Synagis: Uses, Dosage & Side Effects

What Is Synagis and What Is It Used For?

Synagis contains the active substance palivizumab, a humanized immunoglobulin G1 (IgG1κ) monoclonal antibody produced using recombinant DNA technology. As a monoclonal antibody, palivizumab is a highly specialized protein that has been engineered to recognize and bind to a single, specific molecular target. In the case of Synagis, this target is the A antigenic site on the fusion (F) protein of respiratory syncytial virus (RSV), one of the most important surface proteins responsible for the virus's ability to enter and infect human respiratory epithelial cells.

Respiratory syncytial virus (RSV) is one of the most common and significant causes of lower respiratory tract infections in infants and young children worldwide. According to the World Health Organization (WHO), RSV is responsible for an estimated 33 million episodes of acute lower respiratory infection in children under 5 years of age annually, leading to approximately 3.2 million hospitalizations and 60,000 to 120,000 in-hospital deaths globally each year. In temperate climates, RSV follows a seasonal pattern, with epidemics typically occurring between November and March in the Northern Hemisphere (and May to September in the Southern Hemisphere). While the majority of healthy, full-term infants who contract RSV will experience only mild, cold-like symptoms, certain groups of children are at substantially increased risk for developing severe disease requiring hospitalization, intensive care, or mechanical ventilation.

The RSV F protein is essential for viral entry into host cells. It mediates the fusion of the viral envelope with the host cell membrane, allowing the virus to deliver its genetic material into the cell and begin replicating. By binding to the A antigenic site on the F protein with high affinity, palivizumab effectively neutralizes the virus—preventing it from attaching to and fusing with respiratory epithelial cells. This mechanism of action blocks viral entry, inhibits viral replication, and prevents the spread of infection within the lower respiratory tract. In vitro studies have shown that serum concentrations of palivizumab above approximately 40 μg/mL reduce RSV replication in the lungs by approximately 99%.

It is important to understand that Synagis provides passive immunity, meaning it supplies the body with pre-formed antibodies rather than stimulating the immune system to produce its own antibodies (as a vaccine would). This distinction is clinically important: passive immunity provides immediate but temporary protection that wanes as the antibody is naturally metabolized and eliminated from the body. The serum half-life of palivizumab in children is approximately 18 to 20 days, which is why monthly injections are required throughout the RSV season to maintain protective antibody levels.

Synagis is specifically indicated for the prevention of serious lower respiratory tract disease caused by RSV in the following high-risk pediatric populations:

• Premature infants: Children born at 35 weeks of gestational age or earlier who are less than 6 months old at the onset of the RSV season. Premature infants have immature lungs and immune systems, making them particularly vulnerable to severe RSV disease including bronchiolitis and pneumonia.
• Children with bronchopulmonary dysplasia (BPD): Infants and children under 2 years of age who have required medical treatment (supplemental oxygen, bronchodilators, diuretics, or corticosteroids) for BPD within the 6 months before the expected RSV season. BPD, also known as chronic lung disease of prematurity, leaves the lungs damaged and more susceptible to respiratory infections.
• Children with hemodynamically significant congenital heart disease (CHD): Infants and children under 2 years of age with hemodynamically significant CHD, including those with cyanotic heart defects, those requiring medical treatment for congestive heart failure, and those with pulmonary hypertension. These children are at increased risk of severe RSV disease due to compromised cardiovascular and pulmonary function.

The efficacy of palivizumab was established in two pivotal randomized, double-blind, placebo-controlled clinical trials. The IMpact-RSV trial enrolled 1,502 high-risk premature infants and children with BPD, demonstrating a 55% reduction in RSV-related hospitalizations in the palivizumab group compared with placebo (4.8% vs. 10.6%, p < 0.001). The second pivotal trial enrolled 1,287 children with hemodynamically significant CHD and showed a 45% reduction in RSV-related hospitalizations (5.3% vs. 9.7%, p = 0.003). These results established Synagis as the first monoclonal antibody approved for the prevention of an infectious disease.

Synagis was first approved by the U.S. Food and Drug Administration (FDA) in June 1998 and subsequently received marketing authorization from the European Medicines Agency (EMA). It is now approved in more than 80 countries worldwide. Synagis represented a landmark in pediatric medicine as the first monoclonal antibody specifically developed to prevent a viral infection, paving the way for a new paradigm in passive immunoprophylaxis for vulnerable pediatric populations.

What Should You Know Before Your Child Receives Synagis?

