The epidemiology of COVID-19 outbreaks in farms in Ontario, Canada, in the pre-vaccine year of the pandemic

By Bhavana KunkalikarReviewed by Danielle Ellis, B.Sc.Jun 7 2023

In a recent study published in the Canada Communicable Disease Report, researchers discussed the farm outbreaks of coronavirus disease 2019 (COVID-19) in Ontario from January to December 2020.

Background

Amid the COVID-19 outbreak, agricultural workers were considered an essential workforce and allowed to operate in person as they played a vital part in cultivating and gathering food. Farm work involves working closely without physical barriers.

About 32% of farms in Ontario are indoor greenhouses. Greenhouse farms provide different conditions than outdoor fields, with varying humidity, ventilation, and temperature. These conditions can create a favorable environment for viral transmission.

In September 2020, the Ontario Ministry of Health created the "COVID-19 Guidance: On-Farm Outbreak Management" to address the specific challenges farm workers face during the pandemic. The guidelines offer suggestions for safe practices in transportation, worksites, and shared accommodations.

About the study

In the present study, researchers examined the occurrence of farm outbreaks in Ontario between January and December 2020 and investigated the patterns of outbreaks based on the farm type and season.

Information related to COVID-19 outbreaks on farms as well as laboratory-confirmed cases associated with those outbreaks, were collected from Ontario's Public Health Case and Contact Management Solution (CCM), a reporting system for COVID-19 cases and contact management. Data related to laboratory-confirmed COVID-19 cases detected within the general Ontario population was also acquired. On February 9, 2021, Public Health Ontario extracted data entered by staff at 34 local public health units (PHU).

In September 2020, the Ontario Ministry of Health provided guidance that outlines what constitutes a COVID-19 on-farm outbreak. An on-farm outbreak of COVID-19 occurs when there is at least one COVID-19 case in a congregate living region or two cases in the workplace, whether symptomatic or asymptomatic, along with evidence of COVID-19 spread in either of these areas. The study involved outbreaks that occurred between January 1, 2020, and December 31, 2020.

The study considered COVID-19 cases confirmed in laboratories and associated with farm outbreaks between January 1 2020 and January 31 2021, to account for outbreaks still ongoing after December 31, 2020. Outbreaks were deemed closed either if they bore a "declared over date" in CCM or if five months had passed since the outbreak began.

Ontario's COVID-19 farm outbreaks were described using descriptive statistics. The study calculated proportions for various classes of outbreak-associated cases, including age, gender, medical risk factors, outcomes, clinical presentation, and PHU. Outbreaks and outbreak-related cases were also analyzed by classifying them by season and calculating the average, median, and range of the duration, number, and size of outbreaks for each season.

The study also calculated the percentage of farm outbreaks as well as outbreak-associated cases among greenhouses in each season. The outbreaks among the top three PHUs were displayed using an epidemiologic curve, showing the number of outbreak-associated infections over the given period.

Results

The team noted that 64 farm outbreaks resulted in 2,202 outbreak-related cases. The outbreaks varied in size, ranging between one and 240 cases. A total of 63 outbreaks had at least two cases, while six outbreaks had 100 or more cases. The duration of the outbreaks varied between zero and 128 days. Furthermore, around 37 farm outbreaks were reported on greenhouses.

Almost 69% of all farm outbreaks in Ontario took place in three PHUs, namely Haldimand-Norfolk Health Unit, Windsor-Essex County Health Unit, and Chatham-Kent Public Health. The farm outbreaks reached their highest point in May 2020 and December 2020. Between late June and early September 2020, there were few instances of outbreaks on farms.

COVID-19 farm outbreaks were analyzed by season, revealing that the spring and fall had the highest total number of outbreaks. However, the spring had the highest number of outbreak-related cases, followed by the fall and summer. Outbreaks that began in the spring had the longest duration, followed by those in the summer and fall. Farm outbreaks were more common in greenhouse farms during the spring and summer seasons, as opposed to the fall season. Outbreaks were primarily concentrated in Windsor-Essex, an area with a high concentration of agricultural farms, irrespective of the time of year.

Most cases linked to the outbreak were males with a median age of 35. Almost 221 cases had comorbidities, and 121 met high-risk criteria. Most cases showed symptoms, with 688 cases being asymptomatic and 139 cases lacking symptom information. Sixteen individuals were hospitalized due to outbreak-associated cases, eight requiring admission to the ICU, and three deaths.

Conclusion

The risk associated with large and long farm outbreaks has remarkably reduced due to COVID-19 vaccines and workplace infection prevention and control measures during the pandemic. Despite easing public health measures such as indoor masking and the resumption of international travel, farms remain susceptible to COVID-19 outbreaks due to the potential emergence of novel and highly contagious variants of concern. Further research is required to comprehend the involvement of greenhouse work and other potential factors in COVID-19 outbreaks on farms.