a1 Population Biology of Infectious Disease Group, Department of Zoology, University of Oxford, UK
a2 Immunisation Department, Health Protection Agency Centre for Infections, London, UK
a3 Vaccine & Immunisation Research Group, Murdoch Children's Research Institute & School of Population Health, University of Melbourne, Victoria, Australia
A rise in invasive Haemophilus influenzae type b (Hib) infections occurred 8 years after vaccine introduction in the United Kingdom. Aspects of Hib vaccine delivery unique to the United Kingdom have been implicated. The authors developed a fully age-structured deterministic susceptible–infected–resistant–susceptible mathematical model, expressed as a set of partial differential equations, to better understand the causes of declining vaccine effectiveness. We also investigated the consequences of the vaccine's impact on reducing Hib transmission for maintenance of immunity. Our findings emphasized the importance of maintaining high post-immunization antibody titres among age groups at greatest risk of invasive infections. In keeping with UK population-based estimates, low direct efficacy of immunological memory against disease was found, cautioning against over-reliance on evidence of priming alone as a correlate of population protection. The contribution of herd immunity to disease control was reinforced. Possible intervention strategies will be explored in subsequent work.
(Accepted June 18 2007)
(Online publication August 03 2007)
c1 Author for correspondence: Dr J. McVernon, Vaccine and Immunisation Research Group, Murdoch Childrens Research Institute & School of Population Health, University of Melbourne, Level 5, 207 Bouverie Street, Carlton, Victoria, 3053, Australia. (Email: firstname.lastname@example.org)