A possible cluster of bird-flu infections in Missouri has grown to include eight people, in what may be the first examples of person-to-person transmission in the United States, the Centers for Disease Control and Prevention reported on Friday.

If confirmed, the cases in Missouri could indicate that the virus may have acquired the ability to infect people more easily. Worldwide, clusters of bird flu among people are extremely rare. Most cases have resulted from close contact with infected birds.

Health officials in Missouri initially identified a patient with bird flu who was hospitalized last month with unusual symptoms. The patient may have infected one household member and six health care workers, all of whom developed symptoms, according to the C.D.C.

Investigators have not yet confirmed whether any of those seven individuals were infected with the virus, called H5N1, leaving open the possibility that they had Covid or some other illness with flulike symptoms.

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Still, the news alarmed experts.

“We should be very concerned at this point,” said Dr. James Lawler, co-director of the University of Nebraska’s Global Center for Health Security.

“Nobody should be hitting the panic button yet, but we should really be devoting a lot of resources into figuring out what’s going on.”

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