In the case of FcRL4+ B-lymphocytes with an effector-memory phenotype, a significant increase in this population was detected following the first and second vaccine doses when compared with the nonvaccinated sample from recovered COVID-19(+) participants (p < 0.0001 and p = 0.0037, respectively), and an increase in the second vaccine dose was detected when compared with the nonvaccinated samples from the COVID-19(−) participants (p = 0.035) (Figure 5C). This evidence concerns the gene FCRL4 and COVID-19.