Across Western Europe, hospitals overflow with people sickened by the new coronavirus and morgues struggle to keep up with the mounting number of dead. In nations to the east, however, infections have largely been kept under control and governments are starting to loosen lockdowns.

The difference is stark. By Sunday, Spain had lost 350 people per million of its population to Covid-19, Italy 322, Belgium 314, France 202 and Britain 145, according to Johns Hopkins University data. Romania, in contrast, had lost 15 per million,...