By genetically tweaking the constituent live virus, scientists have created a vaccine against influenza in which the virus is capable of activating the immune system but cannot replicate in healthy cells - an approach that may become more widely used for generating live virus vaccines adapted to other viruses. The vaccine proved effective in mice, guinea pigs and ferrets. A major challenge in developing viral vaccines is incorporating enough of the virus to elicit an immune response, while not allowing the virus to run rampant through the body, infecting healthy cells. To overcome this issue, Longlong Si and colleagues modified the genetic code of influenza A virus so that it could only infect and replicate in a cell line they engineered to be dependent on an unnatural amino acid; critically, their modified virus - though still just as potent in terms of activating the immune system - cannot replicate in conventional cells. In mice, administering the modified, infected cells in the form of a vaccine offered full protection against influenza. The new vaccine was found to offer an antibody response comparable to an existing live-virus vaccine, and a second dose further increased antibody titers by a factor of six to eight. Similar beneficial effects were seen when the viral vaccine was tested against several different strains of influenza, and tested in guinea pigs and ferrets. These types of virus vaccines can be potentially adapted to almost any virus, the authors say, as long as their genome could be manipulated and packaged in a cell line.