Nirsevimab reduced respiratory syncytial virus infections requiring medical care in healthy premature infants in Phase 2b trial
- Nirsevimab reduced respiratory syncytial virus (RSV) lower respiratory tract infections by 70 percent and related hospitalizations by 78 percent1
- Results published in The New England Journal of Medicine
- The investigational immunization demonstrated sustained protection across a typical five-month RSV season with a single dose1
- Sanofi will host a nirsevimab R&D investor event today at 5 p.m. CET/11 a.m. ET

PARIS – July 30, 2020 – Detailed results from the promising Phase 2b trial for the investigational monoclonal antibody nirsevimab showed a significant reduction in medically attended (inpatient or outpatient) lower respiratory tract infections (LRTI), mainly bronchiolitis and pneumonia, and hospitalizations caused by respiratory syncytial virus (RSV) in healthy preterm infants.

Published in The New England Journal of Medicine, results from this trial demonstrate for the first time that a single dose monoclonal antibody significantly reduced medically attended RSV LRTI in infants through the full RSV season.1

The data for nirsevimab are exciting, as they highlight the potential for this innovative approach to help protect infants from RSV with just one injection for the entire season,” said Dr. Joseph Domachowske, study author, Professor of Pediatrics, Professor of Microbiology and Immunology, and Director of the Global Maternal-Child and Pediatric Health Program at the SUNY Upstate Medical University. “Nirsevimab offers the important potential to reduce hospitalizations and emergency department and in-office visits, which are a significant burden for healthcare systems.”

Nirsevimab is an extended half-life monoclonal antibody (mAb) being developed in partnership with AstraZeneca as a passive immunization, meaning a protective antibody is administered directly to an infant to help prevent RSV. If approved, nirsevimab could set a new standard of care by offering an innovative immunization potentially for all infants during their first RSV season, when they are most at risk for infection or complication.

Phase 2b trial met primary and secondary endpoints
The Phase 2b study was conducted by AstraZeneca at 164 sites in 23 countries. Healthy preterm infants of 29–35 weeks’ gestation were randomized (2:1) to receive a single intramuscular injection of nirsevimab or placebo. Between November 2016 and December 2017, 1447 infants were dosed (nirsevimab, n=966; placebo, n=481) at the RSV season start.1

On the primary endpoint, nirsevimab achieved a statistically significant 70.1% (95% Confidence Interval (CI): 52.3%-81.2%) reduction of medically attended RSV LRTI compared to placebo through 150 days post-dose. Medically attended RSV LRTI occurred in 2.6% (25) of participants in the nirsevimab group and 9.5% (46) in the placebo group. On the secondary endpoint, nirsevimab achieved a 78.4% (95% CI: 51.9%-90.3%) relative reduction in the incidence of hospitalizations due to RSV LRTI compared to placebo through 150 days post-dose. Hospitalization for RSV LRTI occurred in 0.8% (8) of nirsevimab and 4.1% (20) of placebo recipients. The safety profile for nirsevimab was similar to placebo, with no significant hypersensitivity reactions observed.1 Serious adverse events occurred in 11.2% (108) of the nirsevimab recipients and 16.9% (81) of placebo recipients, with none considered product-related. The most common adverse events included upper respiratory tract infection (nirsevimab: 40.8%-placebo: 35.5%), skin and subcutaneous disorders (24.5%-23%), nasopharyngitis (16.9%-19.6%), and gastroenteritis (12.6%-9.6%).1

“It’s encouraging to see from these data that serious complications from RSV were reduced in healthy preterm infants,” said John Shiver, Senior Vice President, Global Research and Development, Sanofi Pasteur. Up to 80 percent of babies who are hospitalized from RSV are otherwise healthy with no prior complications, but currently infants have no approved preventative option to protect them.”

Nirsevimab’s safety and efficacy data have not yet been evaluated by any regulatory authority.

A nirsevimab R&D investor event will be held today at 5 p.m. CET/11 a.m. ET. Sanofi speakers include:

  • Thomas Triomphe, Global Head of Sanofi Pasteur
  • Su-Peing Ng, Global Head of Medical, Sanofi Pasteur
  • Jon Heinrichs, Global Project Head, nirsevimab, Sanofi Pasteur
  • John Shiver, Global Head of Research & Development, Sanofi Pasteur

Joining for the Q&A session:

  • Paul Hudson, Chief Executive Officer
  • John Reed, Global Head of Research & Development
  • Jean-Baptiste de Chatillon, Chief Financial Officer

Additional information about today’s session can be found at:

