Both the 1918 and 1957 pandemics began outside the normal flu season. Yet the pandemic strain effectively crowded out the seasonal viruses, even though there was not much pandemic flu present at the start of the normal flu season.
Hence the uncertainty.
"Because we don't know the mechanism by which this happened in the past, we can't be sure it will happen again," Lipsitch said. "But if I had to guess, I would say that other influenza A strains -- seasonal H1N1 and H3N2 -- will be crowded out this year."
Seasonal influenza B, however, will likely persist, he said.
Dr. Howard Markel, infectious disease expert and medical historian at the University of Michigan at Ann Arbor, said he thought there was a possibility pandemic H1N1 would crowd out seasonal influenza.
"This may be the case given that H1N1 is so hearty in its spread," he said. "But influenza, be it seasonal or novel, is a stealthy and cunning foe, so I would not count out either yet.
"The message I am giving to all my patients is still: make sure you are vaccinated against both H1N1 and seasonal influenza."
Dr. Gregory Poland, director of the Mayo Vaccine Research Group at the Mayo Clinic in Rochester, Minn., said "the most likely scenario, based on history, is little seasonal activity but a third 'wave' of H1N1 this late winter/spring."
But he added that scientists still don't know whether the mass vaccination program against H1N1 has been effective and whether seasonal influenza activity will remain low.
"Time will tell," he said. "Lack of transmission this week or even this coming month doesn't predict what may happen the following month."
"If a late outbreak occurs, and people haven't been immunized, it will be too late to protect them," he warned.
Dr. John Treanor, a professor of medicine and immunology at the University of Rochester in Rocherster, N.Y., said the questions of why flu pandemics stop and how one strain replaces another are among the most interesting in influenza epidemiology.
"The practical application of those two questions is trying to guess what's going to happen next," he said, "and the story illustrates that no one really knows, as experts in the field have widely differing opinions."
Treanor came down on the side of little seasonal influenza activity for the rest of the season.
This article was developed in collaboration with ABC News.