Toronto,   One dose of Ad5-nCoV (Convidecia), a Covid-19 vaccine developed in China, is 57.5 per cent effective against symptomatic Covid-19 and 91.7 per cent effective against severe Covid-19 disease beginning 28 days post-vaccination, finds a new study.

According to a phase 3 randomised controlled trial, published in The Lancet, Ad5-nCoV is safe, with no serious vaccine-related adverse events or deaths reported among trial participants, and that the vaccine induces a robust antibody response.

"Our study suggests that one dose of Ad5-nCoV is highly effective against severe disease -- potentially helping to ease the tremendous strain Covid-19 has put on health systems around the world by keeping people from becoming seriously ill or requiring hospitalisation," said lead author Scott Halperin from the Dalhousie University in Canada.

Developed by CanSino Biologics, Inc. and the Beijing Institute of Biotechnology, Ad5-nCoV is a single-dose viral vector vaccine that can be stored between 2 degrees Celsius and 8 degrees Celsius.

The vaccine has been approved for emergency use in 10 countries, including Argentina, Chile, Mexico and Pakistan, where this current clinical trial took place. Regulatory review is in progress in Russia, which also participated in this clinical trial.

The trial, which is still ongoing, commenced on September 22, 2020, and, by January 15, 2021, had enrolled 36,982 adults 18 years of age and older, of which 36,727 were randomised to receive either a vaccine or placebo injection across 66 enrolment sites at study centres in Argentina, Chile, Mexico, Pakistan and Russia.

The researchers conducted an efficacy analysis once the protocol threshold of 150 laboratory-confirmed (RT-PCR positive) symptomatic Covid-19 at 28 days post-injection was reached on January 15, 2021, at which point there were 21,250 trial participants in the primary efficacy cohort.

The team reported 105 positive Covid-19 cases out of 10,590 participants in the placebo group and 45 positive Covid-19 cases out of 10,660 participants in the vaccine group, resulting in an efficacy of 57.5 per cent at 28 days post-vaccination.

Efficacy against the severe disease was 91.7 per cent at 28 days post-vaccination, where the severe disease was defined as a minimum of one of the clinical signs at rest indicative of severe systemic illness, respiratory failure, evidence of shock, significant acute renal, hepatic, or neurologic dysfunction, or admission to an ICU.

There were no Covid-19-related deaths among vaccine recipients.

As reported in phase 1 and 2 trials Ad5-nCoV was well tolerated and produced high levels of anti-RBD antibodies and neutralizing antibodies.

The majority of the adverse events, including pain at the injection site, headache drowsiness and generalised muscle aches were mild to moderate and occurred within seven days of injection. There were no reports of thrombosis or thrombocytopenia in any study participants.