Abstract Background Although failure of rabies post-exposure prophylaxis is rare, several factors, including improper vaccine or rabies immunoglobulin administration, may increase the risk. Where there is the potential for post-exposure prophylaxis failure, public health units may recommend serology to confirm a therapeutic immune response. Objective This study evaluates which factors increase the risk of failed seroconversion where serology was indicated after rabies post-exposure prophylaxis. Methods Potential lyssavirus exposure notifications where serology was collected were identified from five Southeast Queensland public health units from June 2015 – December 2022. Data on the indication and outcome of serology were extracted from public health case notes. Descriptive analysis was performed comparing serology outcomes by demographic and PEP course characteristics. Modelling for identified risk factors was conducted using hierarchical selection logistic regression analysis. Results 181 potential exposures were included in the study. Giving a rabies vaccine into the wrong site (RR 10.8, 95% CI 2.6—36.4), or vaccine into the same arm within 72 hours of rabies immunoglobulin (RR 5.6, 95% CI 1.9—16.9), increased the risk of non-therapeutic serology after adjusting for age and serology indication. Age over 65 also increased the risk of non-therapeutic serology after controlling for administration errors (RR 4.0, 95% CI 1.3—11.7). Conclusions Administering rabies vaccine into an incorrect site, or into the same arm within 72 hours of rabies immunoglobulin increase the risk of failed seroconversion. In such cases we recommend that the risk is sufficient to invalidate the dose and immediately repeat it without serology. Older age may be an independent risk factor for failed seroconversion.
Harris et al. (Mon,) studied this question.