Chinese health authorities investigating a recent Covid-19 outbreak say they have discovered live coronavirus on frozen food packaging, a finding that suggests the virus can survive in cold supply chains.

On Saturday the Chinese Centre for Disease Control and Prevention (CDC) said it had found traces of live Covid-19 on the outer packing of frozen cod in the eastern coastal city of Qingdao, marking the first time live coronavirus has been detected on the outside of refrigerated goods. Researchers were investigating the source of a recent cluster of cases linked to a hospital in Qingdao. Genetic traces have previously been found in samples of frozen food but no living virus has been isolated before. “It has been confirmed that contact with outer packaging contaminated by the new coronavirus can cause infection,” the agency said in a statement on its website, without specifying where the batch of frozen food came from.

China, which until the Qingdao outbreak had recorded no new local cases in 55 days, has been one of few countries to point to possible transmission through frozen food. When Beijing had a second outbreak in June after the virus had been largely contained, officials suggested the new cluster could have come from imported salmon.

The US Centers for Disease Control and Prevention have said there has been no evidence that “handling food or consuming food is associated with Covid-19”. New Zealand ruled out the possibility that one of its first infections happened at a cold storage facility.

In August China increased inspection requirements for imports of frozen products after traces of the virus were found on frozen chicken wings from Brazil. Researchers have also detected the virus on imported shrimp and other frozen seafood.

The CDC did not say whether the outbreak in Qingdao was caused by the frozen food packaging. The outbreak, the first locally transmitted cases in almost two months, was traced back to two dock workers initially diagnosed as asymptomatic, who are believed to have infected 12 other people at a hospital.

It was not clear whether the dock workers contracted the virus from the packaging of frozen food. Still, the CDC warned those handling frozen products not to have direct contact with the goods.

The CDC said there had been no cases of consumers contracting the virus, and such a risk was low. But some residents have begun calling for a temporary ban on frozen food imports.

The CDC said that from a total of 2.98m samples of food packaging, researchers found 22 samples that tested positive.