Syringes filled with Johnson \& Johnson\'s COVID-19 vaccine in a tray at a mobile vaccination site in Miami.Syringes loaded with the Johnson & Johnson vaccine lie in a tray at a mobile vaccination site in Miami. (Wilfredo Lee / Associated Press) As the Delta variant became the dominant strain of the coronavirus across the United States, all three COVID-19 vaccines available to Americans lost some of their protective power, with vaccine efficacy among a large group of veterans dropping be...