Saturday, 25 May, 2024
HomeCoronavirusSeeding of COVID-19 outbreaks by contaminated frozen food — Laboratory study

Seeding of COVID-19 outbreaks by contaminated frozen food — Laboratory study

It is possible to get COVID-19 from contaminated imported chilled or frozen food, but the risk is very low, said experts. The coronavirus that causes COVID-19 remains viable for at least three weeks at 4°C, said Professor Dale Fisher, a senior consultant in the division of infectious diseases at the National University Hospital-Singapore. According to a Straits Times report, he recently concluded a study that involved putting the SARS-CoV-2 virus on prawn, salmon and pork, and testing its viability after three weeks – an ample timeframe for such food to be exported and sold.

The report says transmission through imported food has become a hot topic following the re-emergence of COVID-19 in New Zealand after 102 days with no cases, leading to a lockdown in Auckland. Six of those infected work at a cold storage facility, raising the possibility that they may have contracted the disease from the imported food before spreading it to others.

China recently reported finding the virus on frozen chicken wings from Brazil, where the pandemic is raging, and on the outer packaging of frozen prawns from Ecuador. In June, it said the virus was found in a Beijing market on a chopping board used to cut imported salmon. There was a cluster of more than 200 cases linked to the market.

Professor Ooi Eng Eong, deputy director of Duke-NUS Medical School's emerging infectious diseases programme, said in the report: "My sense is that although it is certainly possible for SARS-CoV-2 to be transmitted through improperly handled food, the risk is likely to be small." He explained that internationally accepted minimum standards for handling food to prevent transmission of pathogens would also prevent the transmission of this virus.

Associate Professor Hsu Li Yang, an infectious diseases doctor at the National University of Singapore's Saw Swee Hock School of Public Health, said "the probability (of infection from handling food) is infinitesimally small. The likelihood of being infected by another asymptomatic or mildly symptomatic person is far, far higher".

Associate Professor Alex Cook, vice-dean of research at the school, said his bet is on infected people here catching the coronavirus from someone else rather than from imported food as there is a hidden reservoir of the virus here.

While agreeing that the risk is low, Fisher is quoted in the report as saying: "It is about seeding it to one person, and then human-to-human (transmission) takes over again. "We know food processing plants host clusters, so it is likely food becomes contaminated. We know the virus can survive the time and temperatures for the trip."

Professor Teo Yik Ying, dean of the Saw Swee Hock School of Public Health, said the risk to the average consumer is extremely low, but may be higher for people working in the chilled food plants handling imported products every day. "But even for something where the chance is very small, when multiplied across the total number of chilled packages being shipped between locations, a few events will occur over time," he said.

Fisher said in the report that if infection from handling food has "happened a handful of times, then it fits with the concept of unlikely and not common", but does not make it impossible.

Reminding people to wash their hands and cook their food well, he said that "the first person to take the frozen or refrigerated meat out of the box and then touch his mouth" could become a new index case.

Ooi said: "The emphasis should be that proper food hygiene matters both in food centres and at home to prevent common food-borne pathogens. The same hygiene standards should also protect the public from COVID-19, even in the event that the raw food is contaminated with SARS-CoV-2."

An explanation is required for the re-emergence of COVID-19 outbreaks in regions with apparent local eradication. Recent outbreaks have emerged in Vietnam, New Zealand and parts of China where there had been no cases for some months. Importation of contaminated food and food packaging is a feasible source for such outbreaks and a source of clusters within existing outbreaks. Such events can be prevented if the risk is better appreciated.

The WHO advises that it is very unlikely that people can contract COVID-19 from food or food packaging. 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. It is necessary to understand the risk of an item becoming contaminated and remaining so at the time of export, and of the virus surviving the transport and storage conditions.

The clusters of infection of COVID-19 among workers in slaughterhouses and meat processing facilities in many countries can be attributed to factors that promote transmission of virus directly between workers, such as crowding, poor ventilation, and shouting in close proximity due to high ambient noise levels. Workers may go to work when infected, they may live in crowded housing, and travel on crowded transport. Environmental contamination at the work site is likely to be prolonged due to low temperatures, metal surfaces and lack of UV light.

With a significant burden of virus present in infected workers and the environment then contamination of meat with SARS-CoV-2 is possible during butchering and processing. The killing lines in abattoirs generally run at ambient temperature but the process later moves into a controlled environmental temperature of not greater than 12°C for the breakdown of carcasses and meat is maintained at 3–7°C as legislated by food regulations. The processing of meat and poultry is generally carried out manually in crowded conditions. Salmon processing is, in contrast, highly automated with filleting and cutting performed by machines, and minimal handling by workers. However, where such processing is carried out manually in crowded conditions the risk of contamination increases.

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).

Contamination of food is possible, and virus survival during transport and storage is likely. Food transportation and storage occurs in a controlled setting akin to a laboratory. Temperature and relative humidity is consistent and maintained and adverse conditions such as drying out is not permitted for the integrity of the food. In quantifying the viral titre we can reasonably assess a rate of decline in infectivity, which did not occur in any of the conditions we assessed.

We believe it is possible that contaminated imported food can transfer virus to workers as well as the environment. An infected food handler has the potential to become an index case of a new outbreak. The international food market is massive and even a very unlikely event could be expected to occur from time to time.

Efforts to avert the risk of COVID-19 outbreaks seeded by contaminated food must begin at the source; that is food processing premises. These include frequent hand washing, cleaning of food contact surfaces, materials and utensils. Fitness to work protocols should be in place and unwell staff should be excluded. Furthermore, the conditions under which workers in our food chain must be reviewed to ensure that our food is safe. Financial support needs to be given to unwell workers to ensure no disincentives to self-isolation or presenting for a test. PPE usage needs to be overseen and social distancing in and out of the workplace needs to be supported.

In receiving markets at the other end of the supply chain, food cannot be decontaminated, however, added precautions to ensure good hand hygiene and regular cleaning of surfaces and utensils is important. Consumers should wash their hands after touching uncooked products and ensure that food is well cooked.

Our findings, coupled with the reports from China of SARS-CoV-2 being detected on imported frozen chicken and frozen shrimp packaging material, should alert food safety competent authorities and the food industry of a “new normal” environment where this virus is posing a non-traditional food safety risk.

Dale Fisher, Alan Reilly, Adrian Kang Eng Zheng, Alex R Cook, Danielle E Anderson


[link url=""]Full Straits Times report[/link]


[link url=""] Full study in bioRxiv[/link]

MedicalBrief — our free weekly e-newsletter

We'd appreciate as much information as possible, however only an email address is required.