A new plant-based COVID-19 vaccine is nearly 70 per cent effective against symptomatic disease caused by five variants of the coronavirus, according to results of the human clinical trial of the preventive.

The vaccine developed by researchers at Canadian biotechnology company Medicago contains coronavirus-like particles (CoVLP) produced in plants which are combined with an adjuvant (ASO3) that helps vaccines work better.

The phase 3 trial of the vaccine was conducted at 85 centres involving 24,141 adults who were randomly assigned to receive two intramuscular injections of either the CoVLP+AS03 vaccine or placebo 21 days apart.

The researchers confirmed COVID-19 by polymerase chain reaction (PCR) test in 165 participants.

The study, published on Thursday in the New England Journal of Medicine (NEJM), shows that the vaccine efficacy was 69.5 per cent against any symptomatic COVID-19 caused by five variants.

In a statistical analysis, vaccine efficacy was 78.8 and 74.0 per cent against moderate-to-severe disease and among those who were seronegative at the start, respectively, the researchers said.

"The CoVLP+AS03 vaccine was effective in preventing COVID-19 caused by a spectrum of variants, with efficacy ranging from 69.5 per cent against symptomatic infection to 78.8 per cent against moderate-to-severe disease," the authors of the study noted.

In the vaccine group, there were no severe cases of COVID-19, the researchers said.

The median viral load for breakthrough cases was more than 100-fold lower in the vaccine group versus the placebo group, they said.

Solicited adverse events were mainly mild or moderate and transient, and occurred more frequently in the vaccine group versus placebo group, according to the researchers.

Local adverse events occurred in 92.3 and 45.5 per cent in the vaccine group and placebo group, respectively, and systemic adverse events occurred in 87.3 and 65.0 per cent, respectively, they said.

"The potential effect of this plant-based technology in the current pandemic will be greatly influenced by the evolution of the pandemic itself," the authors noted.

"However, the availability and further development of this platform could have important implications for pandemic readiness," they added.

The researchers noted that vaccine efficacy among adults who were 65 years or older could not be determined because of the limited enrollment of participants in this age group.

However, previous evidence suggests that the CoVLP+AS03 vaccine induced similar immune responses in both young and older adults, they said.

Several authors of the study disclosed financial ties to Medicago which funded the study and is the manufacturer of the CoVLP+AS03 vaccine.