Thus, in summary, infection-naïve individuals without pre-existing cross-reactive immunity had a significantly lower humoral response to first and second dose (p < 0.01), and they had a lower rate of CD4 + and CD8 + T-cell response to two vaccine doses both in magnitude and in rate of response (30% and 23% after the first dose, 18% and 5% after second dose, respectively), in comparison with the stronger response observed in all the individuals with pre-existing immunity. Here, CD4 is linked to infection.