Pfizer CEO Albert Bourla filed to sell millions of dollars of his company's stock Monday -- the day the pharmaceutical giant announced positive data about its coronnavirus vaccine.

The company's shares soared after Pfizer and European drug company BioNTech said early data suggests the vaccine could be more than 90% effective.

The transaction was part of a regularly scheduled plan set up by Bourla to periodically sell some of his Pfizer shares.

Bourla sold 132,508 Pfizer shares at a price of US$41.94, according to a filing with the Securities and Exchange Commission. That works out to proceeds of nearly $5.6 million.

A Pfizer spokeswoman said in an email to CNN Business that the sale took place because Pfizer shares hit a predetermined price as part of a plan authorized by Bourla on August 19.

When asked if Pfizer and Bourla thought to cancel the stock sale due to the perception that Bourla might be cashing in on good news, the spokeswoman said that "these are predetermined plans managed through a third-party stock administrator."

Many executives sell stock at predetermined intervals to diversify their portfolios. But they can delay the sales to avoid appearances of capitalizing on one-off events that can boost company's shares.

Bouria told CNN's Sanjay Gupta that he learned of the trial's results Sunday, a day before they were made public.

Shares of Pfizer surged nearly 8% Monday and were flat Tuesday. BioNTech's stock has rallied even more sharply, rising 15% Monday and another 8% Tuesday. Pfizer's stock fell 1% Wednesday.

The Pfizer stock sale comes just a few months after executives at Moderna, a biotech also working on a COVID-19 vaccine, sold shares following the release of promising trial results. Critics accused Moderna of overhyping the vaccine trial results, but the company nonetheless raised US$1.3 billion in a stock sale immediately following the vaccine trial announcement. Executives then executed a series of transactions worth tens of millions of dollars before the company's share price fizzled a week later.

Some former SEC officials called on Moderna to be investigated for potential illegal market manipulation.