Sustained reduction in vaccine-type invasive pneumococcal disease despite waning effects of a catch-up campaign in Kilifi, Kenya: A mathematical model based on pre-vaccination data by Ojal, J, Flasche, S, Hammitt, L, Akech, D, Kiti, M, Kamau, T, Adetifa, I, Nurhonen, M, Scott, J, Auranen, K