Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy, was less sanguine, saying that while previous experience with H5N1 viruses suggests that human infections didn’t lead to sustained person-to-person spread, heightened surveillance is important.

“Any time you’re dealing with H5N1, you sleep with one eye open,” he told STAT.

H5 viruses have long struck fear among scientists who study bird flu because of the economic damage they can wreak and their propensity to occasionally infect people. They are one of two subtypes — H7 viruses are the other — that can become highly pathogenic. That term refers to their deadliness when they infect birds. In the mid-aughts, H5N1 viruses decimated poultry flocks first in Southeast Asia and later into South Asia and North Africa.