A Chinese coronavirus vaccine candidate appears to be safe and induces an immune response in healthy volunteers, according to preliminary study results.

Phase one/two trials of an inactivated SARS-CoV-2 vaccine candidate – CoronaVac – involved more than 700 healthy volunteers aged 18-59 recruited in China between April 16 and May 5.

According to preliminary results published in The Lancet Infectious Diseases, the vaccine appeared to be safe and well tolerated at all tested doses.

The most common reported side effect was pain at the injection site.

Within 14 days of the final dose, researchers detected robust antibody responses after two jabs of the vaccine candidate were given two weeks apart.

This was even the case for the lowest dose tested, three micrograms.

Researchers say antibody levels induced by the vaccine were lower than those seen in people who had been infected by and recovered from Covid-19.

But they add that the vaccine could provide protection from the virus.

The primary objective of the study was to evaluate the immune response and safety of the vaccine, and it was not designed to assess how effective it is at preventing infection with SARS-CoV-2, the virus that causes Covid-19.

Our findings show that CoronaVac is capable of inducing a quick antibody response within four weeks of immunisation

Findings from phase three studies will be crucial for determining if the immune response generated by CoronaVac is sufficient to protect from infection.

According to the paper, 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 only included healthy adults aged 18 to 59 years and further studies will be needed to test the vaccine candidate in other age groups, as well as in people who have pre-existing medical conditions.

Professor Fengcai Zhu, joint lead author of the study, from the Jiangsu Provincial Centre for Disease Control and Prevention, Nanjing, China, said: “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.

“We believe that this makes the vaccine suitable for emergency use during the pandemic.

Like all phase two trials, the results must be interpreted with caution until phase three results are published

“In the longer term, when the risk of Covid-19 is lower, our findings suggest that giving two doses with a one-month interval, rather than a two-week interval, might be more appropriate for inducing stronger and potentially longer-lasting immune responses.

“However, further studies are needed to check how long the antibody response remains after either vaccination schedule.”

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

The authors note several limitations to their study, including that the phase two trial did not assess T cell responses, which are another arm of the immune response to virus infections.

This will be studied in ongoing phase three studies.

Writing in a linked comment, Dr Naor Bar-Zeev, from Johns Hopkins University, who was not involved in the study, said: “Like all phase two trials, the results must be interpreted with caution until phase three results are published.

“But even then, after phase three trial completion and after licensure, we should prudently remain cautious.”