China is pointing blame for a recent locally transmitted case of the omicron variant in Beijing at overseas mail.

Residents in the city are being urged to wear masks and gloves when opening mail from abroad and to limit international purchases as authorities try to figure out how COVID-19 seeped through tight borders ahead of the Winter Olympics that start next month.

The city reported one local infection from the variant. The person hadn’t left Beijing two weeks before testing positive but occasionally handled international mail at work, the Beijing Municipal Health Commission said on Monday, adding that no other people who lived and worked with the patient tested positive.