The food supply is an essential service. Yet, workers in this industry are at high risk of acquiring infection with the novel coronavirus in their workplace unless proper mitigation measures are put in place. A new study, released as a preprint on the medRxiv* server, shows that the risk of infection may be reduced to less than 1% with proper precautions.
The study was spurred by mathematical models showing that workers in enclosed plants were more likely to be infected with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19). The aim was to determine how this risk could best be reduced and by how much.
Essential workers in the food industry have been badly hit by the COVID-19 pandemic right from its early days. In California, for instance, deaths among food and agriculture workers increased by almost 40%.
The primary reasons include working together in close proximity for long periods and enclosed workspaces.
Not only does such transmission cause extreme harm to the workers themselves, and their families, but it impacts the food supply chain, both locally and globally, imperiling food security.
Both direct and indirect transmission has been implicated in the spread of the virus. The former involves infectious droplets and aerosols, while the latter is by fomites, such as contaminated surfaces and hands.
Droplet transmission follows coughing or sneezing, typically within two meters. These droplets quickly fall to the floor or any nearby surface. They may be inhaled by others or fall directly onto the eyes, nose or mouth.
Aerosol transmission is linked to tiny particles released even during breathing or speaking, which can be infectious both close by and far away. Aerosols are less effective in large spaces because they disperse and thus have lower infectivity. Transmission of SARS-CoV-2 via fomites is less common.
The existing guidelines by U.S. Occupational Safety and Health Administration [OSHA], European Union-OSHA), food safety agencies (U.S. Department of Agriculture, USDA, the Food and Drugs Administration, FDA) and the food industry are intended to ensure that symptom screening, adequate physical distancing, mask use, and handwashing, are properly practiced at work.
Moreover, the DA’s Food Safety Modernization Act outlines paid leave for sick workers along with hand and surface disinfection.
While many models show the efficacy of all these measures, which is most important? Little evidence has been provided to answer this question.
The current study uses the mathematical modeling approach called quantitative microbial risk assessment (QMRA) to provide some answers. It has been used to determine the level of risk from this virus in healthcare, wastewater treatment plants, and fomite-based spread at a community level.
The model shows that droplets carry 90% of the infectious load to another person one meter away. At 2 or 3 meters, aerosols make up 30-60%, and fomites about 50%.
In absolute terms, the droplets carry far more infection, at 478 plaque-forming units (PFU), compared to about 44 from fomites, and 7 from aerosols, at one meter. Even at 2-3 meters, aerosols or fomites have only up to 10 PFUs.
Droplets contribute 11-fold higher viral loads and, therefore, carry significantly higher infection risks. The combined risk estimates for transmission by all three routes were 0.98, 0.15 and 0.09 at 1, 2 and 3 meters, respectively. Thus, coughs seem to be responsible for the majority of transmission in such workplaces.
Secondly, the model shows steadily increasing risks, whether combined or individual aerosol- or fomite-associated chances of transmission with increased exposure time from 1 to 8 hours.
At one meter from a coughing colleague, the risk steadily increased, beginning to plateau at five hours and becoming maximum at eight hours of exposure.
However, there were extreme differences between the infection risk between individual exposures, given that the viral titer in the saliva, the frequency of coughing, the depth of inhalation and the rate at which droplets were deposited all affected the infectiousness of the worker.
In terms of the basic reproduction rate R0, the combined risk estimates for exposures of 1-8 hours over distances of 1 or 2 meters were >1. That is, viral spread will continue to happen unless workers maintain physical distancing.
At 3 meters, where only aerosols are active, the R0 falls to below 1, indicating a reduction in transmission.
Actual R0 values in the community were between 2.3 to 11, indicating that these risk estimates could be scaled up to represent the observed transmission in the population.
The highest risk is when droplets or aerosols are expelled within one meter of a vulnerable individual. Thus, increasing the distance beyond this value is associated with large reductions in risk, especially if workers are 2 meters or more apart.
When workers were separated by 2 or 3 meters instead of 1, the risk fell by 97% and 98%, respectively.
Even with 8 hours of exposure, the risk fell by 84% and 91%, at 2 meters and 3 meters, respectively.
Under the same conditions (8 hours of exposure, with one coughing colleague at a distance of 1 meter), the universal use of cloth masks led to a halving of the combined infection risk. The risk fell to a third with surgical mask use.
Double masking (a cloth mask + a surgical mask) cut the risk to slightly over a tenth relative to no mask. With an N95 respirator, almost complete protection was achieved (one in one hundred).
Combined measures are synergistic
The best protection was with a combination of both physical distancing and mask use, which have a synergistic effect. At 1-2 meters distance and 8 hours exposure, any mask reduced the risk by 91%.
When the distance is increased from 1 meter to 3 meters, the total risk over 8 hours falls by 97%. Except for N95 respirators, therefore, the greatest effect is seen with a combination of both interventions.
Increased ventilation was measured by the number of air changes per hour (ACH). With a baseline ACH of 0.1, increases in this parameter to between 2 and 8 led to an average risk reduction by over one-third at 1 meter, but >82% beyond 3 meters.
After 8 hours of exposure, the risk fell by up to 95% if combined with physical distancing of 1-2 meters and by 98% at 3 meters.
Infection via fomites also fell by 62% to 99% with the use of masks, depending on mask type. Reductions by ~90-99% were seen with double masking and N95 respirators.
Hand washing also reduced the risk, with hourly washing or sanitization with alcohol almost abolishing the risk. Zero fomite-related risks were achieved with hourly glove changes following decontaminating handwashing or hourly surface disinfection.
Many of these measures are in place in most food plants. The study shows that bundling these interventions (keeping 2 meters between workers, universal mask use, and 2 ACH with handwashing every hour, plus two surface cleanings per shift) drops the combined infection risk over 8 hours to below 1%.
Even at a 1-meter distance, the use of double masking and 6 ACH decreases the risk to 2%, compared to no intervention.
What are the implications?
“Rank prioritizing these single interventions suggests physical distancing, followed by mask use, and then increasing facility ventilation results in the largest combined risk reductions to a susceptible worker after an 8h-shift with a coughing infected worker.” Combining these strategies leads to the greatest impact.
Modeling with a smaller space indicates that small poorly-ventilated enclosed spaces may induce virus-infected aerosols to accumulate. However, the researchers emphasize, “the use of simpler interventions (masking and distancing) provided greater risk reduction than did increasing ACH alone, which would ultimately be a more expensive engineering investment.”
Thus, a combination of relatively inexpensive but highly effective mitigations of close contact transmission could work well. These findings could help decide which interventions to sustain and which to change or stop after vaccination. High-efficiency particulate air (HEPA) filters or inactivation by far-ultraviolet light can also be evaluated in future work.
This work advances the evidence-base for effective risk mitigation strategies currently implemented by the food industry and can be used to inform best practices for protecting essential workers. Although our model was designed for an indoor food manufacturing setting, it can be readily adapted to other indoor environments and infectious respiratory pathogens.”
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.