All Americans ages 6 months and older should receive one of the new Covid-19 vaccines when they become available this fall, the Centers for Disease Control and Prevention said on Thursday. The recommendation comes as the nation faces a summer wave of Covid, with the number of infections rising in at least 39 states and territories. Most Americans have acquired immunity against the coronavirus from repeat infections or vaccine doses, or both. The vaccines now offer an incremental boost, remaining effective for only a few months as immunity wanes and the virus continues to evolve. Still, across every age group,...