Washington ? Government projections that as many as 1.9 million Americans could die in a global flu epidemic amount to a guess that could prove to be highly inaccurate, several public health experts say.

“The problem with all the numbers is that nobody knows,” said Dr. Arnold S. Monto, a University of Michigan epidemiologist who specializes in infectious diseases.

While agreeing that preparation for a pandemic must be an urgent national priority, Monto and other experts say the death toll cannot be reliably estimated at this point, because no one knows how lethal a mutation of the virus might turn out to be.

“Usually, as a virus adapts to human-to-human transmission, it becomes less virulent,” said Ira M. Longini Jr., who analyzes flu and other types of epidemics as a biostatistics professor at Emory University. “A virus that kills the host cannot transmit itself as well. From the virus’ point of view, it wants the host to live.”