The top executive at Pfizer, a leading producer of COVID-19 vaccines, has tested positive for the virus and says he is experiencing very mild symptoms.

Chairman and CEO Albert Bourla said today that he started taking Pfizer’s Paxlovid pill treatment and is isolating while he recovers.

Bourla has received four shots of Comirnaty, the COVID-19 vaccine developed by the New York drugmaker along with BioNTech. He said in a brief statement issued by the company that he is confident of a quick recovery.

More than 128 million people in the United States have become fully vaccinated with Pfizer’s two-shot vaccine since it entered the market more than a year ago and almost 61 million have received an initial booster shot, according to the Centers for Disease Control and Prevention.

Scientists say the vaccine still offers solid protection against hospitalization and serious illness. But the evolving virus has made it tougher for vaccines to prevent all forms of illness from developing.

Pfizer and another vaccine maker, Moderna, are updating their shots to provide protection against newer versions of the virus for a fall booster campaign.

Paxlovid also has shown in research to be extremely effective at warding off serious illness if it is administered shortly after symptoms start.

Comirnaty and Paxlovid are the top-selling COVID-19 vaccine and treatment on the market. Combined, they brought in nearly $17 billion in sales for Pfizer during the recently completed second quarter.