Dec 7 (Reuters) - Canadian drug developer Medicago's plant-based COVID-19 vaccine candidate, enhanced by GlaxoSmithKline's booster, was 75.3% effective against the Delta variant of the virus in a late stage study, the two companies said on Tuesday.

They said the vaccine's overall efficacy rate against all variants of the coronavirus was 71%, except Omicron, which was not in circulation when the study was underway.

"These are encouraging results given data were obtained in an environment with no ancestral virus circulating. The global COVID-19 pandemic is continuing to show new facets with the current dominance of the Delta variant, upcoming Omicron, and other variants likely to follow," GSK Chief Global Health Officer Thomas Breuer said in a statement.

Last week, infectious disease expert Anthony Fauci said though the focus is on Omicron, 99.9% cases in the United States were due to the Delta variant. "Delta continues to drive cases across the country, especially in those who are unvaccinated."

Medicago will imminently seek regulatory approval for the world's first plant-based COVID-19 vaccine from Health Canada as part of its rolling submission.

The Canadian health regulator in April accepted Medicago's application for a real-time review of the only home-grown COVID-19 vaccine to reach the most advanced level of trials.

Medicago, which uses GSK's adjuvant to boost the efficacy of its vaccine, started the late-stage study in March with over 24,000 participants aged 18 years or above in Canada, United States, United Kingdom, Mexico, Argentina, and Brazil.

The vaccine uses a technology known as virus-like particles, which mimic the structure of the coronavirus, but contain no genetic material from it.

(Reporting by Mrinalika Roy in Bengaluru; Editing by Shinjini Ganguli)