About RSV
RSV, a widespread, contagious virus that infects the respiratory tract,1 is the most common cause of LRTI in infants which include bronchiolitis and pneumonia and results in millions of hospitalizations globally2 and is the leading cause of hospitalizations in children younger than one year in the United States.3 Globally, in 2015, there were approximately 33 million cases of acute lower respiratory infections causing more than three million hospitalizations, and it was estimated that there were 60,000 in-hospital deaths of children younger than five years.3 Globally and in the U.S., up to 80 percent of babies who are hospitalized due to RSV are otherwise healthy.4,5,6 Moreover, medically-attended LRTIs are associated with increased costs to the healthcare system.7 A recent study conducted in the U.S. by the CDC demonstrated that most children hospitalized due to RSV were otherwise healthy; therefore, potentially all infants may benefit from preventative RSV treatment.7,8

Nirsevimab Clinical Trials

In July 2019, Sanofi and AstraZeneca initiated pivotal Phase 3 and Phase 2/3 trials to measure the safety and efficacy of nirsevimab to prevent LRTI caused by RSV in full-term, healthy late preterm and high-risk babies.7,9 The trials will be conducted in more than 350 sites across Northern and Southern Hemispheres.

The full results of the Phase 3 and Phase 2/3 trials are anticipated in 2023.

About Nirsevimab
Nirsevimab is an extended half-life mAb being developed for the prevention of LRTI caused by RSV for use in potentially all infants entering their first RSV season and for children with congenital heart disease or chronic lung disease entering their first and second RSV season. 9,10 

Nirsevimab is a passive immunization, whereby an antibody is given directly to an infant to help prevent RSV, unlike active immunization, in which a person’s immune system is activated to prevent or fight infection.11 Passive immunization has the potential to offer immediate protection.

In March 2017, AstraZeneca and Sanofi Pasteur announced an agreement to develop and commercialize nirsevimab jointly. Under the terms of the agreement, AstraZeneca leads all development activity through initial approvals and retains manufacturing activities and Sanofi Pasteur will lead commercialization activities. In February 2019, AstraZeneca and Sanofi Pasteur’s nirsevimab received Breakthrough Therapy Designation from the US Food and Drug Administration and were granted access to the PRIority MEdicines (PRIME) scheme by the European Medicines Agency.

About Sanofi

Sanofi is dedicated to supporting people through their health challenges. We are a global biopharmaceutical company focused on human health. We prevent illness with vaccines, provide innovative treatments to fight pain and ease suffering. We stand by the few who suffer from rare diseases and the millions with long-term chronic conditions.

With more than 100,000 people in 100 countries, Sanofi is transforming scientific innovation into healthcare solutions around the globe.

Sanofi, Empowering Life


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1 Centers for Disease Control and Prevention. RSV in Infants and Young Children. Accessed November 2019.

2 Shi T, et al. Global, regional, and national disease burden estimates of acute lower respiratory infections due to respiratory syncytial virus in young children in 2015: a systematic review and modelling study. Lancet 2017; 390: 946–58.

3 Plotkin's Vaccines (Seventh Edition), Elsevier, 2018, Pages 943-949; IASR Vol. 39 p207-209: December, 2018 (

4 Hall CB, et al. Respiratory Syncytial Virus-Associated Hospitalizations Among Children Less Than 24 Months of Age. Pediatrics, 132(2). doi: 10.1542/peds.2013-0303 9.

5 Hall CB, et al. “The Burden of Respiratory Syncytial Virus Infection in Young Children,” New England Journal of Medicine. 2009; 360(6):588-98.

6 Arriola, C, et al. Estimated Burden of Community-Onset Respiratory Syncytial Virus–Associated Hospitalizations Among Children Aged <2 Years in the United States, 2014–15. Journal of the Pediatric Infectious Diseases Society. 2019.

7 Leistner R, et al. Attributable Costs of Ventilator-Associated Lower Respiratory Tract Infection (LRTI) Acquired on Intensive Care Units: a Retrospectively Matched Cohort Study. Antimicrobial Resistance and Infection Control, vol. 2, no. 1, 4 Apr. 2013, p. 13., doi:10.1186/2047-2994-2-13.

8 Rha, B., et al. Respiratory Syncytial Virus–Associated Hospitalizations Among Young Children: 2015–2016. Pediatrics. July 2020, 146 (1) e20193611.  DOI:

9 A Study to Evaluate the Safety and Efficacy of MEDI8897 for the Prevention of Medically Attended RSV LRTI in Healthy Late Preterm and Term Infants (MELODY). Accessed November 2019.

10 A Study to Evaluate the Safety of MEDI8897 for the Prevention of Medically Attended Respiratory Syncytial Virus (RSV) Lower Respiratory Track Infection (LRTI) in High-risk Children. Accessed November 2019.