“We’re talking about the possibility of a double flu pandemic,” where a second wave starts before the first is over, said Dr. Eric Toner, a senior scholar at the Johns Hopkins Center for Health Security.

Public health experts are also closely watching reserves of vital medical supplies and medications, many of which are made in China. Some hospitals in the United States are already “critically low” on respirator masks, according to Premier Inc., which secures medical supplies and equipment on behalf of hospitals and health systems. And China is the dominant supplier of the raw ingredients needed for penicillin, ibuprofen and even aspirin — drugs taken daily by millions of Americans and dispensed routinely to hospital patients.

“All the hospitals are taxed with a large flu season and other bugs,” said Dr. Mark Jarrett, the chief quality officer for Northwell Health, which operates 23 hospitals across Long Island and elsewhere in New York. About 400 patients are coming to its emergency rooms each day with flulike symptoms.

“Everybody is at maximum capacity,” Dr. Jarrett said.