While there is no evidence that food or food packaging are significant transmission pathways for Covid-19, there are steps that food companies can take to reduce the risk of contamination, argues Professor Phil Bremer, Dr Catherine McLeod, Dr Joanne Kingsbury and Dr Rob Lake.
Covid-19 has caused significant disruption to the food industry as companies strive to ensure worker safety and maintain regulatory and consumer confidence in their products.
Despite billions of meals having been transported around the world and consumed since the start of the pandemic, there is no definitive evidence that food or food packaging has been a source or a transmission route for SARS-CoV-2.
Although significant Covid-19 outbreaks have occurred at food processing facilities overseas and amongst food service workers, the primary cause of these outbreaks is believed to be due to person-to-person transmission, which has been facilitated by the enclosed nature of the work environment, rather than exposure of workers to contaminated food or packaging material.
While a number of studies have investigated the ability of SARS-CoV-2 to survive and remain infective on food, packaging material or other surfaces, the majority of studies have inoculated the food or surface with far higher concentrations of virus than could be reasonably expected to be naturally deposited by infectious people sneezing. Survival over time has then been assessed under conditions that have generally not reflected natural scenarios.
Nevertheless, when added to foods, SARS-CoV-2 can remain infectious for varying periods of time, dependent on the properties of the food (pH) and the storage conditions (temperature, relative humidity).
For example, SARS-CoV-2 inoculated onto salmon, shrimp and chicken was relatively stable after one day at refrigeration temperatures, while another study stated that SARS-CoV-2 on salmon remained infectious for longer than a week at refrigeration temperatures.
Further, SARS-CoV-2 remained infectious in human milk stored for at least two days at refrigeration or freezing temperatures and for at least eight weeks in ice cream stored at -20°C or -80°C. In contrast, in refrigerated acidic fermented milk infectivity declined over time, and the virus was not detectable after four weeks.
Infectious viral loads from refrigerated mushrooms were significantly reduced after one hour and undetectable after one day, but there was only a modest reduction in infectious viral numbers on apple skins and spinach after one day.
There are however, four important take home points to consider when assessing the potential role of food in the transmission of SARS-CoV-2. The first is that studies which have attempted to try to replicate natural fresh produce contamination scenarios by using low levels of aerosolised SARS-CoV-2 or the handling of produce by Covid-19 infected people, have not resulted in infectious SARS-CoV-2 subsequently being detected on the produce. Second, normal cooking or the application of heat during food processing (pasteurisation) will inactivate the virus.
Third, if SARS-CoV-2 did get on to food, and this food was subsequently consumed before the virus was inactivated it is believed that normal intestinal tract conditions (stomach acid and bile salts) would inactivate the virus. Fourth, infection by SARS-CoV-2 occurs principally via inhalation of aerosolised virus particles.
These four points provide a clear rationale as to why food is unlikely to be a vector or transmission route for SARS-CoV-2.
The risk of SARS-CoV-2 being spread from contaminated food packaging is also considered to be very low. There have been relatively few reports of the SARS-CoV-2 virus being detected on food packaging, and where it has been detected most studies have detected viral RNA rather than infectious virus. While the detection of viral RNA highlights the importance of employing good food hygiene practices to minimise the possibility of food or food contact surfaces becoming contaminated, it does not indicate the presence of an infectious hazard nor imply a risk to humans that handle the packaging.
Studies from China suggesting a genetic linkage between SARS-CoV-2 found on imported cold-chain products/packaging and from workers who were subsequently found to be infected after handling them, were not able to definitively determine the direction of the transmission and alternative transmission routes were possible.
Infectious SARS-CoV-2 has been demonstrated to persist on hard surfaces (e.g. plastic, glass, steel) held at ambient temperatures in the dark for periods ranging from several days to at least a month, although significant reductions in infectious virus numbers occurred over time. In general, SARS-CoV-2 remains infectious on surfaces for longer periods at lower temperatures and humidity levels. The demonstrated stability of the virus on freezing was expected.
In the work place, best practice for managing the risk of SARS-CoV-2 infection amongst workers includes facilitating and encouraging vaccination, implementing routine temperature monitoring, stressing the importance of self-isolating and of workers seeking medical advice and getting a Covid-19 test if they have any symptoms of Covid-19 and/or respiratory illness.
Companies can best protect people, products and packaging by ensuring there is good ventilation, appropriate PPE use, the use of screens, strong workplace bubbles and social distancing, and that there is strict adherence to good hygiene practices.
Because vaccination does not completely prevent infection and vaccinated people might be asymptomatic, the importance of continued strict adherence to the use of PPE and the strict adherence to good hygiene and social distancing practices cannot be over-emphasised. Rapid testing has the potential to complement these fundamental control measures, but it cannot replace them.
While SARS-CoV-2 can remain infectious and/or detectable on processing surfaces, food packaging and in some foods under cold chain conditions for significant periods of time, there is no evidence that food or food packaging are significant transmission pathways for Covid-19.
Aerosols and droplets are by far the dominant transmission route. In the workplace, vaccination with health checks, status reporting, physical distancing, good ventilation, the use of PPE and the following of hygiene practices remain the best means to prevent transmission between workers.
*Professor Phil Bremer is from the Department of Food Sciences, at the University of Otago and is a member of the New Zealand Food Safety Science Research Centre.
*Dr Catherine McLeod is Director of the New Zealand Food Safety Science Research Centre.
*Dr Joanne Kingsbury is a Senior Scientist at the Institute of Environmental Science and Research and is a member of the New Zealand Food Safety Science Research Centre.
*Dr Rob Lake is a Science Leader at the Institute of Environmental Science and Research and is a member of the New Zealand Food Safety Science Research Centre.
*The authors declare that they have no conflict of interest.