'Very eerie': This winter had virtually no flu

Ånestad's hypothesis, largely ignored when he first published it 30 years ago, is still on the fringe. And it doesn't help him predict what swine flu will do next.

No one else can predict with any certainty either.

For example, in each of the three pandemics of the 20th century, most recently in 1968-69, the new virus outcompeted and then replaced the single dominant strain of seasonal flu.

For the last three decades, however, two type A strains have jockeyed for dominance. The H3N2 virus usually came out on top, and it caused the most widespread as well as the most severe illness.

The seasonal H1N1 strain, when it won, was usually mild, particularly for older people.

Most scientists believe that the pandemic H1N1 will replace the seasonal H1N1 because they are most similar. If that happens, then the two flus in future seasons theoretically could, on average, be worse than the experience of recent decades.

Is it possible for the pandemic virus to replace both seasonal strains?

"It is," said Viboud, the NIH scientist.

Would that be a good thing?

No one knows, she said. "We don't have any situation in the past that we can use as a template for what will happen."

Contact staff writer Don Sapatkin at 215-854-2617 or dsapatkin@phillynews.com.