The coronavirus can survive on frozen food packaging, Chinese officials have said after discovering a city outbreak linked to frozen fish.
The Chinese Center for Disease Control and Prevention (CDC) detected and isolated a living sample of the coronavirus on the outer packaging of frozen cod. Only traces of the pathogen had been found on frozen packaging before now.
It was imported to the eastern city of Qingdao, which has been hit by a fresh cluster of cases linked to two dock workers. The men, who were diagnosed in September, were responsible for unloading frozen seafood at the port.
The CDC's finding suggests it is possible for the virus, scientifically called SARS-CoV-2, to be conveyed over long distances via frozen goods.
But scientists warned it is possible both dock workers caught Covid-19 — the disease caused by the virus — from elsewhere, despite the fact there had been zero cases in China for two months before the outbreak.
It cannot be ruled out that the patients contaminated the frozen fish products with coronavirus themselves, rather than the product being the source of infection.
Cold food factories have been at the centre of several coronavirus outbreaks in the UK. But scientists have said it's more likely due to them being crowded and noisy — rather than the cold temperatures.
The Food Standard Agency in the UK and the World Health Organization say the risk of catching the virus from frozen foods is 'extremely low'.
The coronavirus can survive on frozen food packaging, Chinese authorities claim. Pictured, a woman in the frozen food aisle at a supermarket in Beijing
The findings come after officials in Qingdao determined the source of a recent cluster of cases to two dock workers with a positive test. In this photo, a barge pushes a container ship to the dockyard in Qingdao, east China's Shandong province
The CDC — different to the agency in the US with the same name — confirmed the detection of the coronavirus on packaging of imported frozen cod on October 17, during an investigation into the Qingdao outbreak.
According to the CDC, it 'has proved that contact with packaging contaminated by living novel coronavirus could lead to infection', China's state-run news agency, Xinhua, reported.
It is the first time in the world that 'live' traces of the virus have been found on cold food packaging, the CDC said. If the virus is live, it means it has the ability to infect people.
The CDC also said it suggests the coronavirus can spread over borders via imported food.
The outbreak in Qingdao can be traced back to two dock workers who were initially diagnosed as asymptomatic patients in September, during a routine test of the company's staff, the Qingdao Municipal Health Commission said in a statement.
The infected employees, Mr Dong, 40, and Mr Chen, 50, both tested negative on September 8 during a routine screening.
They both worked on the evening of September 19, unloading imported seafood products at the port.
The workers had check-ups in an examination room at the Qingdao Chest Hospital.
Because the room had insufficient disinfection and protection measures, according to a Qingdao health official reporting at a media briefing, the two cases led to an outbreak of 13.
The fish was imported to the eastern city of Qingdao - where officials have determined the source of a recent cluster of cases to two dock workers
The Qingdao outbreak prompted authorities to launch ambitious mass screening on October 12. Pictured: A health worker takes a swab from a resident to be tested for the coronavirus in Qingdao on October 12
The city of Qingdao has reported 13 confirmed infections this week, with most of the cases linked to a local hospital, prompting authorities to test nearly 11million residents within four days to curb the virus spread. A woman in Qingdao is pictured receiving a Covid-19 test
COVID-19 AND FROZEN FOOD: WHERE ELSE HAS IT BEEN REPORTED?
In August Chinese authorities said a sample of frozen chicken wings imported from Brazil had traces of the coronavirus.
Covid-19 was detected on a surface sample taken from a batch imported into the southern Chinese city of Shenzhen on August 11.
The Office of the Shenzhen Epidemic Prevention and Control Headquarters immediately traced and tested people who may have come into contact with the contaminated food.
All of the results have come back negative, a statement from the headquarters reportedly said.
It came after a panic about frozen salmon in Beijing, China.
In June, the city saw a spike of Covid-19 cases - almost all of which were linked to a huge wholesale food market, Xinfadi market.
A 22-year-old man - known to have occasionally cleaned frozen seafood - had tested positive for the virus in Tianjin, near Beijing.
State media said the virus was discovered on chopping boards used for imported salmon, sparking fears across the country with supermarkets and restaurants removing the fish from shelves and menus,
Chinese authorities acted quickly to suspend import of salmon from Europe, and later imports from food premises where outbreaks of Covid-19 have occurred among workers, affecting the US, Germany and Brazil among others.
However Shi Guoqing, deputy director of the Chinese Centre for Disease Control, said there was no evidence that salmon was the source for the virus and was contaminated before it reached the market, the BBC reported.
Singapore's Food Agency has also said there was no evidence that the virus would be transmitted via food or even food packaging - though it said it would monitor 'developments in this area'.
The Qingdao outbreak prompted authorities to launch ambitious mass screening on October 12 after a two-month streak with no new cases in China.
An astonishing 11million residents were tested within four days to curb the virus spread.
The Chinese authorities did not specify how the workers contracted the virus but said they 'shared the common exposure', referring to the imported seafood products.
But the CDC's latest statement does not show solid proof that the two workers in Qingdao caught the virus from the packaging directly, Jin Dong-Yan, a virology professor at the University of Hong Kong, said.
They may have contracted the virus from somewhere else and then contaminated the food packaging they handled, Professor Dong-Yan said.
The CDC said no instance had been found of a consumer contracting the virus by after buying frozen food. And they said the risk of this happening remained very low.
