Terahertz ( Hz). There is what’s called a terahertz band, which runs from .3 to 30 THz, and it is used by medical researchers to examine processes in the body. But the receivers are about the size of a microwave oven, and, more crucially, the range of terahertz signals in air is short. It might be as short as ten yards. A vaccinated person might not be detectable even across a street. Terahertz is out.
Infrared. Infrared lamps are used to keep food warm, so the idea is a
common one. So anything that affects heat will affect the signal. In a Texas parking lot in summer, there will be so much stray heat that the detector would never be able to sort out what was noise from what was a vaccinated person. Any place that’s hot in summer will be hot enough to drown the signal in noise (stray heat). In the winter, when everyone puts on coats and jackets, the signal will be kept under the outer garments, and so will never reach the detector. When a vaccinated person is indoors the signal won’t be able to get out (or from the outside in) because of the walls and windows. Same for cars. Infrared is out
Visible. It’s weird to even bring this up. Anything opaque, like clothes, walls, and anything that would hide infrared will also hide visible light. But it’s also true that if visible light is seen coming from someone’s arm, that’s going to give the chip away. The secret is blown. Visible is out.
Radio. It would have to be in some unallocated part of the spectrum.
Anything in the allocated parts of the spectrum will be picked up by somebody or something, and, remember, the signal isn’t encrypted. If it’s detected at all, it will be understood, and the secret will be blown. It would be almost impossible to tease out the vaccinated people from the regular radio transmissions that exist every day.
The rest. Just about any other frequency is either going to require an
antenna that’s way too big to conceal, or will require way too much energy to produce. In any real-world situation there just won’t be a way to get a signal in or out.