Is the test for irreversibility of tetanus toxoids still relevant?

•A test for irreversibility is mandatory for the safety testing of tetanus vaccines.•Tetanus toxoids are subjected to toxicity testing after 37 °C and 5 °C storage.•It was shown that tetanus toxin loses its toxicity rapidly during 37 °C storage.•The 37 °C storage test for human and veterinary tetanus vaccines lacks relevance. Tetanus vaccines for human and veterinary use are based on toxoids resulting from a formaldehyde-mediated inactivation of tetanus neurotoxin (TeNT). Due to the high toxicity of TeNT, safety tests are mandatory for each batch of these toxoids. One of the tests addresses the irreversibility of inactivation: The toxoid is stored at 37 °C for 6 weeks and then subjected to in vivo toxicity testing. However, we found that TeNT solutions rapidly lose their activity at 37 °C. Accordingly, any active TeNT molecules arising in the toxoid due to reversion events may no longer be detectable after the 37 °C storage period. Furthermore, there is no evidence that a “reversion to toxicity” has ever been observed for tetanus toxoids during vaccine production. Thus, we conclude that the irreversibility test that is prescribed for human and veterinary vaccines has no relevance for vaccine safety.