An early clinical trial of potential coronavirus vaccine, developed by Chinese top military virologist, has shown promising results, successfully triggering an immune system response in over a hundred healthy adults.
The study published in The Lancet on Friday said the phase one trial of the vaccine developed by the Academy of Military Medical Sciences and CanSino Biologics suggested it was safe, South China Morning Post reported.
Major General Chen Wei, the scientist leading the military researchers, said in statement: "These results represent an important milestone."
"The trial demonstrates that a single dose [of the vaccine] produces virus-specific antibodies and T cells in 14 days, making it a potential candidate for further investigation."
"However, these results should be interpreted cautiously. The challenges in the development of a Covid-19 vaccine are unprecedented, and the ability to trigger these immune responses does not necessarily indicate that the vaccine will protect humans from Covid-19."
The vaccine uses a weakened version of adenovirus type 5 (Ad5), which is a common cold virus, as a vector to deliver a genetic material from Sars-CoV-2, the virus that causes Covid 19, to induce an immune response from the recipients.
The human trials began in March, testing 108 healthy adults aged between 18 to 60 years in Wuhan, the city where the disease first emerged.
Volunteers were given a low, medium or high dose of the vaccine, and had their immune systems monitored for four weeks.
In total, 105 of the participants had developed binding antibodies – which attach themselves to the pathogen and can help mark them out to the immune system – 28 days after the vaccination. The study said none had shown any "serious" adverse reaction.
The study also detected another immune response, with 97 recipients showing either a T-cell response – cells that target the virus – or the development of a neutralising antibody after 28 days.
Of these 63 volunteers, including 27 out of the 36 given a high dose, developed neutralising antibodies that could destroy the live coronavirus virus in a test tube.
However, Chen said there had been adverse reactions among the recipients, the most common being a fever, which was reported in 50 cases. Fatigue was recorded in 47 participants, 42 suffered headaches and 18 reported muscle pain.
However, there are limitations to the study. One of the major concerns in vaccine trials is pre-existing immunity.
Since adenovirus type 5 is a common cold virus, about half of the volunteers had pre-existing immunity to that and that slowed down the immune response of the recipients, the study said.
The research only includes the results of the first 28 days after vaccination and it is not yet known how long the antibodies can last.
The study also cannot show whether the vaccine can cause antibody-dependent enhancement, which makes the infection worse by assisting the virus's entrance into the host cells.
This phenomenon is a major concern for scientists developing Covid-19 vaccines as this has been observed with other coronaviruses.
It is also unknown how effective the vaccine will be in protecting the elderly, who are the most vulnerable to the disease, and the study said over-60s would be recruited to take part in the next phase of the trial.
"Therefore, we are unable to predict the protection of the [vaccine] on the basis of the vaccine-elicited immune responses in this study," the researchers wrote.
The researchers also said they were worried that the use of the Ad5 virus as a vector may raise the risk of HIV transmission.
A 2007 trial of a possible HIV vaccine had to be halted because of this problem, but remains unclear why this happened.
The researchers said: "Although the association between HIV-1 acquisition risk and Ad5 vectored vaccine is controversial and its mechanism is unclear, the potential risks should be taken into account in studies with this viral vector delivery platform."
"We plan to monitor the participants in our upcoming phase 2 and phase 3 studies to assess the indication for any such acquisition."
The researchers have recruited around 500 volunteers for the phase 2 trials, which will involve further testing for safety and effectiveness.
China has high hopes for Chen's team in the global race to develop a vaccine.
Earlier, CanSino, the team's commercial partner, confirmed that it had secured permission from Canadian health authorities to start clinical development of the vaccine there.
Meanwhile, British officials have said they hope a genetically engineered vaccine developed by Oxford University will be available by September.
The researchers are currently studying the results of the phase one trials and are looking for more than 10,000 volunteers for the next phase of testing.
The US biotech firm Moderna is also hoping to have a vaccine ready by the autumn after all 45 recipients in an early trial produced antibodies.