The Centers for Disease Control and Prevention recently reported genetic sequences of H5N1 bird flu viruses collected from a severely ill patient in Louisiana. These sequences revealed mutations believed to enhance the virus’s capability to bind to cells in the human upper respiratory tract. This development raises concerns as bird flu viruses typically attach to cell receptors rare in human upper airways, which is why H5N1 infections in humans remain infrequent and do not spread easily among people.

An additional mutation was observed in a virus sample from a critically ill teenager in British Columbia, suggesting that these changes could help the virus adapt to humans. Although Scott Hensley, a microbiology professor from the University of Pennsylvania, cautioned against over-interpreting data from only two severe cases, the report made him raise an eyebrow. “It’s not great news,” he told STAT.

Angela Rasmussen, a virologist specializing in emerging infectious diseases, noted that the absence of mutations in birds is somewhat relieving. However, she warned that the uptick in human cases paints a grim picture, stressing the importance of reducing human infections to avoid creating opportunities for a pandemic virus to emerge.

The CDC’s study found that mutations in the Louisiana patient developed during the infection rather than being transmitted from birds. Similarly, British Columbia health officials couldn’t trace the source of the teen’s infection.

Despite over 65 human H5N1 cases in the U.S. this year—60% linked to infected dairy cows—the virus has not shown an ability for human-to-human transmission. Both severe North American cases involved genotype D1.1 viruses, and no secondary cases have been reported, offering a glimmer of hope amidst the concern. Hensley’s laboratory is currently investigating if these mutations indeed facilitate better binding to human cells.