The company, however, has reportedly said the sale was part of a pre-arranged periodic divestment plan when the stock reaches a certain price.
According to a disclosure with US market regulator SEC, Bourla sold 132,508 shares at $41.94 each on Monday.
An executive vice president at the company, Sally Susman, also sold around $1.8 million in stock, or 43,662 shares.
Pfizer shares rose by more than seven percent after peaking at over 15 percent on Monday, when the drugmaker announced that trials so far had shown its vaccine candidate being developed with German company BioNTech was 90 percent effective.
The company had not responded to a request for a comment from AFP, but a spokesman quoted by CNN described the sales as prearranged.
Sales at that price were previously scheduled as part of their personal financial management, the company said.
Such plans are designed to shield executives from insider trading accusations. Agence France-Presse