Participants were 1,121 unvaccinated adults 18 years and over with confirmed exposure to a person with a case of the virus within the previous eight days.
The candidate, AZD7442, reduced the risk of developing symptomatic COVID-19 by 33% compared to placebo, which was not statistically significant.
All participants had a negative antibody test on the day of dosing to exclude prior infection and another swab at a later date to detect potential infection.
The FTSE 100 group is also studying the candidate as prevention in individuals not already infected, with results yet to be published.
In an analysis of people who had a negative test on the day of dosing, AZD7442 reduced the risk of developing symptomatic COVID-19 by 73%.
“While COVID-19 vaccination efforts have been successful, there is still a significant need for prevention and treatment options for certain populations, including those unable to be vaccinated or those who may have an inadequate response to vaccination,” said Myron J. Levin, professor at the University of Colorado School of Medicine and principal investigator on the trial.