The phase-two clinical trial of the COVID-19 vaccine candidate created by the Academy of Military Medical Sciences and Chinese biotech company CanSino Biologics has found that it is safe and can induce an immune response, according to research published in The Lancet medical journal on Monday.

Also on Monday, The Lancet published the results of phase-one and phase-two clinical trials of a similar adenovirus vectored vaccine developed by scientists at Oxford University and biotech company AstraZeneca. That vaccine also demonstrated success in safety and potency against COVID-19.

Experts have called these results "promising". However, pressing questions remain, such as the longevity of its protection, the appropriate dosage to trigger a strong immune response and whether there are host-specific differences such as age, sex or ethnicity. These questions will be probed in larger scale phase-three trials.

An adenovirus vectored vaccine works by using a weakened common cold virus to introduce genetic material from the novel coronavirus into the human body. The idea is to train the body to produce antibodies that recognize the coronavirus spike protein and fight it off.

In the phase-two trial of the Chinese vaccine, 508 people took part, 253 of them receiving a high dose of the vaccine, 129 a low dose and 126 a placebo.

Ninety-five percent of participants in the high dose group and 91 percent in the low dose group had either T-cell or antibody immune responses 28 days after receiving the vaccine. T-cells can directly target and kill invading pathogens, making them a key part of the human immune response.

The authors emphasized, however, that no participants were exposed to the novel coronavirus after vaccination, so it is still too early to say whether the vaccine candidate can effectively protect against COVID-19 infection.

As for adverse reactions, fever, fatigue and injection-site pain were some of the noted side effects of the Chinese vaccine, though most of these reactions were mild or moderate.

Another caveat was that with the vector for the vaccine being a common cold virus, people may have preexisting immunity that kills the viral carrier before the vaccine can take effect, which could partially hamper the immune responses. Compared with younger people, older participants generally had significantly lower immune responses, the study found.

Chen Wei, who spearheaded work on the vaccine, said in a news release that elderly people might possibly require an additional dose to induce a stronger immune response, but further research would be needed to evaluate that approach.

CanSino, the developer of the vaccine, is in talks on launching phase-three trials in several foreign countries, Qiu Dongxu, executive director and co-founder of CanSino, said at a conference in Suzhou, Jiangsu province, on Saturday.

An accompanying editorial in The Lancet on the two latest vaccine studies called the results of the trials from China and the United Kingdom "broadly similar and promising".