Cases typically involve people exposed to poultry. In the United States, the C.D.C., in partnership with state and local public health departments, is monitoring people who are exposed to H5N1. As of last week, 6,315 people had been monitored; 163 reported symptoms; and one tested positive, according to Dr. Tim Uyeki, the chief medical officer of the C.D.C.’s influenza division.

At the same time, officials at the federal Agriculture Department, which is responsible for the health of farm animals, say they have begun testing potential poultry vaccines and initiated discussions with industry leaders about a large-scale bird flu vaccination program for poultry, which would be a first for the United States.

Farm birds are already vaccinated against infectious poultry diseases, such as fowlpox. But an avian influenza vaccination program would be a complex undertaking, and poultry trade associations are divided over the idea, in part because it might spawn trade restrictions that could destroy the $6 billion poultry export industry. Dr. Carol Cardona, an expert on avian health at the University of Minnesota, said that the fear of trade bans was a huge barrier to the mass vaccination of poultry.

“This is the undeclared war — trade,” Dr. Cardona said.

Avian influenza experts, however, say they believe the Biden administration should move ahead with a vaccination campaign, in part to reduce the risk of a human pandemic. In interviews, several called for the administration to act quickly.

“My own opinion is under the present circumstances, we should be vaccinating the poultry population of the United States against H5N1 — absolutely,” said Robert G. Webster, an expert in avian influenza at St. Jude Children’s Research Hospital in Memphis. Such a campaign could “prevent the inevitable transmission to humans,” he said.