Health authorities in the US are studying seven people who developed influenza symptoms after being exposed to a Missouri bird flu patient, raising the possibility of the first human-to-human transmission of the infection.

None have tested positive for avian influenza and work is underway to see if they have antibodies to the virus that’s been spreading among birds and dairy cows in the US, the Centers for Disease Control and Prevention said Friday in a statement.

Six were health care workers who encountered the first American known to be infected with bird flu without previous exposure to a sick animal, and the other was a close household contact of the patient. All developed mild respiratory symptoms after their exposure.

One tested negative for bird flu and the others weren’t tested for active infections since they weren’t identified quickly enough to definitively diagnose it or rule it out, the CDC said. More than 100 health care workers were exposed to the Missouri patient to varying degrees while the person was hospitalized, the agency said.

“CDC continues to closely monitor available data from influenza surveillance systems, particularly in affected states, and there has been no sign of unusual influenza activity in people, including in Missouri,” the agency said.

There have been 14 cases of avian influenza reported in the US in 2024, including the Missouri patient disclosed in early September. The source of that infection has yet to be identified.

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