JBS Canada spokesman Rob Meijer said the company had implemented more than 100 safety measures at the plant since January, including temperature tests of all workers entering the plant, providing and requiring face masks, and physical partitions on production lines.
"We have been working closely with public health and labour officials each and every day to implement rigorous risk mitigation practices throughout our facility," Meijer said Wednesday in a release.
"It is important to note that the move back to two shifts will not increase the number of employees in the facility at any one time. We will continue to make all decisions based on the best available data and advice from both our team members and public health officials."
Alberta Health said Wednesday that 650 workers at the JBS plant have tested positive for COVID-19 and one has died. Eleven cases remain active and 638 employees have recovered.
The two plants together process about 70% of Canada's beef.
The Alberta government delayed the first phase of its economic reopening plan in both Brooks and Calgary because of the continued threat of infection in the two cities.
The United Food and Commercial Workers union represents workers at JBS and Cargill. It said its members are afraid to go back to work despite changes to safety procedures, but need the money.
"We're still extremely nervous and extremely cautious about any kind of suggestion that the plants are safe," said Michael Hughes from UFCW Local 401.
"In Brooks, 500 people were afraid for their lives because of this terrible virus and we have members at JBS who were sleeping in their cars because they were going to work and didn't want to bring the virus home to their families."