(Adds study details, background)
By Ankur Banerjee
July 20 (Reuters) - A vaccine against the coronavirus developed by CanSino Biologics Inc and China's military research unit appears to be safe and induced immune responses in most subjects in a closely-watched mid-stage study, researchers said on Monday.
The CanSino candidate, Ad5-nCOV, which was tested in 508 subjects, is one of a handful of vaccines that have shown some promise in early human testing prior to much larger trials to demonstrate efficacy. Others also gearing up for such pivotal vaccine trials include Moderna Inc, BioNTech SE in partnership with Pfizer Inc.
Data from a combined early/midstage trial of a vaccine candidate being developed by researchers at the University of Oxford and AstraZeneca was also released on Monday.
A safe and effective vaccine against the novel coronavirus is seen as essential to ending a pandemic that is still raging and has claimed more than 600,000 lives worldwide.
CanSino's vaccine uses a modified common cold virus to carry genetic material from the new coronavirus into the human body, a method also used by the Oxford/AstraZeneca vaccine.
Both vaccines elicited antibody and T-cell immune responses and neither prompted any serious side effects. T-cells are an important component of the immune system's attack against foreign invaders, such as viruses.
Results of both trials were released in the medical journal the Lancet.
Both studies augur well for the large Phase III trials, where the vaccines will be tested on thousands of subjects to assess their efficacy and safety, Naor Bar-Zeev and William Moss, from the International Vaccine Access Center at Johns Hopkins Bloomberg School of Public Health, said in an accompanying editorial.
"Overall, the results of both trials are broadly similar and promising," they said.
Separately, BioNTech and Pfizer said data from an early-stage trial of their experimental coronavirus vaccine showed that it prompted an immune response and was well-tolerated, similar to results seen in prior early test.
CanSino's vaccine received the greenlight to be used by China's military despite not yet undergoing the type of large-scale testing needed to prove its ability to prevent infection.
Lilly Asia Ventures, backed by U.S. drugmaker Eli Lilly and Co, is CanSino's top shareholder, according to Refinitiv data. Lilly shares were up about 0.6%.
Chinese researchers in an earlier peer-reviewed paper noted that the immune responses elicited by the vaccine might be undermined if the inoculated person has already developed high-level adenovirus immunity from a previous infection.
That limitation cropped up again with Monday's results. Study authors said increasing age and high pre-existing adenovirus immunity significantly reduced immune responses to the vaccine.
Pre-existing immunity to the type of virus used to deliver the vaccine "is considered to be the biggest obstacle for the candidate ... COVID-19 vaccine to overcome," study authors said.
In some participants with adenovirus immunity, one injection might be inadequate to induce a high level of immune responses, particularly for people aged 55 years or older, researchers said, suggesting an additional dose given between the third and sixth month after the first as a potential solution.
Participants in the study conducted in Wuhan, China, were found to have adenovirus immunity and were representative of the Chinese adults, the authors acknowledged, adding that such immunity varies globally.
Reporting by Ankur Banerjee in Bengaluru and Roxanne Liu in Beijing; Editing by Shounak Dasgupta, Nancy Lapid and Bill Berkrot