AstraZeneca says its vaccine was 79 percent effective against symptomatic COVID-19 in the final phase of clinical trials in the US, Chile, and Peru.
The British pharmaceutical company released preliminary results on Monday. It is developing the vaccine with Oxford University.
It says of the 32,449 participants, it confirmed symptoms of COVID-19 in 141 cases.
The firm found the vaccine 79 percent effective in preventing symptomatic COVID-19 when it compared participants who received it with those who received a placebo.
It says about 20 percent of participants were 65 years of age or older, and that the vaccine's efficacy on the age group was 80 percent.
An independent group that oversees clinical trials has reviewed these preliminary data with a focus on blood clots.
The AstraZeneca-Oxford vaccine roll-out was temporarily halted in several countries, mostly in Europe, following reports of people developing blood clots after receiving it.
France, Germany, and other countries have resumed AstraZeneca vaccinations after the European Medicines Agency announced last Thursday that an intensive review showed the vaccine to be safe.
The board has found no increased risk of blood clots in participants who had at least one dose of the vaccine.
AstraZeneca will soon apply for emergency use authorization from the US Food and Drug Administration.