Assessing the effect of social contact structure on the impact of pneumococcal conjugate vaccines.

Although pneumococcal conjugate vaccines (PCVs) have greatly reduced diseases caused by vaccine-targeted serotypes (VT) of Streptococcus pneumoniae, vaccine impact may be eroded by the increase in rates of disease caused by non-vaccine serotypes (NVT). Here, we investigated the effect of social contact patterns on the dynamics of vaccine impact in carriage. We developed a neutral, age-structured, Susceptible-Colonized model incorporating VT-NVT co-colonization and verified it against real-world carriage data in children. Using social contact matrices from 34 countries, we assessed the impact of contact patterns on the time required to eliminate VT in a given age group (defined here as the time needed for the relative proportion of VT in overall carriage to drop by 95% after the introduction of vaccination). Finally, we quantified the contribution of various parameters-such as vaccine efficacy, coverage, immunity waning, and population susceptibility-to the dynamics of VT elimination. Our model recapitulated the observed decline of VT carriage and showed that varying the contact structure alone led to different time-to-elimination (3.8-6 years). We found that higher total contact rate and assortativity in children under 5 accelerated VT elimination. Additionally, higher vaccine efficacy and coverage, and slower immunity waning led to shorter time-to-elimination. These findings illuminate the mechanisms controlling the dynamics of vaccine impact and may help predict the impact of PCVs in communities with different contact patterns.