Radio-frequency identification (RFID) transponders can be administered through a syringe needle allowing for what is known as near field communication – this is how pets are microchipped. But these sorts of microchips are not so small that you wouldn’t see them in a vaccine. They would also show up in x-rays. These types of standalone embedded microchip devices can only be tracked using a reader that is close to the human body (usually not more than 10cm away).

New technology based around tiny sensors called smartdust or MOTES have also been developed, but there is no evidence to suggest these are in modern vaccines.

It's not to say they might not have roles in medicine in the future as there is already research into embedding RFID transmitters in tablets to swallow.

At the same time novel innovations are being patented like implantable GPS but these still have significant challenges to overcome before they become functional (for example about 60% of the human body is made up of water, and this water makes it difficult for GPS signals to get a direct Line of Sight to satellites).

There is the potential to use several wireless technologies in tandem to offer a more continuous form of tracking, e.g. a microchip implant in the hand that can be read by an app if you bring your smartphone close enough (if the app on the phone allows for that triggerability).This sort of technology could be used by the health industry for prosthesis.

This technology raises a great number of ethical, legal and social implications, particularly when we are considering the commercialisation of this technology towards epayment or other applications.