New York’s plan for confronting bird flu if it begins spreading among people relies heavily on a vast stockpile of flu treatment medicine. But experts worry the stockpile is missing a key tool.

New York has only one type of flu antiviral drug, Tamiflu, which might prove less effective than hoped against a pandemic strain of bird flu, some experts say. There is another antiviral that might work, even if Tamiflu doesn’t — a drug called baloxavir marboxil.

And yet the state has not stored a single dose of it in case of emergency.

“There could be an Achilles’ heel here,” said Dr. Sean T. Liu, an infectious disease expert at Mount Sinai Hospital in Manhattan.

Given the uncertainty about how bird flu might evolve and what treatments might ultimately work best, some public health experts worry that New York might have too much riding on a single drug, when an alternative is available.

“It is always wise, when there are unknown variables, to have more than one therapeutic option in your toolbox,” said Dr. Howard Zucker, the former state health commissioner, who led the Health Department during the coronavirus pandemic and oversaw the state’s epidemic stockpile until 2021.

The next pandemic, some experts worry, could land soon in the form of H5N1, a flu virus that has long circulated in wild birds, occasionally causing outbreaks at poultry farms. The virus can infect people who come into contact with diseased birds and has killed about half of the 1,000 or so people known to have contracted it.