Absolutely brilliant idea

From Science:

Biocompatible near-infrared quantum dots delivered to the skin by microneedle patches record vaccination

The précis:

Vaccines prevent disease and save lives; however, lack of standardized immunization recordkeeping makes it challenging to track vaccine coverage across the world. McHugh et al. developed dissolvable microneedles that deliver patterns of near-infrared light-emitting microparticles to the skin. Particle patterns are invisible to the eye but can be imaged using modified smartphones. By codelivering a vaccine, the pattern of particles in the skin could serve as an on-person vaccination record. Patterns were detected 9 months after intradermal delivery of microparticles in rats, and codelivery of inactivated poliovirus led to protective antibody production. Discrete microneedle-delivered microparticle patterns in porcine and pigmented human skin were identifiable using semiautomated machine learning. These results demonstrate proof of concept for intradermal on-person vaccination recordkeeping.

That they made it work with a cell phone and an app is pure genius.