BERLIN — From nursing homes in France to hospitals in Poland, older Europeans and the workers who care for them rolled up their sleeves on Sunday to receive coronavirus vaccine shots in a campaign to inoculate more than 450 million people across the European Union. The inoculations offered a rare respite as the continent struggles with one of its most precarious moments since the pandemic began. Despite national lockdowns, restrictions on movement, shuttering of restaurants and cancellations of Christmas gatherings, the virus has stalked Europe into the dark winter months. The spread of a more contagious variant of the virus in Britain has raised such alarm that much of continental Europe rushed to close its borders to travelers coming from the country, effectively plunging the nation as a whole into quarantine. “Today, we start turning the page on a difficult year,” Ursula von der Leyen, the European Commission president wrote on Twitter. “The #COVID19 v...