Contraindications

The primary contraindication to Synagis use is known hypersensitivity (allergy) to palivizumab or to any of the other ingredients in the formulation. The excipients in Synagis include histidine, glycine, and water for injections. If your child has had a confirmed allergic reaction to palivizumab or any component of the product following a previous dose, Synagis must not be administered again.

Signs and symptoms of a severe allergic reaction may include severe skin rash, hives (urticaria), or itching; swelling of the lips, tongue, or face; throat constriction or difficulty swallowing; difficulty breathing, rapid or irregular breathing; bluish discoloration of the skin, lips, or under the fingernails (cyanosis); muscle weakness or limpness; drop in blood pressure; and loss of consciousness. If your child experiences any of these signs after receiving Synagis, seek emergency medical attention immediately. These reactions, while extremely rare, can be life-threatening.

Warnings and Precautions

Before your child receives Synagis, discuss the following with the healthcare provider:

• Current illness: If your child is currently unwell with a moderate or severe illness (particularly with fever), the administration of Synagis may need to be postponed until the child has recovered. Minor illnesses, such as a mild upper respiratory tract infection, do not generally warrant delay of immunoprophylaxis.
• Bleeding disorders: Synagis is administered by intramuscular injection, typically into the thigh muscle. If your child has thrombocytopenia (low platelet count) or any coagulation disorder (bleeding disorder), inform the healthcare provider. In such cases, the injection should be given with caution and appropriate measures taken to minimize the risk of bleeding at the injection site.
• Cardiac bypass surgery: If your child is scheduled to undergo cardiac bypass surgery, be aware that cardiopulmonary bypass significantly reduces serum palivizumab concentrations. An additional dose of Synagis (15 mg/kg) should be administered as soon as the patient is medically stable after the surgical procedure. The child can then return to the originally planned monthly injection schedule.
• Previous reactions: If your child has experienced a mild reaction to a previous dose of Synagis (such as a local injection site reaction), this does not necessarily preclude further doses. However, any significant systemic reaction should be reported to the physician and carefully evaluated before subsequent doses are considered.

Vaccines and Immunizations

Synagis does not interfere with the normal childhood vaccination schedule. Because palivizumab is a monoclonal antibody directed against a specific viral protein (the RSV F protein), it does not affect the child's own immune response to vaccines. All routine pediatric immunizations, including live attenuated vaccines (such as measles-mumps-rubella), inactivated vaccines (such as diphtheria-tetanus-pertussis), and conjugate vaccines (such as pneumococcal and Haemophilus influenzae type b), can be given on schedule during palivizumab prophylaxis. The American Academy of Pediatrics (AAP) has confirmed that palivizumab does not interfere with the immune response to any childhood vaccine.

Use in Special Populations

Synagis is a pediatric medication and is not indicated for use in adults, pregnant women, or breastfeeding mothers. It is specifically formulated and studied for use in infants and young children at high risk for severe RSV disease. The safety and efficacy of palivizumab have not been evaluated in adults, and the medication has no approved indication for use in pregnancy or during breastfeeding.

There is no clinical rationale for administering Synagis to adults, as RSV infections in healthy adults are typically mild and self-limiting. In elderly or immunocompromised adult patients who may be at higher risk for severe RSV disease, other preventive strategies and treatments are available and should be discussed with a healthcare provider.

How Does Synagis Interact with Other Drugs?

One of the practical advantages of Synagis in clinical use is its favorable drug interaction profile. Palivizumab is a humanized monoclonal antibody (IgG1κ) that, like all monoclonal antibodies, is degraded through general protein catabolic pathways (broken down into amino acids by proteolytic enzymes throughout the body) rather than being metabolized by hepatic cytochrome P450 (CYP) enzymes. This fundamentally different metabolic pathway means that palivizumab does not compete with other drugs for liver enzyme-mediated metabolism and is not expected to alter the pharmacokinetics or pharmacodynamics of other medications.

No formal drug interaction studies have been conducted with palivizumab. However, extensive clinical trial experience and more than 25 years of post-marketing surveillance have not identified any clinically significant interactions between Synagis and other medications commonly used in pediatric patients, including antibiotics, bronchodilators, corticosteroids, analgesics, and vaccines.

The following table summarizes the interaction profile of Synagis with medications commonly encountered in the target patient population:

While no formal interactions have been identified, it is always good medical practice to inform your child's healthcare provider about all medications, supplements, and treatments your child is currently receiving. This allows for comprehensive monitoring and ensures the best possible care.

It is important to note that Synagis provides passive immunity through externally supplied antibodies and does not stimulate an active immune response. Therefore, it does not interfere with the efficacy of live or inactivated vaccines. The American Academy of Pediatrics (AAP) and the European Medicines Agency (EMA) have both confirmed that palivizumab can be administered concurrently with routine childhood immunizations on the standard vaccination schedule.

