As the coronavirus spreads, assembly lines may take longer than expected to restart, causing a chain reaction around the world. FRANKFURT — Chinese car and car-parts factories may stay closed longer than expected because of the coronavirus, increasing the chances that assembly lines in Asia, Europe and the United States could grind to a halt because of shortages of components. The hit to the auto industry, which employs eight million people worldwide, comes as output from the world’s factories is already sagging. It is likely to amplify the already alarming human and economic cost of the outbreak. Many automakers had planned to reopen their Chinese factories on Monday, which was already a week later than planned following the traditional Lunar New Year holiday. But several companies including BMW, PSA and Toyota have delayed restarting their assembly lines another week and others appear likely to. Even a relatively brief interruption in the flow of parts and materials could have far-reaching effects, analysts said. “Two weeks is probably the buffer most people have got,” Mark Fulthorpe, a director at the market researcher IHS Markit who follows the auto industry, said Friday. “A...