Possible Cluster of Human Bird-Flu Infections Expands in Missouri

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 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 CDC.

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.

There have been 13 other confirmed cases of bird flu in people since the outbreak in dairy cattle was announced in March. Those cases were all acquired from interactions with infected dairy cattle or poultry.

Missouri has not reported any infected herds. The hospitalized patient was tested as part of routine flu surveillance. The infection worried experts, because the individual had no known contact with cattle or birds.

There have been no H5N1-related deaths reported so far in Americans, and most cases have been relatively mild, suggesting that the virus does not have the high mortality rate of up to 50% seen in Asia.

But the small number of confirmed cases so far makes it difficult to estimate its virulence, said Caitlin Rivers, a public health researcher at the Johns Hopkins Center for Health Security.

No one in the possible cluster has been identified, nor has the hospital at which the initial patient was treated. The investigation is led by Missouri health officials with remote assistance from the CDC.

This article originally appeared in The New York Times .