Results from an early-phase clinical trial of a Chinese vaccine candidate, CoronaVac, revealed that the formulation appears safe and induces an antibody response in healthy volunteers aged 18 to 59 years.
According to the findings of the phase 1/2 randomised clinical trial, published in The Lancet Infectious Diseases journal, the vaccine candidate could induce an antibody response in participants within 28 days of the first immunisation, by giving two doses 14 days apart.
The researchers, including those from the Jiangsu Provincial Center for Disease Control and Prevention in China, also found the optimum dose to generate the highest antibody responses, while taking account of side effects and production capacity.
While the average levels of neutralising antibodies induced by CoronaVac was lower than levels seen in people who have previously had COVID-19, the scientists still believe the formulation could provide sufficient protection against the virus based on their experience with other vaccines and data from animal model studies.
Since the study only included healthy adults aged 18 to 59 years, the researchers said further studies will be needed to test the vaccine candidate in other age groups, as well as in people with pre-existing medical conditions.
"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 Fengcai Zhu, joint lead author of the study, from the Jiangsu Provincial Center for Disease Control and Prevention.
"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," Zhu added.
In the current study, the scientists reported the results of the phase 1/2 clinical trial which included participants aged 18 to 59 years, and only people who did not have any history of infection with COVID-19.
They said the participants had not travelled to areas with high incidence of the disease, and did not have signs of fever at the time of recruitment.
In the first phase, the scientists said 144 healthy volunteers were enrolled, who 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.
Within each group, they said the participants were randomly assigned to receive either a low dose of the vaccine, a high dose, or a placebo.
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 pain at the injection site.
There was one case of severe allergic reaction within 48 hours of receiving the first dose, the scientists said, adding that it could be possibly related to vaccination.
However, they said the participant was treated and recovered within three days, and did not experience a similar reaction after the second dose.
The study noted that phase 2 of the trial was initiated when all participants in phase 1 had finished a 7-day observation period after their first dose.
In this phase, the scientists said 600 healthy volunteers were enrolled in the study, who 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 placebo.
Citing the limitations of the research, the scientists said the phase 2 trial did not assess T cell responses, which are another arm of the immune response to virus infections.
Between the phase 1 and phase 2 trials, the researchers said there was no difference in reported side effects, however, they said the immune responses were much stronger in the second phase.
"CoronaVac is one of many COVID-19 vaccine candidates that are being explored in parallel. There are a multitude of different vaccine technologies under investigation, each with their own advantages and disadvantages," said Gang Zeng, another co-author of the study from Sinovac Biotech, a biopharmaceutical company based in China.
"CoronaVac could be an attractive option because it can be stored in a standard refrigerator between 2 and 8 degrees centigrade, which is typical for many existing vaccines including flu. The vaccine may also remain stable for up to three years in storage," Zeng said.
However, he added that data from phase 3 studies will be crucial before any recommendations about the potential uses of CoronaVac could be made.