The sample was sequenced by the National Influenza Center in Chile and uploaded to GISAID, an international database of viral genomes, overnight, C.D.C. officials said.
Chile’s Ministry of Health reported the case to the World Health Organization on March 29. The patient, a 53-year-old man, developed respiratory symptoms, including a cough and a sore throat, and was hospitalized when his condition deteriorated, according to the W.H.O.
Investigation into the case is continuing, and how the man became infected remains unclear. But the virus had recently been detected in birds and sea lions in the region where the man lives.
“According to the preliminary findings of the local epidemiological investigation, the most plausible hypothesis about transmission is that it occurred through environmental exposure to areas where either sick or dead birds or sea mammals were found close to the residence of the case,” the W.H.O. reported last week.
It is the 11th reported human case of H5N1 since January 2022, according to the C.D.C., none of which have been associated with human-to-human transmission. Since H5N1 was first detected in birds in 1996, there have been hundreds of human infections globally, mostly in people who were in close contact with birds.
Still, experts have long been worried about the possibility that avian influenza, which is well adapted to birds, might evolve to spread more easily among humans, potentially setting off another pandemic. An H5N1 outbreak on a Spanish mink farm last fall suggests that the virus is capable of adapting to spread more efficiently among at least some mammals. And every human infection gives the virus more opportunities to adapt.
The mutations documented in the Chilean patient are a “step in the wrong direction,” Dr. Lowen said.
This version of the virus has spread rapidly through wild birds in the Americas, sparking regular outbreaks in farmed poultry. The virus has become so widespread in birds that it has repeatedly spilled over into mammals, and “continued sporadic human infections are anticipated,” the C.D.C. wrote in a recent technical report.
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