Pfizer Begins Testing COVID-19 Booster Shot With Pneumococcal Vaccine.br The drugmaker began administering its first coronavirus booster shots on May 24.br Pfizer is conducting a trial to see if it is safe to administer a third dose of the vaccine... .br ... in tandem with a br pneumococcal vaccine candidate. .br Pneumococcal vaccines help br to prevent pneumonia.br 600 adults over the age of 65 br will participate in the study.