The CDC notes that the poultry worker from Ohio had been hospitalized with respiratory and nonrespiratory symptoms.
The mutation doesn't appear to reduce viral fitness, as evidenced by its rapid spread, the authors say.
A 2024 outbreak of H5N1 on 3 unrelated poultry farms in the Czech Republic suggests airborne transmission, which is difficult to prove.
The farm workers from different households experienced symptoms, but declined testing, before their cats became seriously ill.
Also, Canada's vaccine advisers issue preliminary guidance on use of the vaccine in high-risk groups.
Six countries have reported H5N5 in birds and mammals since the first of the year.
The women, ages 72 and 56, were both from Guangdong province in southern China.
The infected woman, who is hospitalized, was likely exposed to H5N1 through contact with her infected backyard poultry flock.
In other developments, Zoetis announced it has received USDA's conditional approval for its updated poultry vaccine.
As the virus struck more layer and other commercial poultry farms, state wildlife officials continue to warn about wild bird detections.
