Research suggests immunisation against bird flu strains that do not usually threaten humans may provide the nearest thing yet to a "universal" flu vaccine.
Exposure to an unfamiliar flu strain causes the immune system to release broadly effective antibodies, the study shows.
The discovery raises the possibility of vaccinating people against H5N1 and H7N9 bird flu strains, even though neither has yet caused an epidemic.
Both strains have caused serious and sometimes fatal infections in Asia, typically passing from birds to people via poultry. But neither has spread through populations from person-to-person.
Research suggests that although such transmission is possible, it is extremely difficult.
Study leader Dr Ali Ellebedy, director of the Emory Vaccine Centre in Atlanta, United States, said: "Our previous research led us to hypothesise that immune responses to the stem region are likely to be stronger after exposure to ... molecules derived from flu viruses which the human population has been minimally exposed to."