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A cocktail of two monoclonal antibodies against COVID-19, casirivimab and imdevimab, reduced hospitalization and death by 71% versus placebo in high-risk, non-hospitalized patients, a researcher said.

COVID-19 hospitalization or all-cause mortality was significantly reduced through day 29 among high-risk patients who were randomized to the intervention (HR 0.29, 95% CI 0.17-0.49), reported Julie Philley, MD, of the University of Texas Health Science Center in Tyler.

Moreover, treatment with the intravenous monoclonal antibody combination was associated with 4 fewer days of symptom duration compared with placebo.

Casirivimab and imdevimab received emergency use authorization from the FDA in November, which was granted on the basis of phase I/II trial results in non-hospitalized adults with mild or moderate COVID-19. President Trump was treated with this therapy when he had COVID-19 in October 2020.

Philley presented the phase III results of the adaptive trial at the virtual American Thoracic Society annual meeting. She stressed the need for treatments to help reduce viral load in COVID-19 patients, as high viral load has been associated with worse clinical outcomes.

In this trial, the primary outcome was at least one COVID-19-associated hospitalization or all-cause death by day 29. Investigators also amended the protocol from 2,400 mg or 8,000 mg of the cocktail to 2,400 mg or 1,200 mg. A statement from noted the FDA is currently reviewing a request to add the 1,200-mg dose to the current EUA.

High-risk non-hospitalized adults with COVID-19 with symptom onset at least 7 days from randomization, and a positive SARS-CoV-2 test within 72 hours, were randomized to receive one of two doses of the intravenous monoclonal antibody cocktail or an IV placebo. They were then followed for 28 days.

Overall, 4,567 patients were included. They had a median age of about 49, 51% were women, and about a third were Hispanic or Latino. Of the patients, 58% were obese, and 69% had a baseline negative SARS-CoV-2 serum antibody test. All patients had at least one risk factor, including older age, obesity, cardiovascular disease, or chronic lung disease.

Philley noted that the difference between the groups was evident by day 4 and continued through day 29. The reduction in COVID-19-related hospitalizations was consistent across subgroups, including those with high viral loads, as well as patients with baseline seronegative or seropositive status, she added.

Both doses of the intervention were also associated with a shorter median time to symptom resolution (10 days vs 14 days with placebo).

The cocktail was well tolerated, with no safety concerns. Serious adverse events were more common in the placebo group, and there were few hypersensitivity or infusion reactions in the intervention groups, Philley said.

During the question and answer session, one clinician asked about the potential or theoretical risk of viral mutations. Philley noted that the cocktail was effective against variants of concern.

"I think we ... in the healthcare industry have to keep up with different mutations," she said, adding that with other types of mutations, such as the California and Brazilian variants, the cocktail did have "certain neutralization against the spike protein."

Session moderator C. Terri Hough, MD, of Oregon Health & Science University in Portland, asked Philley about longer-term symptoms after COVID, but Philley said there were no data on long-haulers at this time. The cohorts were not available for long-haul investigation, as they were short in duration, she added.

Hough also discussed connecting NIH's planned investment in post-acute COVID-19 syndrome with industry research efforts.

"Creating a follow-up infrastructure on top of short-term studies could be really powerful," she said.

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