For much of this past week, as a dangerous viral outbreak continued to rattle the world, officials seemed keen to offer reassurances. Chinese leaders urged people to get back to work. American officials said they saw no need to evacuate citizens from a quarantined cruise ship in Japan.

In a sign of how unpredictable the crisis still is, and how quickly officials have had to adapt their responses, both of those declarations had at least partly unraveled by Saturday.

Communist Party leaders said people returning to the Chinese capital would have to isolate themselves for two weeks, potentially delaying any economic revival. American officials announced that they would charter a flight to evacuate American citizens from the cruise ship, after citing heightened danger to those on board.

The shifting policies underscore the rapidly evolving terrain of one of the most serious global public health crises in recent years, even as China’s leader, Xi Jinping, said this past week that the crisis was improving. The outbreak has been marked by confusion and conflicting analysis of the available information.