Some retailers are preparing for a rocky back to school season as COVID-19’s impact on shipping remains a big concern.

After lifting a two-month long lockdown in Shanghai on June 1, officials announced new rounds of COVID-19 testing on its 25 million residents this week. Shanghai is the world’s busiest port for container traffic.

Greg Buzek, president of IHL Group, a research firm for retail and hospitality industries, said it takes about 50 days to move from Shanghai through the Los Angeles/Long Beach port complex and onto a truck.

“Which means if it’s not in the water already right now, it is not making it before the back-to-school season,” he said.

During a June webinar hosted by IHL Group, Buzek said if back-to-school items arrive late, retailers will offer sales in September and October.

“It’s not LA and Long Beach being the issue,” he said. “There’s still some truck issues, we’ve got some rail issues there, but it’s not the port itself getting the product off the boats. The problem is Shanghai.”

Buzek said the amount of cargo at sea heading to the United States in mid-June was 30% lower than a year ago and the percentage of shipments arriving on time remain at historic lows.