> I would challenge the term "caution", as it implies giving the vaccine is more risky than not.

I suppose it depends on the alternative - if it's "not having the vaccine" clearly that's worse. If it's "controlling the death rate by prolonging lockdowns" then that's clearly not desirable either, but not quite as bad.

If it's just "give a different vaccine", then that would be no problem. Of course then there are the supply issues it's unlikely that simple.

It may be more risky for a certain population (it looks like younger people). We just don't know yet.

The majority of people getting the vaccinations are older. Perhaps we will see more cases will emerge for the younger age groups.