The author of this study (Philip B. Cowles) seemed skeptical of some claims made about the tetanus toxoid vaccine, so set out to test them.1
The first was the claim that 0.01 unit of antitoxin per milliliter of serum (the level of antibodies against the tetanus toxin in the blood) was enough to protect against the symptoms of tetanus. He used guinea pigs and mice to figure out how much antitoxin was necessary to protect. He seemed to find that 0.1 unit per mL was usually enough, though not always. As another author summarizes,
The second question was how long does it take for a booster shot to raise the level of antitoxin. The common practice was to give a booster when a person presented with a wound of some sort that might expose them to tetanus infection, thinking that the vaccine would raise their antibodies enough to protect. But if that doesn't happen quickly enough, it wouldn't be very useful.
Cowles tested a number of people before and after a booster. Most had about 0.02 unit or less before, and it took 4-5 days to rise up to 0.1 or higher. Is this fast enough? Tetanus seems to develop in 7-10 days, but it's possible the toxin has started working before then, before symptoms show up. So more study would be needed to say a booster is definitely helpful, but it's pretty definitely better than nothing if an antitoxin treatment is unavailable.
As a final addendum, the author mentions the question of whether having tetanus makes one immune to further tetanus. There had been reports of people have tetanus repeatedly, so obviously there isn't always natural immunity, but Cowles describes a man who had recovered from a mild tetanus attack at some point, but didn't seem to have any antitoxin in his blood. So it seems like the tetanus toxin is toxic at lower concentrations than it is antigenic (capable of producing antibodies and immunity), so it's rare to become immune naturally without dying or something. Small sample size though.
References:
The first was the claim that 0.01 unit of antitoxin per milliliter of serum (the level of antibodies against the tetanus toxin in the blood) was enough to protect against the symptoms of tetanus. He used guinea pigs and mice to figure out how much antitoxin was necessary to protect. He seemed to find that 0.1 unit per mL was usually enough, though not always. As another author summarizes,
"This author felt that though 0.10 unit is not sufficiently great to protect all animals against a maximum infection, it is probably much larger than is necessary to care for many infections resulting from wounds judged to be too slight for surgical attention, and probably large enough to care for the majority of injuries that receive surgical treatment."2
The second question was how long does it take for a booster shot to raise the level of antitoxin. The common practice was to give a booster when a person presented with a wound of some sort that might expose them to tetanus infection, thinking that the vaccine would raise their antibodies enough to protect. But if that doesn't happen quickly enough, it wouldn't be very useful.
Cowles tested a number of people before and after a booster. Most had about 0.02 unit or less before, and it took 4-5 days to rise up to 0.1 or higher. Is this fast enough? Tetanus seems to develop in 7-10 days, but it's possible the toxin has started working before then, before symptoms show up. So more study would be needed to say a booster is definitely helpful, but it's pretty definitely better than nothing if an antitoxin treatment is unavailable.
As a final addendum, the author mentions the question of whether having tetanus makes one immune to further tetanus. There had been reports of people have tetanus repeatedly, so obviously there isn't always natural immunity, but Cowles describes a man who had recovered from a mild tetanus attack at some point, but didn't seem to have any antitoxin in his blood. So it seems like the tetanus toxin is toxic at lower concentrations than it is antigenic (capable of producing antibodies and immunity), so it's rare to become immune naturally without dying or something. Small sample size though.
References:
1. Cowles, P. B. Tetanus Immunization. Yale J Biol Med 9, 409–416 (1937).
2. Gold, H. Active Immunization Against Tetanus. Ann Surg 114, 1060–1068 (1941).
2. Gold, H. Active Immunization Against Tetanus. Ann Surg 114, 1060–1068 (1941).
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