Predicting the Impact of Vaccination on the Transmission Dynamics of Typhoid in South Asia: A Mathematical Modeling Study

• Published in: PLoS neglected tropical diseases Vol. 8; no. 1; p. e2642
• Main Authors: Pitzer, Virginia E., Bowles, Cayley C., Baker, Stephen, Kang, Gagandeep, Balaji, Veeraraghavan, Farrar, Jeremy J., Grenfell, Bryan T.
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 01.01.2014; Public Library of Science (PLoS)
• Subjects: Adolescent; Adult; Age; Age Distribution; Aged; Asia - epidemiology; Basic Reproduction Number; Biology; Carrier State - epidemiology; Child; Child, Preschool; Disease transmission; Disease Transmission, Infectious - prevention & control; Distribution; Humans; Incidence; Infections; Influence; Male; Mathematical models; Medicine; Middle Aged; Models, Theoretical; Population; Public health; Studies; Typhoid; Typhoid fever; Typhoid Fever - epidemiology; Typhoid Fever - transmission; Typhoid-Paratyphoid Vaccines - administration & dosage; Typhoid-Paratyphoid Vaccines - immunology; Vaccination; Vaccination - statistics & numerical data; Vaccines; Young Adult; South Asia; Models, Theoretical; Age Distribution; Typhoid Fever; Humans; Middle Aged; Vaccination; Basic Reproduction Number; Child, Preschool; Male; Incidence; Young Adult; Asia; Adolescent; Adult; Aged; Disease Transmission, Infectious; Child; Carrier State; Typhoid-Paratyphoid Vaccines
• ISSN: 1935-2735; 1935-2727; 1935-2735
• DOI: 10.1371/journal.pntd.0002642

Modeling of the transmission dynamics of typhoid allows for an evaluation of the potential direct and indirect effects of vaccination; however, relevant typhoid models rooted in data have rarely been deployed. We developed a parsimonious age-structured model describing the natural history and immunity to typhoid infection. The model was fit to data on culture-confirmed cases of typhoid fever presenting to Christian Medical College hospital in Vellore, India from 2000-2012. The model was then used to evaluate the potential impact of school-based vaccination strategies using live oral, Vi-polysaccharide, and Vi-conjugate vaccines. The model was able to reproduce the incidence and age distribution of typhoid cases in Vellore. The basic reproductive number (R 0) of typhoid was estimated to be 2.8 in this setting. Vaccination was predicted to confer substantial indirect protection leading to a decrease in the incidence of typhoid in the short term, but (intuitively) typhoid incidence was predicted to rebound 5-15 years following a one-time campaign. We found that model predictions for the overall and indirect effects of vaccination depend strongly on the role of chronic carriers in transmission. Carrier transmissibility was tentatively estimated to be low, consistent with recent studies, but was identified as a pivotal area for future research. It is unlikely that typhoid can be eliminated from endemic settings through vaccination alone.