It's not an invalid sentence grammatically, it just probably doesn't mean what you want it to mean.
Your sentence says that people buy medical supplies in case an epidemic has broken out - that is, is already underway (and eg. they don't know about it). If you used present simple here:
Some people buy lots of medical supplies in case an epidemic breaks out.
the sentence would mean something different - that they buy medical supplies anticipating an epidemic sometime in the future. Which sounds more likely to me.
I've drafted an alternative proposal in case they veto ours (they might veto it later, eg. when you present it)
I've drafted an alternative proposal in case they have vetoed ours (the decision is already made, you just don't know it)