Pfizer and BioNTech said Thursday their COVID-19 vaccine was highly effective against the coronavirus variant first detected in South Africa in the latest phase of ongoing clinical trials.

No cases of the disease were observed in South Africa during the phase-three trial study among participants who had received their second dose, the companies said in a statement.

Several coronavirus variants with the potential to be more transmissible have caused global concern over whether existing vaccines will still protect the world from a virus that is constantly mutating.

"In South Africa, where the B.1.351 lineage is prevalent and 800 participants were enrolled, nine cases of COVID-19 were observed, all in the placebo group," the companies said.

The nine strains were sequenced and six of them were confirmed to be of B.1.351 strain, they said.

"The high vaccine efficacy observed through up to six months following a second dose and against the variant prevalent in South Africa provides further confidence in our vaccine's overall effectiveness," said Albert Bourla, chief executive of Pfizer.

Overall, the vaccine was 91.3 percent effective against COVID-19 in the analysis of 46,307 trial participants across several countries.

Global deaths from COVID-19 have topped 2.8 million since the pandemic began a year ago, with vaccines seen as the only real chance of returning to some form of normality.