Two cities in China have found traces of the new coronavirus in imported frozen food and on food packaging, local authorities said on Thursday, raising fears that contaminated food shipments might cause new outbreaks.

A sample taken from the surface of frozen chicken wings imported into the southern city of Shenzhen from Brazil, as well as samples of outer packaging of frozen Ecuadorian shrimp sold in the northwestern Xian city, have tested positive for the virus, local authorities said on Thursday.

The discoveries came a day after traces of the coronavirus that causes Covid-19 were found on the packaging of frozen shrimp from Ecuador in a city in eastern Anhui province. China has been stepping up screenings at ports amid the concerns over food imports.

Shenzhen’s health authorities traced and tested everyone who might have come into contact with potentially contaminated food products, and all results were negative, the city’s notice said.

The Brazilian embassy in Beijing did not immediately respond to a request for comment. Reuters could not immediately reach the Ecuadorean embassy in Beijing.

“It is hard to say at which stage the frozen chicken got infected,” said a China-based official at a Brazilian meat exporter.

The Shenzhen Epidemic Prevention and Control Headquarters said the public needed to take precautions to reduce infection risks from imported meat and seafood.

The health commission of Shannxi province, where Xian city is located, said authorities are testing people and the surrounding environment connected to the contaminated shrimp products sold in a local market.

In addition to screening all meat and seafood containers coming into major ports in recent months, China has suspended some meat imports from various origins, including Brazil, since mid-June.

The first cluster of Covid-19 cases was linked to the Huanan seafood market in the city of Wuhan. Initial studies suggested the virus originated in animal products on sale at the market.

Li Fengqin, who heads a microbiology lab at the China National Center for Food Safety Risk Assessment told reporters in June the possibility of contaminated frozen food causing new infections could not be ruled out.

Viruses can survive up to two years at temperatures of minus 20 degrees Celsius, but scientists say there is no strong evidence so far the coronavirus that causes Covid-19 can spread via frozen food.