The Biden administration has decided that most Americans should get a coronavirus booster vaccination eight months after they received their second shot, and could begin offering third shots as early as mid-September, according to administration officials familiar with the discussions. Officials are planning to announce the decision as early as this week. Their goal is to let Americans who received the Pfizer-BioNTech or Moderna vaccines know now that they will need additional protection against the Delta variant that is causing caseloads to surge across much of the nation.