While thousands of visitors to Shanghai Disneyland on Sunday were queuing for roller coasters and watching fireworks above the fairytale castle, staff quietly sealed the amusement park. People in Hazmat suits streamed in through the gates, preparing to test everyone for Covid-19 before they could leave for the day.

Nearly 34,000 people at Disneyland underwent testing, which ended close to midnight, long after the festivities at the park are usually finished. Ferried home on 220 special buses, all were found Monday to be negative but are still required to isolate at home for two days, and be re-tested for the coronavirus in two weeks.

The shutdown of one of Walt Disney Co.’s most lucrative parks came after a positive case in a woman who traveled to Shanghai from nearby Hangzhou over the weekend. While officials are yet to confirm whether she visited Disneyland, her infection sparked an aggressive contact tracing effort across China, which eventually ensnared the park-goers, their families and Disneyland staff.