Researchers said the findings from large, late-stage studies, or Phase III trials, would be crucial to determine if the immune response generated by CoronaVac was sufficient to protect people from the coronavirus infection.
Late-stage, or phase-three, trials are ongoing to confirm the findings, researchers said, and to test whether the vaccine protects against infection with SARS-CoV-2 in a broad range of people, including people with underlying health conditions. From the 560 volunteers, 240 of them were over the age of 70 years.
"I'd be concerned. that [CoronaVac] seemed to induce lower antibody levels than what we've seen with other vaccines and lower than what you see in most people who have had disease and recovered", says Dr. Gregory Poland, a vaccinologist and professor at the Mayo Clinic in Rochester, Minn. The other group was given a placebo - a shot with an inactive substance.
In contrast, vaccines developed by Pfizer/BioNTech and Moderna use a new technology called synthetic messenger RNA to activate the immune system against the virus and require far colder storage. The human cells will then create proteins to develop antibodies against the coronavirus.
Adenovirus-vectored vaccines are not without controversy.
After lowering expectations for how many millions of vaccines they can produce this year, the companies expect to ramp up their manufacturing early next year.
Vials of the Oxford coronavirus vaccine
The Oxford study was briefly suspended in September because of an unexplained illness in a volunteer.
"It is essential that a COVID-19 vaccine can be effective across a broad age range, particularly in older individuals where they are disproportionately at risk of severe COVID-19 disease".
Both Pfizer and Moderna have arranged deals with the government so that the vaccines will be free to Americans and distributed according to plans worked out between the federal government and the states. About 25 million doses of the Pfizer vaccine may become available in December, 30 million in January and 35 million more in February and March, according to information presented to the National Academy of Medicine this week.
Moderna and Pfizer have not yet published results from their studies in medical papers for review.
Pollard said there is no competition between the various research teams, because several vaccines will be needed to bring the global pandemic under control and allow life to return to normal.