (serious:1, patient.patientsex, primarysource.qualification) from openFDA count queries; serious-only PRR uses the drug serious report total and the term global serious total (N_serious=11,882,979); strata shown only when n ≥ 20
Seriousness and sex counts from openFDA count=serious, patient.patientsex, seriousnessdeath

Page generated 2026-10-08. Label and PubChem text are retrieved automatically and may lag the current label; always check the current prescribing information. OpenPV is a beta research tool of the Open Source Medicine Foundation.