The cultures are living things, but Hammond's right, the cultures are anaerobic. It's true some do die off during their life on the store shelf, but not enough to make a significant difference, at least by the sell by date, and a few weeks afterwards. But generally there'll be enough left to repopulate your gut, if you need it. Also, kefir is better at that than yogurt, since yogurt typically contains only 2 - 4 cultures, while kefir can easily contain 12+