A potential coronavirus vaccine developed in China appeared safe and able to generate an immune response after an early trial in more than 100 people, according to a new study.

The vaccine, called Ad5-nCoV, is being developed by the Chinese company CanSino Biologics, and was one of the first coronavirus vaccines to enter early human trials back in March. Now, there are more than 100 different coronavirus vaccines in development worldwide, with at least eight of those in the process of human trials.

Ad5-nCoV uses a weakened version of a common cold virus — which infects human cells but doesn't cause disease — to deliver a fragment of genetic material from the virus that causes COVID-19. This genetic material provides instructions for making the "spike protein" on the surface of the coronavirus. The idea is that a person's immune system will create antibodies against the spike protein, which will help fight off the coronavirus if the person is later exposed to it.

In the new study, published on May 22 in the journal The Lancet, the researchers tested Ad5-nCoV in 108 healthy people ages 18 to 60 who didn't have COVID-19. Participants received either a low, middle or high dose of the vaccine.

Two weeks after vaccination, participants in all three groups showed some level of an immune response to the virus. By 28 days, nearly all participants had developed antibodies that bound to the coronavirus (but don't necessarily attack the virus), and about half of the participants in the low- and middle-dose groups and three-quarters of participants in the high-dose group developed "neutralizing antibodies," which bind to and disable the virus to prevent it from infecting cells.

The most common side effects were mild pain at the injection site, mild fever, fatigue, headache and muscle pain, the study said.

However, nine participants (two in the low-dose group, two in the middle-dose group and five in the high-dose group) developed a fever of more than 38.5 degrees Celsius, and one participant in the high-dose group developed a high fever along with fatigue, shortness of breath and muscle pain. However these effects lasted no more than 48 hours.

Participants were aware of which dose they received, which may have affected their perceptions of the side effects, according to The New York Times.

"These results represent an important milestone," the study's senior author Wei Chen from the Beijing Institute of Biotechnology in Beijing said in a statement. "However, these results should be interpreted cautiously. 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."

The researchers have now started a larger, phase 2 study of the vaccine involving 500 participants who will be given a low or intermediate dose of the vaccine, or a placebo. This study will also include participants over 60 years old, and will look at side effects up to six months after vaccination.