Beyfortus (Nirsevimab): Uses, Dosage & Side Effects

What Is Beyfortus and What Is It Used For?

Beyfortus contains the active substance nirsevimab, a recombinant human immunoglobulin G1 kappa (IgG1κ) monoclonal antibody produced using recombinant DNA technology in Chinese hamster ovary (CHO) cells. It was jointly developed by Sanofi and AstraZeneca and received European Medicines Agency (EMA) marketing authorization in October 2022, followed by U.S. Food and Drug Administration (FDA) approval in July 2023. It is the first monoclonal antibody approved for the prevention of RSV disease in the general infant population.

Respiratory syncytial virus is a common respiratory pathogen that infects nearly all children by the age of two. While most RSV infections cause mild, cold-like symptoms in older children and adults, the virus can cause severe illness in infants, particularly in those under six months of age. RSV is the leading cause of bronchiolitis (inflammation of the small airways in the lungs) and pneumonia (lung infection) in infants worldwide, and it is the most common reason for hospitalization among infants under one year of age. The World Health Organization estimates that RSV causes approximately 33 million lower respiratory tract infections, 3.6 million hospitalizations, and over 100,000 deaths annually in children under five years of age globally.

How Beyfortus Works

Nirsevimab works by binding to the prefusion conformation of the RSV fusion (F) protein, a surface glycoprotein that the virus requires to enter and infect human cells. The RSV F protein exists in a metastable prefusion form before it mediates viral entry; nirsevimab locks this protein in its prefusion state, preventing the conformational change necessary for the virus to fuse with the host cell membrane. By neutralizing the virus at this critical step, Beyfortus blocks RSV from establishing infection in the respiratory tract.

Unlike traditional vaccines, which require the recipient's immune system to recognize an antigen and generate its own antibodies over a period of weeks, Beyfortus provides passive immunization by delivering ready-made antibodies directly into the bloodstream. This distinction is particularly important for young infants, whose adaptive immune systems are immature and may not mount a robust response to conventional vaccination. Beyfortus has been engineered with a triple amino acid substitution (YTE modification) in the Fc region of the antibody, which extends its half-life to approximately 63 to 73 days, allowing a single dose to provide sustained protection throughout the typical 5-month RSV season.

Prior to the development of nirsevimab, the only available monoclonal antibody for RSV prevention was palivizumab (Synagis), which required monthly injections throughout the RSV season and was limited to high-risk infants due to cost and logistical constraints. Beyfortus represents a significant advance because its extended half-life allows a single-dose regimen, and its approval encompasses all infants, not just those at high risk. In the pivotal MELODY trial published in the New England Journal of Medicine, a single dose of nirsevimab reduced medically attended RSV-associated lower respiratory tract infections by 74.5% compared to placebo in healthy late-preterm and term infants.

Approved Indications

Beyfortus is indicated for the prevention of RSV lower respiratory tract disease in the following populations:

• All neonates and infants born during or entering their first RSV season. This broad indication means that any infant, regardless of underlying health conditions or gestational age at birth, is eligible to receive Beyfortus before or during the RSV season.
• Children up to 24 months of age who remain vulnerable to severe RSV disease through their second RSV season. This includes children with chronic lung disease of prematurity (bronchopulmonary dysplasia), hemodynamically significant congenital heart disease, severe combined immunodeficiency or other forms of severe immunocompromise, and those with neuromuscular conditions that impair airway clearance.

The Centers for Disease Control and Prevention (CDC) and the American Academy of Pediatrics (AAP) jointly recommend Beyfortus for all infants under 8 months of age born during or entering their first RSV season, with additional recommendations for high-risk children aged 8 to 19 months entering their second RSV season. In the European Union, the EMA-approved indication covers neonates and infants during their first RSV season, and children up to 24 months who remain at high risk for their second season.

What Should You Know Before Your Child Receives Beyfortus?

Contraindications

Beyfortus must not be administered to children with known hypersensitivity (allergy) to nirsevimab or to any of the excipients in the formulation. The inactive ingredients include L-histidine, L-histidine hydrochloride, L-arginine hydrochloride, sucrose, polysorbat 80 (E433), and water for injections. Although rare, any history of a severe allergic reaction to a previous dose of Beyfortus or to any component of the formulation is an absolute contraindication to further administration.

