The only way to be certain is the proof it by making a starter. Fizz in the tube can be leftovers and not an indication of live yeast. If you lost 99% of the yeast it will take an extra day for the ferment to start. In theory, one cell is all it takes; but it will take a long time.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk