Residents in Guangzhou, Guangdong province, who received international couriers between Sunday and Wednesday, have been urged to undergo nucleic acid testing in three days starting Thursday.

The testing is free at the testing venues in communities, with residents asked to present proof of having received international couriers during the period or a text messages from the local center for disease prevention and control confirming the matter, according to the city's COVID-19 epidemic control authorities.

The authorities reminded people to minimize receiving overseas mail from places that have a high epidemic risk and to take caution when opening international parcels.

In Shenzhen, Guangdong province, some people have found their health code turn yellow from green after receiving international parcels. The code will turn green again with a negative nucleic acid testing result.

Source tracing for some latest infections in Shenzhen and Beijing has pointed to the possibility of contaminated international couriers.