Depends what you mean by "against medical advice". It was not using the vaccines as tested and designed to be used and hence the drug companies making their reservations clear, but they did seek expert opinion on what the likely outcome would be - and that advice seems to have turned out right.
The problem was that the government's failure to control covid meant it was running wild, so they decided to vaccinate as many people as they could with one shot instead of go for the double, as the experts reckoned the single shot would give decent protection from covid at least short term. And indeed, observation suggests ~90% reduction in likelihood of severe covid symptoms for people who took the one shot (Pfizer ~85% AstraZeneca >90%) , if they became infected. So that informed risk paid off. In the longer term about generating lasting immunity, that's not clear. Maybe it means we'll have to have three shots in the end, or something.