Evaluation of Risk Factors for Failed Seroconversion in the Management of Potential Lyssavirus Exposures.

Background: Although failure of rabies postexposure prophylaxis is rare, several factors, including improper vaccine or rabies immunoglobulin administration, may increase the risk. Where there is the potential for postexposure prophylaxis failure, public health units may recommend serology to confirm a therapeutic immune response. This study evaluates which factors increase the risk of failed seroconversion in cases where serology was indicated after rabies postexposure prophylaxis. Methods: Potential lyssavirus exposure notifications where serology was collected were identified from 5 Southeast Queensland public health units from June 2015 to 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. Modeling for identified risk factors was conducted using hierarchical selection logistic regression analysis. Results: One hundred eighty-one potential exposures were included in the study. Giving a rabies vaccine in the wrong site (relative risk [RR], 10.8; 95% CI, 2.6-36.4) or a vaccine into the same arm within 72 hours of rabies immunoglobulin (RR, 5.6; 95% CI, 1.9-16.9) increased the risk of nontherapeutic serology after adjusting for age and serology indication. Age ≥65 also increased the risk of nontherapeutic 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 increases 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.