Florida’s firebrand surgeon general is calling for use of mRNA COVID-19 vaccines to be stopped.

Joseph Ladapo, M.D., wrote a letter to Robert Califf, M.D., commissioner of the Food and Drug Administration (FDA), and Mandy Cohen, M.D., MPH, director of the Centers for Disease Control and Prevention (CDC), last month. He cited a preprint study from October, which was not peer reviewed or published in a medical journal, claiming there are “nucleic acid contaminants” in both the Pfizer and Moderna mRNA vaccines.

Dr. Ladapo claims, without sufficient backing, that these contaminants can lead to the development of cancer, and damage a person’s blood, heart, lungs, liver, kidney and other organs.

The FDA replied to the surgeon general that there was no evidence the shots posed these types of danger, according to a statement from the Florida Department of Health on Wednesday.