GENETICALLY modified tomatoes containing edible vaccine are to be used to challenge two of the world’s most lethal viruses. The aim is to create affordable vaccines for HIV and the hepatitis B virus (HBV) that could be easily grown and processed in the countries where they are most needed. So far, none of the 90 or so potential vaccines against HIV have proved successful and, though a vaccine already exists for HBV, it is too expensive to be used by poorer countries.