New Delhi: A Phase III randomised controlled trial has shown that one dose of Ad5-nCoV (Convidecia), a Covid-19 vaccine developed in China, is 91.7 per cent effective against severe Covid-19 disease and 57.5 per cent effective against symptomatic Covid-19 disease beginning 28 days after vaccination. 


The findings of the study were recently published in The Lancet.


No Serious Adverse Events


Ad5-nCoV vaccine is safe, and caused no serious vaccine-related adverse events or deaths among trial participants, the Lancet report said. Also, the vaccine was observed to induce a robust antibody response.


Ad5-nCoV is a single-dose viral vector vaccine developed by CanSino Biologics Inc., and the Beijing Institute of Biotechnology. It can be stored between 2 degree Celsius and 8 degree Celsius. As many as 10 countries, including Argentina, Chile, Mexico, and Pakistan, have approved the vaccine for emergency use. Russia participated in the clinical trial as well, and regulatory review is in progress in the country.


‘Highly Effective' Against Severe Covid-19 Disease


Quoting Dr Scott Halperin, the lead author of the study, a statement issued by The Lancet said the study suggests that one dose of Ad5-nCoV is highly effective against severe disease, and can potentially help to ease the tremendous strain put on health systems worldwide by Covid-19. 


He said the vaccine can keep people from becoming seriously ill or requiring hospitalisation, and added that the vaccine could help provide improved access to vaccination because it is effective against severe disease after a single dose. Low-income and middle-income countries could be benefitted by the vaccine, because it can be a challenge for people in those countries to obtain both doses of a vaccine. 


The trial was started on September 22, 2020, and is still going on — conducted across 66 enrolment sites at the study centres in Argentina, China, Mexico, Pakistan, and Russia, according to the report.


57.5% Effective Against Symptomatic Covid-19


Convidecia was found to be 57.5 per cent effective against symptomatic Covid-19 at 28 days post vaccination, the report said.


An efficacy of 91.7 per cent was found against severe Covid-19 disease at 28 days post vaccination. Severe disease was defined as a condition in which at least one clinical sign indicative of severe systemic illness was observed. These included respiratory failure, evidence of shock, significant acute renal, hepatic, or neurologic dysfunction, or admission to an ICU. No deaths related to Covid-19 were reported among the participants.


It was found in Phase I and Phase II trials that Convidecia was well tolerated and produced high levels of anti-RBD antibodies and neutralising antibodies.


The study found most of the adverse events of the vaccine to be mild to moderate and to occur within seven days of injection. These included pain at the injection site, headache drowsiness, and generalised muscle aches. 


Further Research Required


Dr Joanne Langley from Dalhousie University said more research is needed to determine Ad5-nCoV's effectiveness and durability over a longer period of time as well as its effectiveness against variants of concern including Omicron.


Additional secondary outcomes, including efficacy against symptomatic infection and efficacy against PCR-negative, seroconversion-positive cases, will be analysed.


Also, research is being conducted to explore the relative efficacy of a single versus two-dose regimen of Convidecia, the authors noted.


Some limitations of the study include the fact that the majority of the study participants were from Pakistan and Mexico. Also, people with unstable medical conditions, people who were pregnant, and children were excluded from the study.


Further research will be conducted to determine the vaccine's effectiveness and durability against other variants of concern.