The United States on Wednesday promised to buy 500 million more Covid-19 vaccine doses to donate to other countries as it comes under increasing pressure to share its supply with the rest of the world.The White House is hosting a four-hour virtual summit aimed at boosting global vaccination rates and the administration of President Joe Biden wants to show it is leading by example.“To beat the pandemic here we need to beat it everywhere,” Biden said, kicking off the summit, which includes…