Cellular Immunological Memory T Cells and IL15RA Gene Polymorphism in COVID-19 Vaccinated Individuals from Southern Brazil

<b>Background:</b> The development of safe and effective vaccines against SARS-CoV-2 was crucial for controlling COVID-19 and establishing long-lasting immune memory in the population. <b>Methods:</b> This study evaluated cellular immune memory in individuals vaccinated with different regimens in Rio Grande do Sul using flow cytometry. Additionally, the rs2228059 polymorphism in the <i>IL15RA</i> gene was genotyped. A total of 62 participants were randomly recruited. <b>Results:</b> A decrease in memory T cell subsets in response to SARS-CoV-2 stimuli was observed in total CD3⁺, CD4⁺, and CD8⁺ T cells. Regarding the timing of the last vaccine dose, 94.4% of participants had received their final COVID-19 vaccination at least two years prior to recruitment. The rs2228059 polymorphism was genotyped in 443 individuals from the Rio Grande do Sul population. Among participants who received the ChAdOx1/ChAdOx1/BNT162b2 vaccination regimen and carried the TT genotype, an increase in CD8⁺ naive, CD8⁺ effector and CD4⁺ naive subsets was observed in stimulated cells. Although preliminary, the results suggest no major differences between vaccination regimens. <b>Conclusions:</b> The progressive reduction in memory T cell counts supports the need for booster doses, which is essential not only in the context of new emerging variants but also especially to maintain adequate cellular immune protection.