The positivity rate is 55% in emergency shelters and food distribution centers where MSF conducted tests last June and early July, compared to just 12% recently in Paris.
MSF said it is the first study in Europe to focus on the poorest citizens in a region, adding immigrants, representing 90% of the more than 800 people examined.
“The effects show an incredibly high prevalence. The main explanation for why the situation is in the shelters and being packed together, leading to infection groups,” Corinne Tower of MSF told AFP.
Authorities have placed many migrants in gymnasiums and other transit shelters to take them out of makeshift camps and the streets during the two-month national closure from mid-March to mid-May.
And in two centres that house migrant workers, the test positivity rate has reached 89%, according to MSF.
Fitness firm Sante Publique France said the overall positivity rate for France hovered between five and 10% last week.