The Food and Drug Administration said the U.S. milk supply is safe, despite this week’s finding of bird flu fragments in 20% of commercial milk samples.

The majority of milk samples that tested positive for the strain of avian flu known as H5N1 were in areas with infected dairy herds.

“To date, the retail milk studies have shown no results that would change our assessment that the commercial milk supply is safe,” the FDA said Thursday.

The FDA and U.S. Department of Agriculture said pasteurization and the diversion or destruction of milk from sick cows has kept the U.S. milk supply safe.

On Tuesday, the USDA said it had found the H5N1 virus in livestock in Idaho, Kansas, Michigan, New Mexico and Texas.

Highly Pathogenic Avian Influenza, also known as bird or avian flu, can be transmitted by wild birds to domestic poultry and other bird and animal species, the FDA said. They do not normally infect humans, though sporadic infections in people have occurred.

The FDA is currently conducting egg inoculation tests to determine if infectious virus is present in milk. Early research from the National Institutes of Health indicates there is no infectious virus in milk sold commercially.

“Positive results do not necessarily represent actual virus that may be a risk to consumers,” the FDA said in a statement on its website. “Additional testing is required to determine whether intact pathogen is still present and if it remains infectious, which would help inform a determination of whether there is any risk of illness associated with consuming the product.”

The Centers for Disease Control has not found any cases of H5N1 beyond the one known case related to direct contact with infected cattle.