Chinese pharmaceutical firm CanSino Biologics Inc's vaccine shows 57.5 percent effectiveness against symptomatic COVID-19 and 91.7 percent against severe disease four weeks or longer after one dose, while its protection appears lower in older people, peer-reviewed results from a large trial show.

A study published on Thursday in the Lancet medical journal said that the CanSino's single-dose adenovirus vaccine, Convidecia, has 57.5 percent efficacy against symptomatic COVID-19.

The result is based on a randomized, double-blind and placebo-controlled phase III clinical trial with analysis of 150 cases in a cohort of over 20,000 healthy adults, wrote researchers.

The shot's efficacy against any symptomatic infection beginning 28 days after injection was much lower in those aged 60 and older, at 17.5 percent. Efficacy against severe disease was 76.1 percent for the age group.

The authors cautioned that the older group's small sample size made the analysis for that subgroup more likely to be imprecise compared with the overall efficacy.

Richard Kennedy, a vaccine researcher at the Mayo Clinic who wrote an editorial accompanying the study, said the 17.5 percent figure indicates that additional doses might be necessary for the elderly.

Although the study did not look at how recent variants such as Delta and Omicron would affect the vaccine's effectiveness, an earlier lab test by CanSinoBIO has seen encouraging data.

The company found that a combination of a two-dose inactivated vaccine followed by an inhaled regimen of CanSinoBIO's Convidecia can provide 10-fold of neutralizing antibody level compared to people inoculated with three doses of inactivated vaccines, although a slightly decrease of neutralizing antibody level is detected against a "pseudo-virus" mimicking Omicron variant compared to the original COVID-19 virus.

An ongoing UK study led by the University of Oxford has previously said that mixing COVID-19 vaccines gives better immune response.

The single-shot Ad5-nCoV vaccine has been approved for use in countries such as China, Mexico and Pakistan.

The overall efficacy beginning 14 days after one dose was higher than at 28 days, at 63.7 percent against symptomatic COVID-19 and 96.0 percent against severe disease.

There were no vaccine-related serious adverse events or deaths from the trial or reports of vaccine-induced thrombotic thrombocytopenia (VITT), extremely rare cases of clotting with low platelets that have been associated with other adenovirus-vector vaccines.

(With input from Reuters)