WASHINGTON/ABOARD AIR FORCE ONE, Sept 7 (Reuters) - President Joe Biden on Thursday will present a six-pronged strategy intended to fight the spread of the highly contagious coronavirus Delta variant and increase U.S. COVID-19 vaccinations, the White House said on Tuesday. The United States, which leads the world in COVID-19 cases and deaths, is struggling to stem a wave of infections driven by the variant even as officials try to persuade Americans who have resisted vaccination to get the shots. Rising case loads have raised concerns as children head back to school, while also rattling investors and upending company return-to-office...