From Scientific American:
Keeping track of vaccinations remains a major challenge in the developing world, and even in many developed countries, paperwork gets lost, and parents forget whether their child is up to date. Now a group of Massachusetts Institute of Technology researchers has developed a novel way to address this problem: embedding the record directly into the skin.
Along with the vaccine, a child would be injected with a bit of dye that is invisible to the naked eye but easily seen with a special cell-phone filter, combined with an app that shines near-infrared light onto the skin. The dye would be expected to last up to five years, according to tests on pig and rat skin and human skin in a dish.