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RHEDA-WIEDENBRUCK, Germany — An “unfavorable mix” of conditions inside a German meat processing plant promoted aerosol transmission of the coronavirus over long distances, a recent study found, shedding light on how meat processing facilities in Iowa and around the globe became hotspots for COVID-19 outbreaks.

A joint study conducted by the Helmholtz Centre for Infection Research, the University Medical Center Hamburg-Eppendorf and the Heinrich Pette Institute investigated the conditions that led to a coronavirus outbreak at the largest meat processing complex in Germany. More than 1,500 workers at the Tönnies plant in Rheda-Wiedenbruck tested positive for COVID-19, according to a report from CNN.

Originating from a single employee, researchers discovered the virus was transmitted via aerosols (respiratory particles suspended in the air) to other workers in a radius of more than 26 feet. The study found the climate conditions and airflow inside the plant were factors that promoted such efficient aerosol spread of the virus.

“Our results indicate that the conditions within the deboning work area of the meat processing plant — namely the low temperature, low fresh air supply and constant air circulation through the air-conditioning system in the hall, together with hard physical work — promoted the aerosol transmission of SARS-CoV-2 particles over greater distances,” said Professor Adam Grundhoff, co-author of the study and research group leader at the Heinrich Pette Institute. “Under these conditions, a distance of 1.5 to 3 meters alone is obviously not sufficient to prevent transmission.”

Grundhoff said it is very likely these factors in general have played a significant role in coronavirus outbreaks at meat processing plants around the world.

Recommendations for Meat Processing Plants

Coronavirus outbreaks have infected hundreds of workers at meat processing facilities in Iowa, resulting in multiple deaths. More than 1,000 workers at a Tyson Foods pork processing plant in Waterloo tested positive for COVID-19, according to Black Hawk County health officials. More than 500 workers were also infected at a Tyson Foods plant in Columbus Junction, the Associated Press reported. Three Waterloo Tyson employees and two Columbus Junction employees died from COVID-19, according to the AP.

As outbreaks grew in Tyson’s plants, the company implemented measures to limit the spread of the virus. Tyson installed workstation barriers between workers, supplied face shields, started conducting health screenings and required social distancing when possible.

The CDC recommends social distancing of at least 6 feet and the use of face coverings in crowded spaces, but this may not be enough to prevent aerosol transmission of the coronavirus inside meat processing plants, according to the study of the Tönnies outbreak.

“Our findings indicate that a physical distance of 2 meters (6.5 feet) does not suffice to prevent transmission in environmental conditions such as those studied here; additional measures such as improved ventilation and airflow, installation of filtering devices or use of high-quality face masks are required to reduce the infection risk in these environments,” researchers recommended.

In addition, they said frequent and systematic screening of workers is essential to prevent future outbreaks. If another outbreak does occur, researchers say immediate action must be taken to quarantine all workers in a radius that may significantly extend beyond 2 meters around the infected person because transmission can occur over longer distances. However, researchers say exact distances of transmission via aerosols are likely to vary substantially depending on a facility’s layout and operation conditions.

Last week, Tyson Foods announced it will launch a new nationwide COVID-19 monitoring strategy that involves increased testing of workers without coronavirus symptoms, as well as those who show certain symptoms or have been in close contact with someone who has the virus.

“What we’re adopting is a strategic, ongoing approach to combating COVID-19,” said Donnie King, Tyson Foods group president and chief administrative officer. “It involves weekly testing of team members at our facilities to monitor for the presence of the virus. By using data science to test a statistically sound sample of team members, we have a better chance of staying ahead of any potential virus spread and protecting our teams and communities.”

Reuters reported in July that Tyson is also researching airflow and testing ultraviolet air treatment systems in its plants after concerns arose over airborne spread of the virus.

The Aerosol Debate

The subject of aerosol transmission of the coronavirus is still up for debate. Throughout most of the pandemic, the World Health Organization said aerosol transmission was unlikely to be a primary mode of transmitting the virus. However, the agency recently expanded its guidance to include circumstances where aerosol transmission could be possible.

In its updated guidance, the World Health Organization wrote, “There have been reported outbreaks of COVID-19 in some closed settings, such as restaurants, nightclubs, places of worship or places of work where people may be shouting, talking, or singing. In these outbreaks, aerosol transmission, particularly in these indoor locations where there are crowded and inadequately ventilated spaces where infected persons spend long periods of time with others, cannot be ruled out. More studies are urgently needed to investigate such instances and assess their significance for transmission of COVID-19.”

The agency’s updated guidance was released after a paper published in the medical journal Clinical Infectious Diseases appealed to the medical community and the World Health Organization to recognize the potential for airborne transmission of the coronavirus. The paper was co-signed by 239 other scientists.

The World Health Organization has maintained it monitors emerging evidence regarding modes of coronavirus transmission and will update its guidance whenever new information becomes available.

Ramifications for Other Industries

The researchers of the Tönnies outbreak say their findings have “imminent” implications for the meat and fish processing industries and may reach well beyond these industries. They say their findings point to the importance of air quality and airflow in confined spaces to prevent future superspreading events of the virus.

“Our study sheds light on SARS-CoV-2 infections in a workspace where various factors come together that allow transmission over relatively long distances,” said Melanie Brinkmann, professor at the Technische Universitat Braunschweig and research group leader at the Helmholtz Centre for Infection Research. “The important question is now, under which conditions transmission events over longer distances may occur in other areas of life.”