Cohort profile: The Kilifi Vaccine Monitoring Study by Adetifa, I, Bwanaali, T, Wafula, J, Mutuku, A, Karia, B, Makumi, A, Mwatsuma, P, Bauni, E, Hammitt, L, Nokes, D, Maree, E, Tabu, C, Kamau, T, Mataza, C, Williams, T, Scott, J