COVID-19 clusters emerged early in the pandemic in slaughterhouses in western France, posing a risk of community transmission. This study assesses the effect of the presence of slaughterhouses on the epidemiological dynamics of COVID-19 in Brittany and the Pays de la Loire region between July 8, 2020, and December 7, 2021. Three health indicators were examined: incidence, positivity rates, and testing rates. An ecological study was conducted using IRIS as the spatial unit of analysis. Tests and cases from the Population Screening Information System (Sidep) were aggregated by IRIS (Statistical Information Cluster), week, sex, and age group. Two exposure indicators were constructed, distinguishing between poultry slaughterhouses and meat slaughterhouses. The numbers of cases and tests per week and per IRIS were regressed against the slaughterhouse exposure indicators while controlling for the following adjustment factors: household size, municipal urbanization level, and four variables measuring social disadvantage. A random effect was assigned to the department. An interaction between the temporal trend and the exposure indicators for poultry and meat slaughterhouses was accounted for using a tensor product. COVID-19 vaccination coverage was used for sensitivity analyses. For each indicator studied—incidence, positivity rate, or testing rate—results were reported as rate ratios (RR) for an increase from the 1st to the 3rd quartile of each exposure indicator. The study area covered 3,682 IRIS (7,248,566 residents): 1,894 (51.4%) were considered exposed to meat processing plants and 1,518 (41.2%) to poultry slaughterhouses. Positive associations suggest that exposure to beef slaughterhouses contributed to an amplification of the epidemic during the summer of 2020, when viral circulation was low. This effect diminished in October 2020 with the increase in viral circulation. Over the first 9 weeks studied, the associations were strong regardless of the indicator examined: the increase in the positivity rate associated with increased exposure ranged from 57.2% (week 1, RR of 1.572 [1.424–1.735]) to 18.8% (week 9, RR of 1.188 [1.136–1.242]). Weaker associations were observed during the third wave of the epidemic amid high viral circulation. In week 41, the RR was 1.047 [1.036–1.058] for testing and 1.090 [1.061–1.120] for incidence. For poultry slaughterhouses, the effect of exposure was estimated to be constant over time with RR values close to 1: 1.016 [1.004–1.029] for incidence, 1.017 [1.005–1.028] for positivity, and 1.008 [1.004–1.013] for screening. The hypotheses explaining the differences in impact between the two sectors remain to be validated. These results underscore the need to prepare for future respiratory outbreaks by combining targeted actions aimed at slaughterhouse workers—particularly those in the butchery sector—with prevention measures extended to populations living near these sites, in order to limit community spread.