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 that he started taking Pfizer’s Paxlovid pill treatment, and he’s staying in isolation while he recovers. Bourla said Monday in a brief statement from the drugmaker that he has received four shots of Comirnaty, the COVID-19 vaccine developed by both Pfizer and German drugmaker BioNTech, and he is confident that he will recover quickly.