As with any injectable medicine, appropriate medical treatment and supervision should be available for immediate use in the event of an anaphylactic reaction following administration. Healthcare providers administering Beyfortus should be trained in the recognition and management of anaphylaxis, and observation for at least 15 minutes after injection is recommended as standard practice.

Warnings and Precautions

Before your child receives Beyfortus, inform the healthcare provider about the following conditions:

• Bleeding disorders or low platelet count (thrombocytopenia): Since Beyfortus is given as an intramuscular injection, it should be administered with caution in children with thrombocytopenia, bleeding disorders, or those receiving anticoagulant therapy. The injection may cause bleeding or bruising at the injection site in these patients. Subcutaneous administration may be considered as an alternative in exceptional circumstances.
• Immunocompromised states: While Beyfortus can be used in immunocompromised children, the efficacy and duration of protection may be reduced in children with severe immunodeficiency, as they may metabolize the antibody differently.
• Conditions causing excess protein loss: In children with chronic conditions that cause significant protein loss through urine or the gastrointestinal tract, such as nephrotic syndrome or protein-losing enteropathy, the protective antibody levels from Beyfortus may decrease more rapidly. These children may require closer monitoring or earlier re-dosing decisions in consultation with their specialist physician.

Use in Specific Populations

Beyfortus is specifically designed for use in infants and children up to 24 months of age. It is not indicated for use in adults, adolescents, or children aged 2 to 18 years, as the safety and efficacy have not been established in these age groups. There is no relevance for pregnancy or breastfeeding considerations, as Beyfortus is a pediatric medicine administered directly to the infant, not to the mother.

For pregnant women seeking to protect their newborns against RSV, maternal RSV vaccination (such as Abrysvo) during pregnancy is an alternative strategy. However, current guidance from the CDC and AAP recommends that infants should receive either maternal RSV vaccination or Beyfortus, but generally not both. Parents should discuss with their pediatrician which approach is most appropriate for their situation, taking into account factors such as timing of birth relative to the RSV season and availability of each product.

Age and Weight Considerations

Beyfortus dosing is based on the child's body weight at the time of administration, not on age alone. For their first RSV season, infants weighing less than 5 kg receive 50 mg (one 0.5 ml injection), while those weighing 5 kg or more receive 100 mg (one 1 ml injection). For children entering their second RSV season who qualify based on high-risk criteria, the dose is 200 mg (given as two separate 100 mg injections at different sites). There are no published clinical data on the use of Beyfortus in children who weigh more than 30 kg.

How Does Beyfortus Interact with Other Drugs?

As a monoclonal antibody that targets a viral protein rather than a human metabolic pathway, Beyfortus does not interact with the cytochrome P450 enzyme system or other drug-metabolizing pathways. Formal drug interaction studies have not been conducted because the mechanism of action of nirsevimab makes pharmacokinetic interactions with other medications extremely unlikely. Monoclonal antibodies are cleared through intracellular catabolism (proteolytic degradation) rather than hepatic or renal metabolic pathways, which eliminates the common mechanisms by which conventional drugs interact with each other.

Co-administration with Vaccines

One of the most important practical considerations for Beyfortus is its compatibility with the standard childhood immunization schedule. Clinical trials and post-marketing data have confirmed that Beyfortus can be administered at the same time as routine childhood vaccines without any reduction in the efficacy of either Beyfortus or the co-administered vaccines. When given concurrently, Beyfortus and vaccines should be administered at different injection sites (for example, one in each thigh).

Interaction with Other Monoclonal Antibodies

If a child has previously received palivizumab (Synagis), another anti-RSV monoclonal antibody, Beyfortus should not be administered during the same RSV season. Both products target the RSV F protein, and co-administration has not been studied. However, Beyfortus can be used in subsequent RSV seasons for children who previously received palivizumab. Conversely, if Beyfortus has already been administered for the current RSV season, palivizumab should not be given.

Regarding maternal RSV vaccination (Abrysvo), current U.S. guidance recommends that infants born to mothers who received the RSV vaccine at least 14 days before delivery generally do not need Beyfortus, as they are likely to have adequate transplacental antibody protection. However, this recommendation may vary by country and clinical situation, and healthcare providers should make individualized decisions based on the timing of maternal vaccination relative to delivery, the infant's gestational age, and the proximity of the RSV season.

