Since the time of Louis Pasteur, vaccines have worked on the principle that injecting dead or weakened pathogens into the body allows the immune system to learn to fight them. But vaccines from weakened microbes require constant refrigeration until use, and those from microbes killed by heat or chemicals provoke a weaker immune response, requiring occasional boosters. But new research reviving an old concept--killing microorganisms via gamma radiation--seems to show that such irradiated vaccines can trigger powerful immunity.

Sandip Datta of the University of California, San Diego, and his colleagues first bombarded a colony of Listeria monocytogenes--a bacteria found in food that can cause health problems for mothers, babies and the immunocompromised--with 600,000 rads of gamma rays. The irradiated bacteria showed no signs of growth despite being introduced into a warm soy broth and allowed to sit.

But when injected into 10 mice, the irradiated bacteria helped keep eight of them alive after they were exposed to a dose of live listeria that killed 20 other mice (10 unvaccinated controls and 10 innoculated with a heat-killed version of the vaccine). "Irradiation is a technically simple process that retains structural features of the bacterial pathogen," Datta explains. "Therefore, a strong immune response is induced in the vaccinated host."

In fact, the irradiated vaccine is able to trigger a long-lasting immunity in its host; the mice were still resistant to listeria more than a year after vaccination. A freeze-dried version of the irradiated vaccine also proved effective, raising hopes for versions that would be easy to store, transport and reconstitute on site. And, if it proves effective in creating vaccines for other pathogens, irradiation could provide a quicker way to respond to epidemics or other threats, the researchers argue in the paper presenting the finding in today's Immunity. "[Irradiated vaccines] would be very safe, simple and inexpensive to produce," notes co-author Eyal Raz. "This might not be the ideal vaccine, but its practicality is beyond imagination."