Limitations of the rabbit pyrogen test for assessing meningococcal OMV based vaccines

The rabbit pyrogen test was developed in the early 1900's to detect contaminating pyrogens in parenteral medicines. Since its conception alternative methods with improved sensitivity, relevancy and which are ethically more acceptable have been developed. However, the test is a current Pharmacopeial method and is used to evaluate the pyrogen content of some vaccines. In this article the limitations and pitfalls of using the test to measure pyrogenicity of vaccines containing outer membrane vesicles are described. The method is unsuitable as a safety test for these products due to the high levels of endotoxin present in the vaccine which generate a pyrogenic response in rabbits when delivered intravenously without any dilution. Its use as a consistency test is also ambiguous as the test gives a qualitative rather than quantitative response and the rabbit models are highly variable. In addition there is evidence that measuring the temperature rise of the animals over three hours does not capture the maximum fever response. Finally the article considers the use of alternative methods and the validity of animal models when applying a consistency based approach for assessing the quality of licensed products.