This trial has 5,197 participants who had not been exposed and showed a 77-percent reduced risk of developing symptomatic disease with no severe cases recorded.
Drug firm AstraZeneca on Friday announced positive results from a trial of a treatment for COVID-19 symptoms.
The drug, made from a combination of two antibodies, was initially developed as a treatment for those who had already been exposed to the disease.
A new trial of 5,197 participants who had not been exposed showed a 77-percent reduced risk of developing symptomatic disease, with no severe cases recorded, Astra said in a statement.
A previous trial of the AZD7442 drug had shown it only reduced the risk of developing symptoms by 33 percent, which it concluded in June was not statistically significant.
The data show that one dose could “quickly and effectively prevent symptomatic COVID-19 “, said Myron Levin, principal trial investigator.
“With these exciting results, AZD7442 could be an important tool in our arsenal to help people who may need more than a vaccine to return to their normal lives.”
It is hoped that the drug could be used alongside vaccines for those who need more protection, affording up to 12 months of defence.
Participants in the trial were adults who were poor responders or intolerant to vaccines, or who had increased risk of infection because of their locations or circumstances.
The US government has funded the development of AZD7442 and has agreements to receive 700,000 doses.
The company will now send the data to health authorities for potential emergency use authorisation or conditional approval.
AstraZeneca already makes the most widely offered vaccine in the UK, although those under 40 are offered Pfizer or Moderna instead because of concerns over possible side effects.