People who were exposed to Covid and received under-the-skin injections of Regeneron's synthetic antibody treatment were 81 percent less likely to develop the disease compared to those on a placebo, the company said Monday.
The trial, which was jointly run with the US National Institutes of Health, enrolled 1,505 people with household contacts who had tested positive for Covid-19.
The volunteers themselves tested negative at the start of the trial, and were then randomly assigned to receive either one dose of REGEN-COV -- which is a 1,200 milligram combination of asirivimab and imdevimab -- or a placebo.
After 29 days, 1.5 percent of people who received the treatment developed symptomatic Covid, compared to 7.8 percent for those on placebo.
This equates to a risk reduction of 81 percent, which compares favorably with authorized Covid vaccines.
The participants were ethnically diverse, 31 percent had at least one risk factor, the median age was 44 years old and the age range was 12 to 92 years.
No one who received the treatment was hospitalized or had to go to an emergency room for Covid-19, while four who received the placebo did.
Side effects occurred in 20 percent of people on the drug and 29 percent on placebo, while serious side effects occurred in one percent of participants of both groups.
"These data suggest that REGEN-COV can complement widespread vaccination strategies, particularly for those at high risk of infection," said Myron Cohen, a University of North Carolina at Chapel Hill scientist who is leading NIH research on Covid monoclonal antibody treatments.
He added that the treatment retained its potency against emerging coronavirus variants of concern during lab tests.
Despite promising results, widespread use of monoclonal antibodies against Covid has been hampered by a lack of public awareness about what they are, and the fact that the main way of administering them has been through an intravenous drip that takes at least 20 minutes.
By switching to four subcutaneous injections, Regeneron hopes to remove this impediment.
Antibodies are infection-fighting proteins made by immune systems in response to pathogens.
Vaccines train the body to produce antibodies without being exposed to the live infection, while synthetic versions can also be cultivated in bioreactors to produce mass treatments.