It is less clear what those antibody tests mean for real life, however, because immunity functions on a continuum. With some pathogens, such as the varicella-zoster virus (which causes chicken pox), infection confers near-universal, long-lasting resistance. Natural infection with Clostridium tetani, the bacterium that causes tetanus, on the other hand, offers no protection—and even people getting vaccinated for it require regular booster shots. On the extreme end of this spectrum, individuals infected with HIV often have large amounts of antibodies that do nothing to prevent or clear the disease.
There is a big difference with tetanus compared with other infections. The amount of toxin that produces tetany is extremely tiny. Even a small infection that the immune system has yet to react to can be lethal. To produce a long-lasting response against the bacterium would require a lethal number of bacteria.
Vaccination with the toxoid results in antibodies against the toxin, not the bacteria.