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S. typhi Vi capsular polysaccharide vaccine-induced humoral immunity in travellers with immunosuppressive therapy for rheumatoid disease

Introduction
Typhoid fever is a global health problem, causing significant morbidity and mortality. Currently, the most widely used vaccine is the typhoid Vi capsular polysaccharide (Vi-PS) vaccine. While epidemiological studies on its efficacy have been performed in children in endemic countries, there are no efficacy studies evaluating its use in travel medicine. Response to vaccination may differ in travellers receiving immunosuppressive therapy. This study investigates the humoral response to Vi-PS vaccination in travellers receiving immunosuppressive therapy for rheumatoid disease.


Methods
We recruited patients from the LUMC rheumatology outpatient clinic and travellers from the travel clinic who had previously received Vi-PS vaccination and also immunosuppressive therapy for rheumatoid disease. We analysed blood samples acquired from 42 patients over a period of 3 years. We estimated the length of persistence of protective titres using the survival analysis using multiple cut-off values for protection and measured titre half-life and the influence of immunosuppressive medication on titre half-life using mixed models.


Results
Anti-Vi-PS antibody levels stayed above 10 EU/ml for a mean of 13.3 years, above 15 EU/ml for a mean of 10.1 years and above 20 EU/ml for a mean of 8.6 years after Vi-PS vaccination. Titre half-life was 7.5 years (95% CI 5.0-14.7 years, P < 0.001). No significant influence of medication on titre half-life was found.


Conclusion
Both persistence of protective antibody titres and titre half-life are longer than expected based on other studies. This warrants further study in adult volunteers, both in healthy individuals and patients suffering from rheumatoid disease.