Pfizer begins testing COVID-19 booster shot Researchers also testing how it works with other common vaccines Updated: Tags: JACKSONVILLE, Fla. – Biopharmaceutical company Pfizer has administered its first COVID-19 booster shots. It’s a third dose Americans could take yearly -- like the flu shot -- to prevent contracting new strains of the coronavirus. Researchers are also testing how it works with other common vaccines “That’s what we’ve been wondering about the booster shot, you know, if there’s going to be a booster every fall that we have to get, is it something we can combine with another vaccine?” said Dr. Sunil Joshi, the president of the Duval County Medical Society Foundation.