A Covid-19 vaccine, which is being developed by US pharmaceutical giant Pfizer and German biotech firm BioNTech, is 90% effective in preventing infections in ongoing Phase 3 trials, the companies announced on Monday.

"The first set of results from our Phase 3 Covid-19 vaccine trial provides the initial evidence of our vaccine's ability to prevent Covid-19," Pfizer chairman and CEO Albert Bourla said in a statement.

"We are a significant step closer to providing people around the world with a much-needed breakthrough to help bring an end to this global health crisis.We are reaching this critical milestone in our vaccine development program at a time when the world needs it most," he added.

According to preliminary findings, protection in patients was achieved seven days after the second of two doses, and 28 days after the first. The findings are based on an interim analysis conducted after 94 participants contracted the illness. The trial will continue until 164 cases have occurred.

The companies said they expect to supply up to 50 million vaccine doses globally in 2020, and up to 1.3 billion doses in 2021, based on supply projections.

President Donald Trump meanwhile hailed the announcement in a tweet.

"Stock market up big, vaccine coming soon. Report 90% effective. Such great news!" he tweeted.