What Is the Correct Dosage of Synagis?

Synagis should always be administered exactly as prescribed by the child's healthcare provider. It is given as an intramuscular (IM) injection by a trained healthcare professional in a clinical setting (hospital, clinic, or doctor's office). Synagis is not intended for self-administration at home, unlike some other monoclonal antibody products. The precise dosing is based on the child's body weight, and the correct volume must be carefully calculated for each administration.

Standard Dosage

The recommended dose for all eligible pediatric patients is 15 mg per kilogram of body weight, administered once monthly via intramuscular injection throughout the RSV season. In most temperate regions of the Northern Hemisphere, the RSV season typically runs from November through March, meaning that most children will receive approximately 5 monthly injections per season. The exact timing and number of doses may vary depending on the local RSV seasonality, and healthcare providers should follow local epidemiological guidelines.

*Injection volumes exceeding 1 mL should be divided and administered as two separate injections at different sites to minimize discomfort.

The volume of Synagis (100 mg/mL solution) to be administered is calculated using the following formula: Volume (mL) = body weight (kg) × 0.15. For example, a child weighing 3 kg would receive 3 × 0.15 = 0.45 mL per monthly injection. The healthcare provider should weigh the child before each dose and recalculate the volume to account for growth during the RSV season.

Administration

Synagis is administered by intramuscular injection, preferably into the anterolateral aspect of the thigh. The gluteal muscle (buttock) should not be used routinely as an injection site because of the risk of damaging the sciatic nerve. Standard aseptic technique should be observed during the injection procedure. The solution should be inspected visually before administration: it should be clear or slightly opalescent. Do not use the product if it appears discolored or contains visible particulate matter.

Each vial of Synagis is intended for single use only and contains no preservatives. The solution should be drawn into the syringe and administered promptly after preparation. Any unused portion remaining in the vial must be discarded in accordance with local requirements for pharmaceutical waste disposal. Both the 0.5 mL and 1 mL vial sizes are slightly overfilled to ensure that the full labeled dose (50 mg or 100 mg, respectively) can be withdrawn.

After Cardiac Bypass Surgery

If a child who is receiving Synagis prophylaxis undergoes cardiac bypass surgery, an additional dose of 15 mg/kg should be administered as soon as the child is medically stable after the procedure. Cardiopulmonary bypass has been shown to significantly reduce serum palivizumab concentrations, potentially leaving the child inadequately protected against RSV. After receiving the post-surgical dose, the child should continue with monthly injections at the interval originally planned from the first dose of the season.

Missed Dose

If your child misses a scheduled monthly injection, contact the healthcare provider as soon as possible to arrange a make-up dose. Each injection of Synagis provides protection for approximately one month. Delays in dosing may leave the child with subprotective antibody levels and increased vulnerability to RSV infection. After receiving the missed dose, resume the regular monthly schedule from that date. Do not give a double dose to compensate for a missed injection.

Duration of Prophylaxis

Synagis prophylaxis is continued throughout the RSV season, which in most Northern Hemisphere temperate regions spans approximately 5 months (November through March). In some tropical or subtropical regions, RSV circulation may follow a different pattern, and the timing and duration of prophylaxis should be determined based on local epidemiological data. Healthcare providers should consult national or regional guidelines for the most up-to-date recommendations regarding the duration of Synagis prophylaxis in their geographic area.

What Are the Side Effects of Synagis?

Like all medicines, Synagis can cause side effects, although not all children who receive it will experience them. The safety profile of Synagis has been well characterized through extensive clinical trials involving more than 2,800 children, as well as over 25 years of post-marketing surveillance encompassing millions of doses administered worldwide. Overall, Synagis is considered to have a favorable safety profile, and the benefits of RSV prophylaxis in high-risk infants are well established to outweigh the risks of potential side effects.

The side effects observed in clinical trials and post-marketing experience are categorized below by their frequency of occurrence, based on the standard frequency classification used in European Union regulatory documents:

Rash and fever are the most frequently reported side effects with Synagis. In clinical trials, rash occurred in approximately 12% of children receiving palivizumab compared with 10% of those receiving placebo. Fever was also observed at similar rates. These side effects are generally mild, transient, and resolve without specific treatment within a few days. They are common in the target population of young infants regardless of medication use.

Injection site reactions (pain, redness, and swelling at the injection site) are a predictable consequence of intramuscular injection and are generally mild and short-lived. Rotating injection sites between the left and right thigh from month to month can help minimize cumulative discomfort at any single site.

