Nirsevimab is an option for preventing lower respiratory tract infections caused by the Respiratory Syncytial Virus (RSV) in newborns and infants. RSV is a common virus that can cause serious lung infections, particularly in the very young. Nirsevimab is a monoclonal antibody that provides direct, temporary protection against the virus. It is not a vaccine that stimulates the body’s own immune system for a lasting defense.
Understanding Nirsevimab’s Mechanism
Nirsevimab provides what is known as passive immunity. Unlike a vaccine that prompts the infant’s immune system to produce its own antibodies, nirsevimab delivers ready-made antibodies directly to the body. These manufactured antibodies target the fusion (F) protein on the surface of the RSV virus, which the virus uses to enter and infect human cells.
By binding to this protein, the antibody blocks the virus from entering respiratory cells and neutralizes it. The antibodies are engineered with a modification that extends their half-life, allowing a single dose to provide protection for about five months, which typically covers an entire RSV season.
Safety Findings from Clinical Trials
The safety of nirsevimab was evaluated in large-scale clinical trials involving thousands of infants before its approval. The primary studies, including MELODY and MEDLEY, compared infants who received nirsevimab to those who received a placebo or another preventive antibody, palivizumab.
Data from these studies showed that the overall incidence of adverse events was similar between the nirsevimab and comparison groups, with most events being mild to moderate. A pooled analysis confirmed that the type and frequency of adverse events were consistent across different populations, including healthy term, premature, and infants with congenital heart or chronic lung disease. Serious adverse events were rare and occurred at similar rates in both the nirsevimab and placebo groups.
Common Side Effects and Regulatory Oversight
The most commonly reported side effects of nirsevimab are consistent with clinical trial findings. These are typically mild and temporary, including pain, redness, or swelling at the injection site, and in some cases, a rash or a low-grade fever. In clinical trials, rash occurred in about 0.9% of infants who received nirsevimab, while injection site reactions were observed in 0.3%. These reactions are generally not serious and resolve on their own.
Although rare, a serious allergic reaction (anaphylaxis) is possible with any injectable medication. Parents should be aware of the signs, which can include hives, swelling of the face or throat, and difficulty breathing, and seek immediate medical attention if they occur.
Regulatory bodies like the FDA and CDC reviewed the safety data before approval and use post-market surveillance systems to monitor nirsevimab’s safety as it is used more widely. These agencies affirm that the benefits of protecting infants from severe RSV outweigh the known risks.
Contraindications and At-Risk Populations
The primary contraindication for nirsevimab is a history of a severe allergic reaction, such as anaphylaxis, to the medication or any of its components. Minor illnesses like a common cold are not contraindications, though administration may be deferred in cases of moderate to severe illness.
Nirsevimab was studied in infants at high risk for severe RSV, including those born prematurely or with chronic lung disease or congenital heart disease. The MEDLEY trial evaluated its safety in these groups and found a favorable profile comparable to the older antibody, palivizumab.
Because these infants are more susceptible to severe RSV, they are among the groups recommended to receive nirsevimab. The CDC also recommends a dose for children up to 19 months of age entering their second RSV season with qualifying conditions, such as being severely immunocompromised or having cystic fibrosis.