U.S. health officials on Friday said a new study in Michigan suggested the bird flu virus is not causing asymptomatic infections in people, while also announcing two new human cases in Colorado.
Last month, the Michigan Department of Health and Human Services launched a study of workers who were around cows sickened by the bird flu. The researchers drew blood from 35 people.
One goal was to determine if there were people who never had any symptoms but did have evidence of past infections. None of the blood testing showed antibodies that would indicate infections with the highly pathogenic avian influenza (HPAI) A(H5N1) virus circulating among dairy cattle and poultry, the CDC on Friday.
"The lack of antibodies to avian influenza A(H5N1) virus suggests these people were not previously infected with an avian influenza A(H5N1) influenza virus," the agency said. "These data are consistent with demonstrating the seroprevalence to HPAI A(H5N1), even among workers with known exposures, is low."
Many of the workers did show antibodies to seasonal flu, which was used as a control virus in the study. Researchers will continue to sample workers, and the data will be analyzed and prepared for a peer-reviewed publication, the CDC said.
Meanwhile, two more infections tied to a Colorado poultry farm were reported, bringing the total to six. It's the largest outbreak of human bird flu infections in U.S. history, and accounts for most of the 11 cases reported to date. Ten of those cases occurred this year, all among farmworkers and all with mild symptoms.
A bird flu virus has been spreading since 2020 among -- including dogs, cats, skunks, bears, and even seals and porpoises -- in scores of countries. Earlier this year the virus, known as H5N1, was detected in U.S. livestock, and is now circulating in cattle in several states.
Health officials continue to characterize the threat to the general public as low, and the virus has not spread between people. But officials are keeping careful watch, because earlier versions of the same virus have been deadly.