AstraZeneca, Sanofi reveal mixed hand in high-stakes RSV game, with hospitalization miss tarnishing primary endpoint hit

There’s more trouble in the respiratory syncytial virus (RSV) race. Days after GlaxoSmithKline stopped its maternal vaccine studies, AstraZeneca and Sanofi have revealed their antibody prospect failed to reduce infant hospitalization in a phase 3 clinical trial. 

Writing in The New England Journal of Medicine, the researchers explained that the study randomized almost 1,500 healthy late-preterm and term infants to receive anti-RSV antibody nirsevimab or placebo before the start of the RSV season. Two-thirds of the infants got the drug, with the rest receiving placebo.

While, as AstraZeneca and Sanofi disclosed last year, the clinical trial linked nirsevimab to a 75% drop in medically attended RSV-associated lower respiratory tract infection, it failed to show the antibody has a statistically significant effect on hospitalization rates. The rate of hospitalization for RSV-associated lower respiratory tract infection in the nirsevimab arm was 0.6% compared to 1.6% in the control group.

The figures put efficacy on the secondary hospitalization endpoint at 62%, but the difference between the two arms fell short of statistical significance, with the p-value coming in at 0.07. A pre-specified pooled analysis of RSV-associated hospitalizations across the phase 2b and 3 clinical trials painted nirsevimab in a better light, linking it to a statistically significant efficacy of 77.3%.

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It is unclear why the reduction in infections failed to translate into reduced hospitalizations in the phase 3 clinical trial, but the NEJM paper discusses the impact of the pandemic on the secondary endpoint.

Investigators enrolled more than 1,000 participants in the Northern Hemisphere in 2019. A further 462 infants were enrolled in South Africa early in 2020 in anticipation of the Southern Hemisphere RSV season, at which point COVID-19 restrictions caused the “abrupt termination of RSV circulation” and led AstraZeneca to pause enrollment and amend the protocol to allow it to perform the primary analysis on the available data.

“Given that the target enrollment was not reached, the trial had less power than originally planned to evaluate the efficacy of nirsevimab with respect to preventing hospitalizations or the difference in efficacy in subgroups,” the authors of the NEJM paper wrote.

The next step is to find out what regulators make of the data. AstraZeneca and Sanofi have begun making regulatory submissions of a drug that, with its primary endpoint hit and seemingly acceptable safety, has a good shot at coming to market despite the question mark over its effect on hospitalizations.

Editor's note: This story was updated at 1:37 a.m. ET March 4 to correct data on the rates of hospitalization.