COVID-19 transmission between the community and meat processing plants in Ireland: a retrospective modelling study

Outbreaks of COVID-19 in meat processing plants (MPPs) were recorded globally throughout the pandemic. There was speculation that these outbreaks resulted in dissemination of COVID-19 throughout the surrounding county leading to high incidence rates. We aimed to investigate the dynamics of spread between MPPs and their surrounding counties. In this retrospective longitudinal study, data was collected on the number and size of outbreaks in 33 MPPs and county infections in Ireland between March 2020 and May 2021. This data was used to investigate the relationship between outbreaks in MPP and county infection rates through statistical analysis, and the development of a novel SEIR model. We found an association between the number of MPPs present in a county and county incidence rates, however, incidence rates in the counties did not increase as a consequence of an outbreak in a MPP. The model results indicate that county incidence rates in the weeks prior to a MPP outbreak could reliably predict the size of that outbreak in a plant, r(49)=0·62, p < 0·0001, RMSD = 5·6. In Ireland, outbreaks in MPP were strongly correlated with high levels of infection in the surrounding county, rather than being a driver of infection in the county. The modified SEIR model described here can provide an explanation of the generative process required to cause outbreaks of the size and scale that occur in MPP.