Nothing. The idea that rust itself causes tetanus is a misconception.

Tetanus doesn't come from rust, it comes from things being dirty. It lives in soil. The idea of a "rusty nail" is more indicative that it's been out there in the environment a long time and may have picked up contamination. The rust itself is incidental.

If you get a puncture wound from a non-rusty nail or anything similar out in a field you should also probably get a tetanus shot for that as well.