The agency cited recent test results for 2.98million samples taken from businesses across the country, including 670,000 taken from cold-chain food or food packaging.
Only 22 samples from cold-chain food or food packaging tested positive for the virus, the China CDC said.
It also said no 'living virus' was isolated in these samples. Only living viruses can infect people, while samples containing dead viruses could also test positive for traces, Professor Dong-Yan said.
Dr Michael Head, a senior research fellow in global health at the University of Southampton, told MailOnline: 'I think that there always has been a theoretical risk of transmission by packaging on food, similar to examples of people touching door handles or buttons in lifts.
'However, the reality is that the risk of transmission via these routes remains very small, and that close contact with an infectious individual is by far and away the main route by which transmission occurs. Regular and thorough hand-washing will minimise this small risk still further.'
Fomites - objects or materials which are likely to carry infection, such as furniture, door knobs or hospital equipment - are a major source of disease transmission.
A person may get Covid-19 by touching a surface or object that has the virus on it and then touching their own mouth, nose, or eyes - hence the importance of regular hand washing.
However this is not thought to be the main way the virus spreads - inhalation of respiratory droplets, produced when an infected person coughs or sneezes, and or small particles, produced when someone talks or breathes, are the main route of transmission.
How long the virus can survive on surfaces depends on the material as well as the environment it is in.
It has been found to last three to seven days on plastic and stainless steel at room temperature.
Scientists at the Indian Institute of Technology Bombay found that higher temperatures and lower humidity dry out droplets faster, and when droplets evaporate, the virus left behind dies.
But the research only compared the drying time in two different temperatures: 25 degrees Celsius (77 degrees Fahrenheit) and 40 degrees Celsius (104 degrees Fahrenheit), both with 50 per cent humidity.
The evidence on coronaviruses ability to survive in frozen temperature, and on food packaging, is murky.
The World Health Organization has said that it is unlikely that people can contract the coronavirus from food or food packaging, and that 'it is not necessary to disinfect food packaging materials'.
But one study led by the National University of Singapore challenged this statement after assessing the survival of SARS-CoV-2 on samples of meat and salmon at refrigerated and frozen temperatures over three weeks.
It found the level of virus on the food samples remained constant during the entire experiment.
The authors wrote in their pre-print paper: 'Our laboratory work has shown that SARS-CoV-2 can survive the time and temperatures associated with transportation and storage conditions associated with international food trade.
'When adding SARS-CoV-2 to chicken, salmon and pork pieces there was no decline in infectious virus after 21 days at 4°C (standard refrigeration) and –20°C (standard freezing).
'While it can be confidently argued that transmission via contaminated food is not a major infection route, the potential for movement of contaminated items to a region with no Covid-19 and initiate an outbreak is an important hypothesis.'
The CDC has advised that workers who handle, process and sell frozen products should avoid direct skin contact with products that could possibly be polluted.
Staff should not touch their mouth or nose before taking off work garments that could possibly be contaminated without washing their hands and should take tests regularly, the agency said.
The UK has been plagued with Covid-19 outbreaks at leading food factories, with 40 active outbreaks across England in one week in August.
Hundreds of cases were recorded at Greencore, Northampton, which makes M&S sandwiches. And dozens more were reported at a Greggs factor in Bramley, Leeds, at Bakkavor Desserts in Newark, Nottinghamshire, and a chicken factory in Coupar Angus, Scotland.
However the Food Standard Agency boss Dr Sullivan argued the figure of 40 active outbreaks in England should be seen as small when set against the 20,000 food processing plants across the UK.
The FSA's chief executive, Emily Miles, stressed there is no need for alarm, saying: 'The risk of transmission of Covid-19 through the consumption or handling of food, or food packaging, remains very low.
'There have been a small number of Covid 19 outbreaks associated with meat and food processing places and we have been supporting the Health & Safety Executive, Public Health England, Public Health Wales and other bodies in their work to deal with those.'
She added: 'The FSA has to be here to tell the truth about food. People need to be able to turn to us and get the facts about food and Covid 19.
'The risk assessment led by our very brilliant scientists, who have been assessing and re-assessing the evidence that we are learning about Covid 19 is saying that the risk is extremely low.'
Experts have suggested the cold conditions inside food factories may be conducive to the spread of the virus.
Professor Calum Semple, a disease outbreak expert at the University of Liverpool and member of SAGE, told The Telegraph that cold, sunless food factories are ideal conditions.
He said: 'If I wanted to preserve a virus I would put it in a cold, dark environment or a cool environment that doesn't have any ultraviolet light - essentially a fridge or a meat processing facility.'
'The perfect place to keep a virus alive for a long time is a cold place without sunlight.'
But the temperature alone does not appear to be a controlling factor in coronavirus outbreaks.
Dr Michael Head, a global health researcher at the University of Southampton, said he thought close proximity was most likely to be behind the factory outbreaks.
He said: 'Whilst refrigeration may be a contributory factor to the spread of the virus, the key factors are likely to be the number of people close together in indoor conditions.
'Some of these factories have onsite or nearby accommodation where there are several people in each dormitory, they may be transported on a bus to the site of work, and they will be indoors together all day.
'Levels of adherence to measures such as washing hands is uncertain and there is unlikely to be widespread use of PPE.'