Under the skin isn't necessarily problematic. However, the dispersal-rate of the vaccine would be much quicker - with side effects being even more dramatic, which I suspect is not optimal for a situation where you are, ideally, trying not to overwhelm a population's already taxed, environmentally-compromised immune systems. What further makes no sense is the expectation that a dosage meant for one individual will, somehow, magically and adequately provide coverage for five people. Expecting to bring this into reality by fiat is utter insanity. Were this even a remotely viable option, I suspect we'd have heard of it, long, long ago. There are many viable vaccines in production, throughout the world, and as far as I know, exactly none of them have ever hinted at the possibility of "watering down" dosages, let alone dividing them by five. Holy Sh*t.