Purpose: To assess if the common practice of assuming a uniform distribution and constant birth rate for age-stratified infectious disease models produces meaningfully different predictions of the disease burden of meningococcal C disease than a model estimated using real-world population assumptions.
Method: We used an age-stratified mathematical model of meningococcal C transmission, carriage, and disease to examine the effect of changes in population distribution and birth rate assumptions on model predictions. The model was calibrated to US surveillance data for carriage and disease incidence using recently-published contact matrix data that detail age-to-age specific rates of contact. We estimated age-specific transmission rates (ß) and risk of meningococcal disease conditional on carriage (q) at equilibrium for a cohort of 305 million individuals distributed across 100 1-year age cohorts based on two alternative assumptions regarding the baseline population distributions: uniform population distribution with equal numbers of person in each age group (median age: 49), and the current US population distribution (median age 36). We examined three alternative scenarios (1) uniform distribution with a constant birth rate, (2) US population with constant birth rate, and (3) US population with increasing birth rate based on US Census projections. 10-year cumulative incidence of meningococcal C disease was estimated under each scenario; analyses assume no meningococcal vaccination.
Result: The model predicts 10-year cases of meningococcal C disease of 2,100 for the uniform distribution, 2,150 for the US distribution with constant births and 2,210 for the US distribution with increasing births. The model using the uniform distribution predicts disease incidence 3% lower (320 vs. 329) among infants aged <1, 3% lower in adolescent aged 11-17 (505 vs. 518), and 14% lower (126 vs. 146) in young adults aged 18-22 than the US distribution with constant births model. Estimates based on the US population with constant births are 9% lower among infants, <1% lower among adolescents, and <1% lower in young adults than those using the US population with increasing births (363 infants; 519 adolescents, 146 young adults). Conclusion: When modeling transmission of an infectious disease such as meningococcal C for which the prevalence of carriage and disease are concentrated in younger age groups, choice of population distribution and birth rate may have a substantial effect on age-specific estimates of disease incidence.