PARIS—France imposed a month-long lockdown on Paris and parts of the north after a faltering vaccine rollout and spread of highly contagious coronavirus variants forced President Emmanuel Macron to shift course. Since late January, when he defied the calls of scientists and some in his government to lock the country down, Macron has said he would do whatever it took to keep the euro zone’s second-largest economy as open as possible. However, this week he ran out of options just as France and other European countries briefly suspended the use of the AstraZeneca vaccine. His prime minister, Jean Castex, said France was in the grip of a third wave, with the virulent variant first detected in Britain now accounting for some 75 percent of cases. Intensive care wards are under severe strain, notably in Paris where the incidence rate surpasses 400 infections in every 100,000 inhabitants. “The epidemic is getting …