LONDON (Reuters) - England’s test and trace scheme needs improvement and it is hard to run an effective system when there are large and increasing numbers of COVID-19 cases, UK chief scientific adviser Patrick Vallance said on Thursday. “It’s undoubtedly the case that test trace and isolation becomes much more difficult to have an impact once numbers are high. So, it’s much more effective when numbers are low,” he said at a news conference. “It’s very clear that there’s room for improvement.”