Beijing, Nov 18 (IANS): A Chinese vaccine candidate -- CoronaVac -- appears safe and induces an antibody response in healthy volunteers aged 18 to 59 years in an early-phase randomised clinical trial, say researchers.

According to the study, published in the journal The Lancet Infectious Diseases, antibody responses could be induced within 28 days of the first immunisation, by giving two doses of the vaccine candidate 14 days apart.

The study also identified the optimum dose to generate the highest antibody responses, while taking account of side effects and production capacity.

"Our findings show that CoronaVac is capable of inducing a quick antibody response within four weeks of immunisation by giving two doses of the vaccine at a 14-day interval," said study author Fengcai Zhu from the Jiangsu Provincial Center for Disease Control and Prevention in China.

"We believe that this makes the vaccine suitable for emergency use during the pandemic," Zhu added.

CoronaVac is one of the 48 vaccine candidates for Covid-19 that are currently in clinical trials.

It is a chemically-inactivated whole virus vaccine based on a strain of SARS-CoV-2 that was originally isolated from a patient in China.

In the current study, all participants were aged 18 to 59 years and only people who did not have any history of infection with covid-19.

In the first phase, 144 healthy volunteers were enrolled between April 16 and April 25, 2020.

Participants were split into two groups to receive one of two vaccination schedules -- either two injections given 14 days apart or two injections given 28 days apart.

In the phase 1 trial, they said the overall incidence of adverse reactions was similar in the low- and high-dose groups at both vaccination schedules, with the most common symptom being a pain at the injection site.

According to the study, Phase 2 of the trial was initiated when all participants in phase 1 had finished a 7-day observation period after their first dose.

Some 600 healthy volunteers were enrolled in the study between May 3 and May 5, 2020.

Again, participants were separated into two groups for the 14-day and 28-day vaccination schedule and then randomly assigned to receive either a low dose of the vaccine, a high dose, or a placebo.

Between the phase 1 and phase 2 trials, the researchers changed the manufacturing process of the vaccine to increase production capacity.

There was no difference in reported side effects, however, the immune responses were much stronger in the second phase of the study after the new process was introduced.

In addition, the proportion of participants producing antibodies to SARS-CoV-2 was higher after the new process was introduced.

"CoronaVac is one of many Covid-19 vaccine candidates that are being explored in parallel," the researchers noted.