The agency tested blood from health workers who had been exposed to a person with H5N1 bird flu and later developed flu-like symptoms, raising concerns of human transmission.
Scientists from the federal Centers for Disease Control and Prevention say there's no evidence that human-to-human transmission of bird flu occurred among Missourians earlier this year.
The agency tested blood from health workers who had been exposed to a person with the virus and later developed flu-like symptoms.
"To date, human-to-human spread of H5 bird flu has not been identified in the United States," health officials wrote this week in a bulletin outlining the blood test findings. "CDC believes the immediate risk to the general public from H5N1 bird flu remains low, but people with exposure to infected animals are at higher risk of infection."
A Missouri patient who tested positive for the H5N1 caught the attention of federal health experts two months ago. The patient, who had been hospitalized for an unrelated sickness, tested positive for bird flu during routine respiratory tests.
Unlike other people who had tested positive for the virus this year, the Missouri patient had no recorded contact with animals. As of this October, the other 30 people in the U.S. who have tested positive had contact with livestock or poultry. The lack of animal contact raised the possibility the virus could be transmitted from one human to another.
Health officials sent blood from the patient as well as a handful of health workers and a family member who had been exposed to the original case to CDC's headquarters in Atlanta to test the blood for bird flu antibodies. If they found any, it could indicate human transmission.
The blood tests showed the health workers had not caught the bird flu.
"This finding rules out person-to-person spread between the MO case patient and any of the health care workers tested," officials wrote.
Additionally, the scientists found evidence that the initial patient without animal contact and their family member may have caught H5N1 from a common source.
"These similar immunologic results coupled with the epidemiologic data that these two individuals had identical symptom onset dates support a single common exposure to bird flu rather than person-to-person spread within the household," officials wrote in the update.