Probably this is stating the obvious for most people reading this, but saying that th frequency of side effects is unknown means just that - they don't know the rate, ie there haven't been enough occurrences to estimate it with statistical significance.
So it could be low or incredibly low. One thing it can't be is incredibly high - that would mean enough statistics to estimate the frequency by now.
So, it could be low, it could be high but maybe not...nobody knows for sure.
Well I'm sorry, that's not good enough for my daughter.