The first Americans vaccinated against COVID-19 are getting their second dose while Britain is postponing boosters. Exactly how to give the shots varies by country, and debates over what’s best is adding to public confusion. Britain says it’s OK to wait longer between doses than vaccine studies showed. U.S. regulators made clear Monday it is sticking with three to four weeks between doses. Supplies aren’t the only challenge, as the U.S. and other countries are facing logjams in using the scarce shots that already have been raced out.