Late last winter, a committee of vaccine experts designing this season's flu shot considered their choices. They had two, and both seemed bad.

Should they stick with last year's formula, even though a new strain of the bug was ominously building strength? Or should they try to make a new vaccine and risk complications or delays that could result in a shortage or maybe even no vaccine at all?

In the end, the committee voted 17-1 to bring back last year's version, even though they feared they were telling millions of Americans to roll up their sleeves for shots that might not work very well.

Many of them probably agreed with Dr. Theodore Eickhoff of the University of Colorado, who said: "For the first time in many years of participating in these deliberations, I must add I am very uncomfortable with the recommendation."

What Eickhoff and the others dreaded is exactly what happened. That new strain of flu became the dominant variety, accounting for three-quarters of all cases as the disease got an unusually early start.

About 83 million doses of vaccine were made, but no one really knows how much protection from illness it gives. It almost certainly will not be the usual 70 percent to 90 percent, and some experts fear it is below 50 percent.

"We agonized. We asked repeatedly 'Is there another choice?' " remembered Dr. David Stephens, who chaired the panel and heads infectious diseases at Emory University. "The bottom line is, we weren't really given a choice."

Their experience shows the frustrating and often imprecise nature of humanity's labor to stay ahead of this perennial nuisance and sometime killer. The flu virus mutates constantly. The Food and Drug Administration, with the help of its expert committee, must decide in late winter what varieties will be the biggest threats.

"By the time you know what's the right strain, you can't do anything about it," said Dr. Michael Decker, head of scientific affairs at Aventis, one of the three U.S. vaccine makers.

Flu viruses are categorized according to the makeup of their two key proteins, hemagglutinin and neuraminidase, the "H" and "N" in their names. Changes in the virus' hemagglutinin is especially troublesome, since this is the protein the human body aims for when it makes antibodies to fight off the flu.

For five years, the vaccine had protected against an H3N2 strain called Panama. Now that virus had mutated. A version with two differences in its hemagglutinin was causing outbreaks in Asia and had also turned up in Europe and North America.

The FDA's committee met in February and heard the bad news: The current vaccine might not reliably keep people from catching this emerging strain, called Fujian.

Nobody knew if the new strain would die out or gain strength, but Dr. Roland Levandowski, the FDA's flu vaccine expert, warned that new flu variants sometimes spread rapidly.