Neonatal tetanus despite protective serum antitoxin concentration. 1991, Maselle, FEMS Microbiol Immunol.

10 newborns with tetanus in Tanzania. Mothers of all but one have been vaccinated during pregnancy. The level of antibodies of nine of them was above the protective level. The mother of the tenth baby was vaccinated two weeks before giving birth.
Two babies had antibodies levels 100 and 400 times higher than the protective level. The mother of one of them received 14 vaccines during five of her pregnancies; the other mother received six vaccines in the last three years. The baby whose mother was not vaccinated had the level of antibodies three times above the protective level.
The authors conclude that there is no such thing as ‘protective level of antibodies.’