Naturally acquired antibodies to tetanus toxin in humans and animals from the Galápagos islands. 1983, Veronesi, J Infect Dis.

The level of tetanus antibodies of 57 tested residents of Galapagos Islands was above the protective level. None of them had been vaccinated. Two of them have had tetanus in the past. This contradicts the accepted dogma that having had the disease does not give immunity.
They also checked nine animals (horses, cows, donkeys and a dog) and all of them had sufficient levels of antibodies.
The authors believe that immunity is produced by swallowing bacterial spores that multiply in the intestines. While the skin wounds act like booster vaccines.
It is reported here that most of the 59 unvaccinated people had protective levels of antibodies.
It is reported here that 80% of the 410 of unvaccinated Indians had antibodies in their blood.
The occurrence of natural immunity in 20 out of 48 children and adults in Mali is reported here.
It is reported here that out of 166 unvaccinated people only two did not have antibodies.