Pfizer and BioNTech have begun studying whether adding a third shot, or a booster shot, to their two-dose COVID-19 vaccine could increase the vaccine’s effectiveness against new mutant strains of the virus, the companies announced Thursday (Feb. 25), just days after top FDA officials encouraged vaccine developers to begin testing booster shots as soon as possible to prepare for potentially resistant strains. The companies also are currently in talks with FDA and the European Medicines Agency about setting up a...