BRUSSELS—The European Union’s executive body proposed Monday that the bloc’s 27 nations impose more travel restrictions to counter the worrying spread of new coronavirus variants but make sure to keep goods and workers moving across EU borders. Amid concerns related to the production and delivery of COVID-19 vaccines, the European Commission urged EU nations to reinforce testing and quarantine measures for travelers as virus mutations that are more transmissible threaten to overwhelm European hospitals with new cases. More than 400,000 EU citizens have already died from the virus since the pandemic first hit Europe last year. “The start of the EU vaccination campaign kicked off the beginning of the end of the pandemic,” EU Justice commissioner Didier Reynders said. “At the same time, new, more transmissible variants of the virus have surfaced. There is currently a very high number of new infections across many member states. And there is an …