One in six dairy products in U.S. retail stores contained signs of inactive bird flu virus this summer, regulators said, slightly lower than the numbers seen in a different survey when the pathogen was first found in the nation's dairy herds.

None of the 167 samples, which included milk, , hard cheese, butter, cream cheese and aged raw milk cheese, contained viable H5N1 , the U.S. Food and Drug Administration said.

The results show pasteurized dairy remains safe to consume. It was the second FDA survey, after the first in April found inactive viral particles in one fifth of sampled . The latest effort tested a wider array of foods from more states, leading the FDA to caution that the results may not be directly comparable.

"Obviously, there's a great deal of concern on the potential and real impacts to human health," said Eric Deeble, deputy under secretary for Marketing and Regulatory Programs at the Department of Agriculture. "I do feel like the response is adequate." None of the raw milk products that were tested contained any H5N1 genetic material.

The FDA said that indicated the samples came from cows that weren't infected, and doesn't mean raw milk is safe for consumption. Cows that test positive are removed from the commercial food supply. A test of hamburgers that were intentionally inoculated with the virus found that cooking made them safe for , said Emilio Esteban, the USDA's under secretary for food safety.

Thirteen people have tested positive for the virus this y.