What Is the Correct Dosage of Beyfortus?

Beyfortus is administered exclusively by healthcare professionals as an intramuscular (IM) injection. The recommended injection site is the anterolateral aspect of the thigh (the outer front part of the upper leg). The gluteal muscle (buttock) should not be used as a routine injection site due to the risk of sciatic nerve injury. The dose is determined by the child's body weight at the time of administration and the RSV season in question.

First RSV Season

Second RSV Season (High-Risk Children Only)

Optimal Timing

The timing of Beyfortus administration is crucial for ensuring adequate protection throughout the RSV season. In temperate climates of the Northern Hemisphere, the RSV season typically runs from October or November through March or April. In the Southern Hemisphere, it usually occurs from April through September. In tropical and subtropical regions, RSV may circulate year-round or have less predictable seasonal patterns.

The recommended approach is to administer Beyfortus shortly before the anticipated start of the RSV season. For infants born before the RSV season, the injection is ideally given in the weeks leading up to the season's onset. For infants born during the RSV season, Beyfortus should be given as soon as possible after birth, ideally before hospital discharge or at the earliest postnatal visit.

After Cardiac Surgery

Children who undergo cardiac surgery (cardiopulmonary bypass) may experience a significant reduction in circulating antibody levels due to hemodilution and the use of extracorporeal circuits. If a child has previously received Beyfortus and subsequently undergoes cardiac surgery during the RSV season, an additional dose may be considered to restore protective antibody levels. The decision to re-dose should be made by the treating physician in consultation with the child's cardiologist and infectious disease specialist, weighing the remaining duration of the RSV season and the child's clinical condition.

Missed Dose

If a child misses the optimal window for Beyfortus administration before the RSV season, the injection should still be given as soon as possible, provided the RSV season is still ongoing. Even partial-season protection offers significant clinical benefit. However, if the RSV season has already ended, administration should be deferred until the next RSV season. There is no concept of a "catch-up" dose outside of the RSV season.

Overdose

There is no specific clinical experience with Beyfortus overdose. In the event of accidental overdose, the child should be monitored for any signs or symptoms of adverse effects, and appropriate symptomatic treatment should be initiated as necessary. Because nirsevimab is a monoclonal antibody with a highly specific target (RSV F protein), overdose is unlikely to cause systemic toxicity. There is no specific antidote for nirsevimab overdose; management would be supportive and guided by the clinical scenario.

What Are the Side Effects of Beyfortus?

Like all medicines, Beyfortus can cause side effects, although not every child will experience them. The safety of nirsevimab has been evaluated in multiple clinical trials involving more than 3,000 infants and young children, as well as through extensive post-marketing surveillance since its introduction. The overall safety profile is highly favorable, with the vast majority of reported adverse events being mild and self-limiting.

In the pivotal phase 3 MELODY trial (which enrolled 1,490 healthy late-preterm and term infants) and the phase 2b MEDLEY trial (which included preterm infants and those with chronic lung disease or congenital heart disease), the incidence of adverse events was similar between the nirsevimab group and the placebo group. No safety signals of concern were identified during these trials or in subsequent real-world surveillance across multiple countries.

Understanding the Side Effect Profile

The favorable safety profile of Beyfortus can be attributed to its highly targeted mechanism of action. Unlike broad-spectrum immunosuppressive agents or drugs that interact with human metabolic pathways, nirsevimab specifically targets the RSV fusion protein, which has no human homologue. This means the antibody does not cross-react with human tissues or interfere with the body's normal physiological processes. Additionally, nirsevimab is a fully human antibody (not chimeric or humanized from animal sources), which further reduces the risk of immunogenic reactions.

Injection site reactions are the most intuitively expected side effect of any intramuscular injection and are generally attributable to the physical process of injection rather than the specific drug product. These reactions are typically mild and resolve spontaneously within one to two days. Applying a cool compress to the injection site can help alleviate discomfort.

Fever, when it occurs, is usually low-grade (below 38.5°C / 101.3°F) and transient. Parents and caregivers should monitor the infant's temperature and consult their pediatrician if fever exceeds 38.5°C, persists for more than 48 hours, or is accompanied by other concerning symptoms such as poor feeding, excessive irritability, or difficulty breathing.

