Do Australian immunoglobulin products meet international measles antibody titer standards?

The effectiveness of passive immunisation post-exposure to measles appears subject to a dose-response effect. New Zealand and the United Kingdom have increased the recommended dose of polyclonal human immunoglobulin for post-exposure prophylaxis within the last decade in response to concerns about decreasing levels of measles antibodies in these products. This study used the plaque-reduction neutralization test (PRNT) to measure the titer of measles-specific antibodies in Australian immunoglobulin products for post-exposure prophylaxis and compared the utility of an enzyme-linked immunosorbent assay (ELISA) to the PRNT in available Australian and international samples: Australian intramuscular (n = 10), Australian intravenous (n = 28), New Zealand intramuscular (n = 2), Hizentra (subcutaneous)(USA) (n = 3), and Privigen (intravenous)(USA) (n = 2). Measles titres in Australian IM and IV immunoglobulins ranged from 51 to 76 IU/mL and 6 to 24 IU/mL respectively, as measured by PRNT calibrated to the WHO 3rd international standard. ELISA titres were variable but higher than PRNT titres in all tested samples. Measles antibody titres in Australian immunoglobulin products meet consensus-prescribed international thresholds. Development of a convenient, standardized, readily accessible assay for determination of measles titres in immunoglobulin products would be useful for future studies and facilitate international comparisons.