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Viral infections that attack the lungs are among the leading reasons infants are hospitalized. A new treatment may offer protection against these infections that is as effective and safe as existing options but requires far fewer injections. The Phase III SMART clinical trial investigated the safety and efficacy of clesrovimab (brand name ENFLONSIA), a long-acting medicine designed to protect babies from respiratory syncytial virus (RSV).
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During the first RSV season, a single dose of clesrovimab was found to be as well tolerated as the current standard treatment, palivizumab, which requires up to five monthly injections.
Clesrovimab proved to be just as effective as the standard treatment in preventing RSV-related infections that required medical attention. During the first RSV season, these infections occurred in 3.2% of infants receiving clesrovimab, compared with 3.4% of infants receiving the standard treatment.
The most common side effects of clesrovimab were irritability and pain at the injection site, and rates were very similar to those seen with palivizumab.
The findings are published in JAMA Pediatrics.
Single-dose protection
Respiratory syncytial virus spreads easily from person to person, usually through coughing or sneezing, and infects the lungs and airways of people of all ages. For most people, it causes mild, cold-like symptoms, but in babies, the infection can become more serious.
Some infants, during their first bout with RSV, develop pneumonia or bronchiolitis, which can lead to frequent doctor visits and even hospitalization.
Clesrovimab, or ENFLONSIA by Merck & Co., Inc., is a monoclonal antibody, a specially designed protein that acts like a targeted defense system, recognizing and attaching to one specific part of a virus. The antibody molecule locks onto a specific protein found on the surface of RSV, blocking the virus from entering the body's cells.
The Phase II CLEVER trial evaluated clesrovimab in more than 2,400 healthy babies, including those born early (preterm) and those born at full term. The study found that clesrovimab reduced the risk of serious RSV lung infections and medical visits by more than 90% for up to six months, with side effects similar to those seen with a placebo.
How the trial was structured
In this Phase III randomized trial, researchers studied the safety and effectiveness of clesrovimab in high-risk infants across 110 sites in 27 countries and territories over two consecutive RSV seasons. The study included 997 infants entering their first RSV season who were eligible to receive the standard treatment, palivizumab.
The infants were divided into two equal groups. One group received a single 105 mg dose of clesrovimab on day 1, followed by a placebo (saline) injection on day 28 to ensure the study remained double-masked. This meant that neither the researchers nor the families knew which treatment the infants had received during the initial phase of the trial.
The second group received monthly doses of palivizumab throughout their first RSV season, reflecting the current standard approach for RSV prevention. A smaller group of 276 children, still at high risk for severe illness, received a single 210 mg dose of clesrovimab about a month before their second RSV season.
Comparable results with fewer shots
A single injection of clesrovimab emerged as a strong defense against severe RSV infections in high-risk infants, offering protection for up to two years. It was also well tolerated, with side effects reported less often than with palivizumab (75.3% vs. 79.6%).
Clesrovimab matched the protection offered by the current standard treatment, with RSV-related infections requiring medical attention seen in 3.2% of infants receiving clesrovimab, compared with 3.4% receiving palivizumab. Severe cases leading to hospitalization were also rare and occurred at similar rates in both groups: 1.0% with clesrovimab and 1.7% with palivizumab.
After receiving the 210 mg dose of clesrovimab in the second season, 7.3% of infants experienced an RSV-related lung infection requiring medical attention.
The trial report makes a case for clesrovimab being generally well tolerated in infants at increased risk of severe RSV disease, while also supporting its use in children who continue to face a higher risk of severe RSV during their second season.
The one-dose approach could make a real difference for families, offering protection throughout the RSV season without the burden of returning to the hospital multiple times for repeat injections.
Written for you by our author Sanjukta Mondal,edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—thisarticle is the result of careful human work. We rely on readers like you to keep independent science journalism alive.If this reporting matters to you,please consider a donation (especially monthly).
More information: Heather J. Zar et al, Clesrovimab in Infants at Increased Risk For Severe Disease During 2 RSV Seasons, JAMA Pediatrics (2026). DOI: 10.1001/jamapediatrics.2026.2760
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This story was originally published on Medical Xpress.