When to Seek Immediate Medical Attention

Although extremely rare, severe allergic reactions (anaphylaxis) can occur with any biological product. Parents and caregivers should seek immediate emergency medical care if they observe any of the following symptoms after administration of Beyfortus:

• Difficulty breathing or wheezing
• Swelling of the face, lips, tongue, or throat
• Severe skin rash with hives (raised, itchy welts)
• Pale or blue skin color
• Rapid heartbeat or weak pulse
• Unusual drowsiness or unresponsiveness

Reporting Side Effects

Reporting suspected side effects after a medicine has been authorized is critically important for ongoing safety monitoring. Healthcare professionals and patients/caregivers are encouraged to report any suspected adverse reactions to their national pharmacovigilance authority. In the United States, reports can be submitted through the FDA's MedWatch program. In the European Union, reports can be filed through the national competent authority in each member state. In the United Kingdom, the Yellow Card Scheme operated by the MHRA is the reporting mechanism. Reporting allows regulatory authorities to continuously evaluate the benefit-risk balance of approved medicines.

How Should Beyfortus Be Stored?

Beyfortus is supplied as a ready-to-use solution in a pre-filled syringe and does not require reconstitution, dilution, or mixing. As a biological product (monoclonal antibody), proper storage is essential to maintain its efficacy and safety. The storage and handling of Beyfortus are the responsibility of the healthcare provider, pharmacy, or healthcare facility administering the product. Parents and caregivers do not need to store Beyfortus at home.

The key storage requirements are as follows:

• Refrigerate at 2°C to 8°C (36°F to 46°F): Beyfortus must be kept continuously refrigerated. It should not be stored at room temperature for extended periods.
• Protect from light: Store the pre-filled syringe in its original carton until ready for use. Light exposure can degrade the antibody protein.
• Do not freeze: Freezing can damage the protein structure of the monoclonal antibody and render it ineffective. If Beyfortus has been accidentally frozen, it must be discarded and not administered.
• Do not shake: Vigorous shaking can cause protein aggregation and denaturation, which may reduce efficacy or increase the risk of immunogenicity. If the syringe has been accidentally shaken, it should not be used.
• Do not expose to direct heat: Elevated temperatures can accelerate protein degradation.
• Use within 8 hours after removal from refrigerator: Once Beyfortus has been removed from refrigerated storage, it should be administered within 8 hours. If not used within this window, the syringe must be discarded.

Before administration, healthcare professionals should visually inspect the solution. Beyfortus should appear as a clear to opalescent, colorless to yellow liquid. The syringe should not be used if the solution is cloudy, discolored, or contains large particles or foreign matter. Small amounts of translucent to white proteinaceous particles may be present and are acceptable, as this is normal for protein-based biological products.

Each pre-filled syringe is for single use only. Any unused product remaining in the syringe after administration should be disposed of in accordance with local regulations for pharmaceutical waste. The pre-filled syringe should not be reused, even if medication appears to remain in the device. Healthcare facilities should document the product name and batch number for traceability of biological products, as required by regulatory guidelines.

What Does Beyfortus Contain?

Active Ingredient

The active substance in Beyfortus is nirsevimab, a recombinant human immunoglobulin G1 kappa (IgG1κ) monoclonal antibody. Each pre-filled syringe contains a precisely measured dose:

• 50 mg/0.5 ml syringe: Contains 50 mg of nirsevimab in 0.5 ml of solution (concentration: 100 mg/ml)
• 100 mg/1 ml syringe: Contains 100 mg of nirsevimab in 1 ml of solution (concentration: 100 mg/ml)

Inactive Ingredients (Excipients)

The formulation of Beyfortus includes carefully selected excipients that maintain the stability and integrity of the monoclonal antibody protein:

Packaging

Beyfortus is available in the following presentations:

• 1 pre-filled syringe (50 mg or 100 mg) without needles
• 5 pre-filled syringes (50 mg or 100 mg) without needles
• 1 pre-filled syringe packaged with two separate needles of different sizes (to accommodate different patient sizes)

Not all pack sizes may be available in every country. The pre-filled syringes are made of Type I glass and fitted with a bromobutyl rubber plunger stopper. The tip cap contains no latex. Healthcare providers should select the appropriate needle gauge based on the infant's size and the recommended injection technique for intramuscular administration in the pediatric population.