The new study marks the first time the vaccine was tested in humans.
The findings are preliminary, and based on observations made over the first 28 days after the vaccine was administered. The final results are to be evaluated in six months.
"These results represent an important milestone," co-author Wei Chen, of the Beijing Institute of Biotechnology in China, said in a statement.
"However, these results should be interpreted cautiously [because] the challenges in the development of a COVD-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," he said.
Currently, more than 100 candidate COVID-19 vaccines are in development worldwide. However, the new adenovirus type 5 vectored COVID-19, or Ad5-nCoV, is the first to be tested in humans.
It uses a weakened common cold virus, adenovirus, which infects human cells but is incapable of causing disease, to deliver genetic material that codes for the SARS-CoV-2 spike protein to the cells.
They then produce the spike protein and allow the immune system to create antibodies to fight the coronavirus. SARS-CoV-2, the new coronavirus -- is the virus that causes COVID-19.
Chen and colleagues assessed the vaccine's safety and its ability to generate an immune response by using three different doses in 108 healthy adults between 18 and 60 years old who had not been infected with SARS-CoV-2.
Participants were enrolled from one site in Wuhan, China, and assigned to receive either a single intramuscular injection of the new vaccine at a low dose of 5 × 1010 viral particles per 0.5 milliliters, a middle dose of 1×1011 viral particles per 1.0 ml. or a high dose of 1.5 x 1011 viral particles per 1.5 ml.
The researchers tested the volunteers' blood at regular intervals following vaccination to see whether the vaccine stimulated both arms of the immune system -- the one that produces neutralizing antibodies that can fight infection and could offer a level of immunity and the cell-mediated portion, which depends on a group of T cells, rather than antibodies, to fight the virus.
The ideal vaccine would generate both antibody and T cell responses to defend against SARS-CoV-2.
The researchers found that the Ad5 vaccine candidate was well tolerated at all doses with no serious side effects within 28 days of vaccination. The most common adverse reactions were mild pain at the injection site, fever, fatigue, headache and muscle pain, they added.
Within two weeks of administration, the authors noted, all dose levels of the vaccine triggered some level of immune response in the form of binding antibodies -- 44 percent in the low-dose group, 50 percent in the medium-dose group and 61 percent in the high-dose group.
In addition, some participants had detectable neutralizing antibodies against SARS-CoV-2: 28 percent in the low-dose group , 31 percent in the medium-dose group and 42 percent in the high-dose group, they found.
After 28 days, more than 95 percent of participants in all groups had a four-fold increase in binding antibodies, and 50 to 75 percent had neutralizing antibodies, they said.
The Ad5-nCoV vaccine also stimulated a rapid T cell response in the majority of volunteers, which was greater in those given the higher and middle doses of vaccine, the authors said.
However, they noted that both the antibody and T-cell response could be reduced by high pre-existing immunity to adenovirus type 5 -- the common cold virus vector/carrier.
The Ad5-nCoV vaccine will need to pass through Phase II and Phase III clinical trials in humans with similar positive results before it can be made available for widespread use.
A randomized, double-blind, placebo-controlled Phase II trial of the vaccine has been started in Wuhan with 500 healthy adults already enrolled.
"[Our] trial demonstrates that a single dose of the new adenovirus type 5 vectored COVID-19 vaccine produces virus-specific antibodies and T cells in 14 days, making it a potential candidate for further investigation," Chen said.
"This result shows a promising vision for the development of COVID-19 vaccines, but we are still a long way from this vaccine being available to all."