Lu Jiaying hasn’t tested positive for Covid, and hasn’t experienced any symptoms.

But at 7 p.m. on Sunday, the 34-year-old technology worker and resident of central Shanghai, was ordered to board a bus for a mass quarantine center.

Nearly 16 hours later, Ms. Lu, still aboard the bus in a head-to-toe protective suit, hadn’t eaten a bite, sipped any water or gone to the bathroom, a casualty of the chaos that has seized China’s biggest and most prosperous city amid a surge in Covid infections.

“My only wish is to leave this place alive,” she told a Wall Street Journal reporter at 8:30 a.m. on Monday. Hours earlier she posted an online selfie showing sweat beads on her forehead and fog covering her plastic face shield inside the sealed bus.