In the waning days of April, as federal officials were declaring a public health emergency and the world seemed gripped by swine flu panic, two rival supercomputer teams made projections about the epidemic that were surprisingly similar  and surprisingly reassuring. By the end of May, they said, there would be only 2,000 to 2,500 cases in the United States.

May’s over. They were a bit off.

On May 15, the Centers for Disease Control and Prevention estimated that there were “upwards of 100,000” cases in the country, even though only 7,415 had been confirmed at that point.

The agency declines to update that estimate just yet. But Tim Germann, a computational scientist who worked on a 2006 flu forecast model at Los Alamos National Laboratory, said he imagined there were now “a few hundred thousand” cases. (At their peaks, epidemics are thought to double in as little as three days, which could drive the number into the millions, but Dr. Germann said he would not use such a rapid doubling rate unless it was a cold November and no countermeasures, like closing schools, were being taken.)

What went wrong?

The leaders of both the Northwestern University and Indiana University teams seemed a bit abashed when they were asked that last week.