LONDON (Reuters) - The British government is working through complicated ethical issues posed by COVID-19 vaccine certification before deciding how the so-called passports can best be used, Prime Minister Boris Johnson said on Monday. Johnson said there would be no need for people to prove they had been vaccinated for the reopening of shops and pub gardens next week, adding that the idea of vaccine passports for international travel was likely and its use domestically would be piloted at some big events. “I want to stress that there are complicated, ethical, and practical issues ... raised by the idea of COVID status certification ... using vaccination alone,” Johnson told a news conference. “You’ve got to be very careful in how you handle this, and don’t start a system that’s discriminatory.”