Apnea (temporary pauses in breathing) has been reported in premature infants following Synagis administration. This is a known risk in the premature infant population, particularly in neonates born before 32 weeks of gestation who may have immature respiratory control centers. Healthcare providers should monitor premature infants for an appropriate period after the injection, especially following the first dose. The relationship between Synagis and apnea episodes is difficult to distinguish from the baseline apnea rate in this population.

Allergic reactions: While very rare, serious hypersensitivity reactions including anaphylaxis have been reported during post-marketing surveillance. Symptoms may include difficulty breathing, hypotension (low blood pressure), swelling of the face or throat, and widespread rash. Healthcare facilities administering Synagis should have appropriate medical equipment and trained personnel available to manage anaphylaxis. The decision to continue prophylaxis after a hypersensitivity reaction should be carefully evaluated by the treating physician.

How Should You Store Synagis?

Proper storage of Synagis is essential to maintain the stability and efficacy of the monoclonal antibody. Palivizumab is a biological product (a protein-based medication) that can be degraded by exposure to extreme temperatures, freezing, or prolonged light exposure. Incorrect storage may render the medication ineffective without any visible change in its appearance.

The following storage conditions must be maintained at all times:

• Temperature: Store in a refrigerator at 2°C to 8°C (36°F to 46°F). Maintain the cold chain from the pharmacy or hospital pharmacy to the point of administration.
• Freezing: Do not freeze Synagis. Freezing can damage the protein structure of the monoclonal antibody and compromise its efficacy. If the product has been accidentally frozen, do not use it—discard it and obtain a replacement.
• Light protection: Keep the vial in its outer carton to protect it from light. Prolonged exposure to direct light can degrade the protein.
• Expiry date: Do not use Synagis after the expiry date stated on the carton and vial label (indicated as “EXP”). The expiry date refers to the last day of the stated month.
• Child safety: Store out of the sight and reach of children.
• After opening: Synagis vials are for single use only and contain no preservatives. Once the vial has been punctured and the solution drawn into a syringe, it should be administered immediately. Do not store a prepared syringe for later use. Discard any unused portion.

Healthcare professionals should inspect the solution visually before administration. Synagis solution for injection is a clear or slightly opalescent liquid. Do not use the product if it is discolored, turbid, or contains visible particles. Do not mix Synagis with any other medications or diluents. Do not dilute the solution.

Unused or expired medication and waste materials should be disposed of in accordance with local pharmaceutical waste disposal regulations. Do not dispose of medicines in household waste or via wastewater. Ask your pharmacist or healthcare provider about proper disposal procedures.

What Does Synagis Contain?

Understanding the complete composition of Synagis is important for healthcare providers and caregivers, particularly for children who may have known allergies or sensitivities to specific pharmaceutical ingredients. Synagis has a relatively simple formulation, containing the active monoclonal antibody in a buffered aqueous solution with minimal excipients.

Active Substance

The active substance in Synagis is palivizumab, a humanized monoclonal antibody of the IgG1κ subclass. Each milliliter of Synagis solution for injection contains 100 mg of palivizumab. Palivizumab is produced using recombinant DNA technology in a mammalian cell expression system and subsequently purified through a series of chromatographic and filtration steps to achieve high purity.

• The 0.5 mL vial contains 50 mg of palivizumab
• The 1 mL vial contains 100 mg of palivizumab

Both vial sizes are slightly overfilled to ensure that the full labeled dose can be accurately withdrawn using standard injection technique.

Inactive Ingredients (Excipients)

The excipients in Synagis serve to stabilize the monoclonal antibody protein and maintain the appropriate pH and osmolality of the solution:

• Histidine: An amino acid used as a buffering agent to maintain the pH of the solution within the optimal range for protein stability.
• Glycine: An amino acid used as a stabilizer to protect the monoclonal antibody from degradation during storage.
• Water for injections: The sterile aqueous vehicle in which the active substance and other excipients are dissolved.

Synagis does not contain preservatives, latex, sucrose, or polysorbate. The formulation is free from animal-derived components other than the cell line used in manufacturing (recombinant DNA technology). If your child has known allergies to any of the listed ingredients, inform the healthcare provider before administration.

Physical Appearance and Packaging

Synagis solution for injection is a clear or slightly opalescent liquid. It is available in single-use glass vials with a rubber stopper and an aluminum crimp seal. Each carton contains one vial. The product is available in two presentations:

• 0.5 mL vial containing 50 mg of palivizumab
• 1 mL vial containing 100 mg of palivizumab

Not all pack sizes may be marketed in every country. Check with your local pharmacy or healthcare provider for availability in your region.