As COVID-19 vaccinations continue to roll out, momentum builds with strong phase III data for what could become the fourth and fifth walls of defense in the U.S. With three vaccines already authorized, research reported in March offers hope for a second adenovirus vector vaccine candidate with Astrazeneca plc’s AZD-1222, as well as for the first protein subunit vaccine option with Novavax Inc.’s NVX-CoV2373.