There’s no evidence that a policy allowing for mass expulsions prevents the spread of COVID-19, they argue. And it may, in fact, have the opposite effect: by rounding up and detaining hundreds of thousands of migrants in large groups, Customs and Border Patrol (CBP), which does not offer COVID-19 testing for migrants, may actually be stoking the transmission of the disease. Migrants often spend days and weeks in crowded facilities before they’re transported and expelled…

There has been no scientific research conducted on the question of whether Title 42 has affected the spread of COVID-19, according to both Zard and Katherine McCann, a senior program officer at the Program on Forced Migration and Health. A CDC spokesperson did not answer TIME’s questions on whether the agency was studying the impact internally.

A DHS spokesperson says the agency refers anyone in their custody who is exhibiting symptoms to local health systems for appropriate testing, diagnosis and treatment. CBP does not offer COVID-19 testing before expelling migrants unless they show signs of illness.

Lawyers and humanitarian aid workers at organizations like Al Otro Lado, Las Americas Immigrant Advocacy Center and Global Response Management argue that Title 42 has exacerbated the spread COVID-19 among migrant populations on the Mexican side of the border. The problem, they tell TIME, is particularly acute when CBP has detained large groups of migrants in one part of the border for days or weeks, then transported them to another part of the border to expel them.