Cryopreservation-related loss of antigen-specific IFNγ producing CD4+ T-cells can skew immunogenicity data in vaccine trials: lessons from a Malaria vaccine trial substudy by Ford, T, Wenden, C, Mbekeani, A, Dally, L, Cox, J, Morin, M, Winstone, N, Hill, A, Gilmour, J, Ewer, K