According to shipping analytics firm Windward, 20% of the world’s roughly 9,000 active container ships are currently sitting in traffic jams outside congested ports. Close to 30% of that backlog alone is in China—double the domestic congestion rate in February—where a virulent Omicron wave is snarling supply lines.

Ships have been piling up outside Shanghai, the world’s largest port, and other container docks across China as authorities have forced multiple cities into lockdown to counter the country’s worst COVID outbreak since the pandemic began.

Shanghai has been under lockdown for close to four weeks, having recorded over 35,000 COVID cases since March. Residents in the city of 25 million have struggled to secure food and access to health care as authorities prohibit them from leaving their homes.

Windward says the lockdowns in China, which began in mid-March, have nearly doubled the number of container ships loitering off the country’s coast. As of April 19, Windward recorded 506 vessels awaiting berthing space at Chinese docks, up 195% from the 260 halted offshore in February.

Before the lockdowns started, congestion at China’s ports accounted for only 14.8% of the global container backlog, versus roughly one-third now, Windward says.

Shanghai port authorities claim that the port is operating normally and has suffered zero delays thanks to a closed loop system keeping staff on-site, despite numerous shipping analytics firms providing data to the contrary.

Beyond the port of Shanghai, restrictions on cross-border trucking and movement throughout the city have prevented workers from off-loading and on-boarding shipping containers, creating delays.