With the FDA’s granting of emergency use authorization (EUA) to Johnson & Johnson (J&J), there is now a third vaccine – and the first requiring only a single shot – against COVID-19 for adult Americans. Though it packs less of an efficacy punch, the EUA allows J&J’s Ad26.COV2.S to join mRNA vaccines from Moderna Inc. and Pfizer Inc./Biontech SE as protection against the virus.