ELISPOT quantification of IFN-γ-producing PBMC stimulated by each of 3 peptide pools spanning the NS3 protein. Expressed as mean spot forming units (SFU) above baseline (mean media control) for each sample. Samples with mean SFU below background (mean media control + 3x SD) are set to zero for display. *, productive rechallenge infection. SC, seroconversion; FV, falling viremia; CH., rechallenge inoculations; nd, not done. NZP1 rechallenge data for horses N and P were not done.