Nirsevimab Significantly Reduces RSV-Induced Lower Respiratory Tract Infections in Infants


A single-dose injection of nirsevimab before RSV season was proven to reduce infants’ risk of lower respiratory tract infection by 74.5%.

A single-dose injection of nirsevimab before RSV season was proven to reduce infants’ risk of lower respiratory tract infection by 74.5%.

Respiratory syncytial virus (RSV) is a highly contagious respiratory illness with a high disease burden in infants and older adults. Though the US Centers for Disease Control and Prevention (CDC) reports nearly all infants contract RSV before the age of 2, the virus is the leading cause of hospitalizations in infants under 12 months old.

Sanofi announced a single dose of their investigational immunization nirsevimab reduced the risk of RSV lower respiratory tract infections (LRTI) by 74.5% in infants. This success makes nirsevimab the first investigational, long-acting antibody to protect all infants in their first RSV season.

Nirsevimab is a monoclonal antibody with an extended half-life that attacks the RSV protein. The inoculation has been in development by Sanofi and AstraZeneca since March 2017.

Today, The New England Journal of Medicine (NEJM) published the phase 3 trial results, demonstrating nirsevimab protected healthy infants against LRTI caused by RSV throughout the entire 150-day RSV season.

“These data show for the first time, the potential to significantly protect all infants through their first RSV season with a single-dose immunization,” said Mene Pangalos, the executive vice president of Biopharmaceuticals Research & Development at AstraZeneca. “We look forward to working with health authorities to bring nirsevimab to infants as quickly as possible.”

The phase 3 MELODY trial met its primary endpoint, proving nirsevimab was more effective than placebo at reducing the risk of medically attended LRTI, such as RSV-induced bronchitis or pneumonia, in infants.

The infants included in the study were healthy, born at term or late preterm (≥ 35 weeks gestational age), and entering their first RSV season. In a 2:1 ratio, infants were randomly assigned to receive a single intramuscular injection of nirsevimab or placebo before the beginning of the RSV season, for a total of 994 infants in the nirsevimab group and 496 in the placebo group.

The study found medically attended LRTI in 1.2% (n=12) of the nirsevimab recipients, and 5% (n=25) of placebo recipients), leading the investigators to conclude nirsevimab was associated with a 74.5% risk reduction in LRTI due to RSV.

The secondary endpoint of the MELODY trial was hospitalization for LRTI up to 150 days after nirsevimab injection. There was an observed reduction of RSV-associated hospitalizations, though at 62.1%, the risk reduction was not statistically significant. Among the nirsevimab infants, 0.6% were hospitalized, in comparison with 1.6% of the placebo infants.

Among the infants with data available 361 days after inoculation, antidrug antibodies were detected in 6.1% (58/951) of the nirsevimab recipients and 1.1% (5/473) of the infants who received placebo.

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