Oxford University, which partnered with AstraZeneca to develop the vaccine, said its Phase III trial results will be available by Christmas.
The Phase II trial conducted between May 30 and Aug. 8 among 560 adults, including 240 adults who were 70 years old or older, found that an initial dose of AstraZeneca's vaccine "was safe and well-tolerated with a lower reactogenicity profile in older adults than in younger adults," the results read.
The results were particularly encouraging because COVID-19 disproportionately affects older adults. Immune responses to the vaccine were more similar across all age groups after a boost vaccine, or an extra dose of a vaccine after an earlier initial dose.
"The reason that we’re so delighted is that we’re seeing the immune responses look exactly the same, even in those who are over 70 years of age," Oxford Professor of Pediatric Infection and Immunity Dr. Andrew Pollard, told the BBC.
Adverse effects of the vaccine were limited and mild, and fewer adverse effects were recorded after a boost vaccine.
Phase II vaccine trials provide important preliminary data but don’t prove whether they ultimately prevent people from getting sick. Oxford and AstraZeneca are waiting for the results of Phase III trials on thousands of people around the world to show whether their vaccine is safe and effective.
Two other drugmakers, Pfizer and Moderna, this week reported preliminary results from late-stage trials showing that their COVID-19 vaccines were almost 95% effective.
The Associated Press contributed to this report.