Pfizer and BioNTech Are Testing a Third Booster Shot to Bolster Its Covid-19 Vaccine Against New Variants Posted byAndrea Michelson February 25, 2021 Leave a comment on Pfizer and BioNTech Are Testing a Third Booster Shot to Bolster Its Covid-19 Vaccine Against New Variants The companies plan to offer booster shots to those who participated in their initial clinical trial.