Spatial Variability in the Persistence of Pneumococcal Conjugate Vaccine-targeted Pneumococcal Serotypes Among Adults

Invasive pneumococcal disease is a leading cause of morbidity worldwide. Pneumococcal conjugate vaccine effectively reduces the number of cases caused by vaccine-targeted serotypes among children who receive the vaccine and adults who are not directly vaccinated. Recently, there has been a debate as to whether adults should receive the same conjugate vaccine as children. In settings where vaccine uptake in children is high, the vaccine serotypes cause a small fraction of disease cases, and direct vaccination might have a small effect. However, direct vaccination might be warranted if geographic regions or subpopulations exist where the targeted serotypes persist at higher levels than expected. To detect such geographic variability, new methodology is required. We introduce an innovative, spatially varying change points model, combined with spatially varying intercepts and slopes, to jointly determine whether the beginning date of the vaccine-associated decline, the initial baseline proportion of invasive pneumococcal disease cases caused by vaccine-targeted serotypes, and/or the rate of decline of vaccine-targeted serotypes vary in the adult population across Connecticut, 1998-2009. Results indicate that there is substantial spatial variability in the pattern with which vaccine-targeted serotypes decline, suggesting that the fraction of invasive pneumococcal disease cases that could have been preventable by direct vaccination of adults in Connecticut during the study period differed over time and space. The newly developed model is shown to outperform a number of competitors in terms of explanatory and predictive ability.