in my opinion, first, it is the whole biotech sector got dumped since Hillary made her speech to lower the medicine cost. The dumped IBB from 400 to 290. Second, NVAX followed the whole sector down, but in a much more violent way. Third, I think lots of people got margin call these days, also some stop loss got triggered.
Simple like that.
There is nothing rational that supports the drop from 15+ to 6.5, will be back to 10+ in days.
whenever the stock rises, they may sell... pretty bad plan, should just sell to some big investors at some discount and let this issue go away instead of selling from time to time into any possible rally