Tetanus bacteria, or Clostridium tetani (c.tetani), is an anaerobic bacteria, meaning it can’t survive in oxygenated environments. Sometimes a deep wound cannot get oxygen and may be more prone to allowing c. tetani to multiple.
A very deep wound or a cut on rusty metal doesn’t automatically mean you have been exposed. Tetanus is primarily found in soil or manure and rarely around the home. In the days before automobiles were invented, horses in the streets and around our homes increased the likelihood of coming into contact with manure. This is no longer the case unless we live in a farm environment.
If the wound being treated bleeds, there is also less likelihood of an infection. Blood carries oxygen, but, as stated above, c. tetani can only thrive in an anaerobic environment.
Even if a deep puncture wound that does not bleed was caused by an object that had the bacteria on it, the act of giving a vaccination AFTER the exposure is of no value. The vaccine does not instantly kill the bacteria; the vaccine takes about two weeks for seroconversion to take place. Seroconversion is the production of measurable antibodies the body develops to a pathogen. Giving a tetanus shot after an injury provides no benefit.