Golden State Foods Corp. in Industry, S&S Foods in Azusa and a Mission Foods facility in Commerce are all now closed.
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County health officials say they each had more than 40 cases and failed to notify the county as required.
The shutdowns come days after Los Angeles Apparel, the site of the county's worst COVID-19 outbreak, was allowed to reopen. The business' facility in South L.A. saw a COVID-19 outbreak that led to more than 300 workers contracting the virus, resulting in the death of four people.