Household contacts of people who have tested positive for coronavirus at Kepak in Merthyr over the weekend have been told to self-isolate.
No outbreak has been declared and no further action is being taken, such as closing schools or workplaces.
Testing at the meat production facility identified 101 confirmed cases out of the 810 people tested on Saturday.
This brings the total number of cases identified at the site since April to 130.
Heather Lewis, consultant in health protection for Public Health Wales, said clusters were expected in settings such as workplaces during the recovery phase of the pandemic.
"Any contacts have been instructed to self-isolate to prevent spread to the wider community," she said.
"We are confident that all appropriate action is being taken to control this cluster.
"A Health and Safety Executive (HSE) visit has been carried out at the site and HSE officers were satisfied that Kepak Merthyr are taking all reasonably practicable measures to prevent the spread of coronavirus within the workforce."
She reiterated the importance of observing social distancing guidelines, washing hands regularly and working from home where possible.
"When travelling you should also avoid car sharing with people outside your household, and ensure social distancing on entry and exit from your workplace," she said.
"If you or a member of your household develop symptoms of cough, fever or change in sense of taste or smell, you must book a test for Covid-19 promptly to help control the spread of infection."
The multi-agency team investigating the incident at Kepak includes Merthyr Tydfil County Borough Council, Cwm Taf Morgannwg University Health Board, the Health and Safety Executive, the Food Standards Agency and Welsh government.