A study from Yale University today in JAMA Network Open reveals substantial protection by the preventive drug nirsevimab (Beyfortus) against infant respiratory syncytial virus (RSV) infections, hospitalizations, and severe disease.

The test-negative case-control study used inpatient, outpatient, and emergency department data from the Yale New Haven Health System to estimate efficacy of the RSV monoclonal antibody drug in 3,090 infants seen from October 1, 2023, to May 9, 2024.

In total there were 680 RSV-positive cases (22.0%) and 2,410 RSV-negative controls (78.0%). RSV-positive patients were slightly younger than RSV-negative patients, at 6.1 months versus 6.9 months. Nirsevimab uptake was 10.7% (330 patients). Of the RSV-positive patients, nirsevimab use was documented in 21 patients and in 309 RSV-negative controls.

Protection peaked at 2 weeks

Adjusted effectiveness of nirsevimab was 68.4% (95% confidence interval [CI], 50.3% to 80.8%) against medically attended RSV infection, 61.6% (95% CI, 35.6% to 78.6%) against outpatient visits, and 80.5% (95% CI, 52.0% to 93.5%) against hospitalizations.

Against severe RSV disease, nirsevimab had a very high efficacy of 84.6% (95% CI, 58.7% to 95.6%).

Protection against infections peaked at 2 weeks post-administration, when the antibody was estimated to be 79.3% effective (95% CI, 63.4% to 90.6%). By 14 weeks, that percentage dropped to 54.8% (95% CI, 16.3% to 74.7%).

This pattern of waning effectiveness was observed across all clinical outcomes and was consistent with data from clinical trials.

“This pattern of waning effectiveness was observed across all clinical outcomes and was consistent with data from clinical trials,” the authors wrote.

Interestingly, protective effectiveness was observed against all-cause lower respiratory tract infections (LRTIs) (49.4%) and all-cause LRTI hospitalization (79.1%) during the peak months of RSV season (November and December 2023) when compared with other studies, the authors said.

“Given that RSV was the predominant virus during these months (RSV positivity rate, 39.3% in our sample), these estimates largely reflect the effectiveness of nirsevimab against RSV,” the authors wrote.

The authors said their findings support US guidelines recommending nirsevimab for all infants entering their first RSV season.