An inactivated vaccine candidate against COVID-19 developed by China has shown to provide "highly efficient protection" against the disease in rhesus monkeys with just two doses, a new study said.

In a long-term toxicity test spanning over three weeks, the animals showed only local irritation at the injection site, and the reaction was gone after two weeks. No other adverse effects were recorded, meaning the vaccine has proved to be safe in animal testing, according to a study published on Saturday in the journal Cell.

The vaccine, called BBIBP-CorV, is being developed by the Chinese Center for Disease Control and Prevention, along with the Beijing Institute of Biological Products Co, the Chinese Academy of Medical Sciences, Tsinghua University and the Peking Union Medical College.

It is the fourth vaccine in the world reported to provide immunological protection against the novel coronavirus in rhesus monkeys, one of the closest relatives to humans and a crucial bench mark in drug and vaccine testing, the journal said.

Three vaccines, two inactivated and one adenovirus vector-based, were made by Chinese scientists, and the last one, a nucleic acid vaccine, was created by United States biotech company Moderna.

An inactivated vaccine is a type of well-established vaccine that uses a dead version of the pathogens to trigger the immune system. It is widely considered to be very safe because deactivated pathogens cannot cause illness, but it typically can't provide immunity as strong as live vaccines, so several booster shots over time are needed.

In the study, researchers said they tested the vaccine on various animals including mice, guinea pigs, rabbits and nonhuman primates, all of which reported high levels of neutralizing antibodies that can provide protection against the novel coronavirus.

Two doses, each with 2 micrograms of the vaccine, could provide highly efficient protection in rhesus monkeys without detecting antibody-dependent enhancement, an adverse effect in which the neutralizing antibodies enhance the virus' entry into host cells instead of inhibiting the infection.

Moreover, the vaccine candidate has good genetic stability and efficient productivity, meaning it is relatively easy for mass production, they added. However, the vaccine still needs further research to assess its efficacy and safety in human clinical trials.

On May 30, the National Vaccine and Serum Institute reported to have finished building the world's largest inactivated COVID-19 vaccine production facility that is capable of producing over 3 million doses per batch and can manufacture an estimated 100 to 120 million doses per year.

These Chinese-made inactivated vaccines would hit the market in late December or early next year at the soonest, the institute said.

Wang Zhigang, minister of science and technology, said in a news briefing on Sunday that China currently has five vaccine candidates in clinical trials, with some having entered phase two of such trials, which primarily evaluate the vaccine's safety and efficacy.

Zeng Yixin, deputy director of the National Health Commission, said in a news briefing in mid-May that these vaccine candidates are set to complete their phase two trials in July. Phase three trials would also study the candidate's safety and potency, albeit on a much larger recipient pool.