BRUSSELS -- The European Union (EU) will authorize a contract for the purchase of up to 300 million doses of the COVID-19 vaccine jointly developed by German company BioNTech and its American partner Pfizer, European Commission President Ursula von der Leyen said on Tuesday.

"This is the most promising vaccine so far," she said on Twitter. The contract, due to be reached on Wednesday, will be the fourth one the EU secures with a pharmaceutical company to buy vaccines for COVID-19.

The BioNTech/Pfizer vaccine candidate has proved to be more than 90 percent effective in preventing COVID-19 infection in participants with no proven previous SARS-CoV-2 infection, the two companies announced on Monday.

As the world is struggling to contain the COVID-19 pandemic, countries including Germany, China, Russia, the United Kingdom and the United States are racing to find a vaccine.

According to the website of the World Health Organization, as of Nov. 3, there were 202 COVID-19 candidate vaccines being developed worldwide, and 47 of them were in clinical trials.