One team leader, Dr. Wayne A. Marasco of Harvard, said it began by screening a library of 27 billion antibodies he had created, looking for ones that take aim at the hemagglutinin “spikes” on the shells of flu viruses.

Antibodies are proteins normally produced by white blood cells that attach to invaders, either neutralizing them by clumping on or tagging them so that white cells can find and engulf them. They can be built in the laboratory and then “farmed” in plants, driving prices down, Dr. Marasco said.

The flu virus uses the lollipop-shaped hemagglutinin spike to invade nose and lung cells. There are 16 known types of spikes, H1 through H16.

The spike’s tip mutates constantly, which is why flu shots have to be reformulated each year. But the team found a way to expose the spike’s neck, which apparently does not mutate, and picked antibodies that clamped onto it. Once its neck is clamped, a spike can still penetrate a human cell, but it cannot unfold to inject the genetic instructions that take over the cell’s machinery to make more virus.

The team then turned the antibodies into full-length immunoglobulins and tested them in mice.

Immunoglobulin  antibodies derived from the blood of survivors of an infection  has a long history in medicine. As early as the 1890s, doctors injected blood from sheep that had survived diphtheria to save a girl dying of it. But there can be dangerous side effects, including severe immune reactions or accidental infection with other viruses.

The mice in the antibody experiments were injected before and after receiving doses of H5N1. In 80 percent of cases, they were protected. The team then showed that their new antibodies could protect against both H1 and H5 viruses. Most of the flu this season is H1, and experts still fear that the lethal H5N1 bird flu may start a human pandemic.

However, the other seasonal flu outbreaks each year are usually caused by H3 or B strains, so flu shots must also contain those. But there is always at least a partial mismatch because vaccine makers must pick from among strains circulating in February since it takes months to make supplies. By the time the flu returns in November, its “lollipop heads” have often mutated.