Tetanus is required for every injury incurred from an object, road injury, and specially more so for metal. Any tetanus shot taken once, gives you immunity for 6 months. Within that 6 months if you get an injury, you can well assume that you have protection. But having said that, it is a food precaution to take if you think that it has been too intrusive or injury has been very extensive to go ahead and take another tetanus shot, because if the first was taken only for a scrape, and after that you have met with a road accident, it’s going to be humungous exposure. In that case it is always good to take another tetanus shot and stay away from that deadly disease.