Despite respiratory syncytial virus’ (RSV) status as a leading cause of infant death, there is currently no approved prophylactic measure for healthy pre-term infants entering their first RSV season.1 Therefore, researchers analyzed the efficacy of a single-dose of nirsevimab to prevent RSV in a study that included 164 sites in 23 countries. Results of the analysis found that infants given nirsevimab had lower rates of RSV-associated lower respiratory tract infections and hospitalizations compared with infants given a placebo, according to a study published in the New England Journal of Medicine.2

Healthy infants who were born at a gestational age between 29 and 35 weeks were eligible to participate. Infants who met local, national, or American Academy of Pediatrics recommended guidelines to receive RSV prophylaxis, had an acute illness at the time of randomization, had a previous RSV infection, or had received palivizumab or any other RSV monoclonal antibody or vaccine, including maternal vaccines, were excluded.

A total of 1447 infants received intramuscular injections 2-months before the RSV season: 966 in the nirsevimab 50 mg group and 481 in the placebo group. Of the 1447,  1417 completed the 150-day efficacy period. The randomized groups were further stratified by age (≤3 months, >3 months to ≤6 months, or >6 months) and location (northern or southern hemisphere).

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Incidence of RSV-associated lower respiratory infections was 70.1% lower in the nirsevimab group compared with the placebo (2.6% vs 9.5%, respectively; 95% confidence interval, 52.3 to 81.2). Infants in the nirsevimab group were also 78.4% less likely to be hospitalized for an RSV-associated lower respiratory infection compared with the placebo group (0.8% vs 4.1%, respectively; 95% CI, 51.9 to 90.3).

The study authors noted that reduction in RSV-associated lower respiratory infections and associated hospitalizations with nirsevimab was consistent regardless of hemisphere, participant age at randomization, sex, race, gestational age, and potential enrolled siblings. Researchers also found that infections with RSV A and RSV B subtypes were approximately equal among participants who had a medically attended RSV-associated lower respiratory tract infection.

All patients who were admitted to the intensive care unit during their hospitalization (n=5) or received assisted ventilation (n=4) were in the placebo group. Participants in the nirsevimab group with a medically-attended, RSV-associated lower respiratory tract infection required supplemental oxygen less frequently (n=4) than infants in the placebo group (n=15).

Participants in the nirsevimab group experienced a 23.5% lower rate of medically-attended lower respiratory tract infection from any cause up to 150 days after injection compared with participants in the placebo group (19.7% vs 25.8%, respectively). A lower rate of hospitalization for any respiratory illness was observed in the nirsevimab group compared with the placebo group (5.5% vs 9.5%, respectively), reflecting a 42.5% lower incidence.

Adverse events were reported at similar rates in both groups. Serious adverse events occurred in 11.2% of patients in the nirsevimab group and 16.9% of patients in the placebo group, though researchers concluded that none of these events were directly related to the injection.

“This trial of a monoclonal antibody with an extended half-life showed that a single intramuscular dose of RSV immunoprophylaxis could protect infants against RSV-associated lower respiratory tract infection requiring medical attention,” researchers said.

The study was supported by AstraZeneca and Sanofi Pasteur. Disclosures for the authors are available in the link to the full article.


1. Shi T, McAllister DA, O’Brien KL, 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(10098):946-958.
2. Griffin MP, Yuan Y, Takas T, et al. Single-dose nirsevimab for prevention of RSV in preterm infants. N Engl J Med. 2020;383(5):415-425.