Changes in T lymphocyte Subsets Following COVID-19 Infection and Vaccination Walid Mohamed Attiah1 , Neveen George El Antouny 1 , Heba Shafeak Abd El Khalik1 , Rania Mohammad Abdullah2 , Ahmed Abdulsaboor Mohammed2* ABSTRACT Background: Several SARS-CoV-2 vaccines have been developed, and understanding their impact on immune system is of paramount importance. This study aimed to investigate changes in T Lymphocyte subsets following Covid19 vaccination vs in covid-19 patients. Methods: This prospective cohort study was conducted on 82 individuals divided into two equal groups of covid patients and healthy recipients of covid vaccines. Patients were subjected to detailed history taking, clinical examination, routine lab and measurement of CD-4 cells, CD-8 cells. Results: TLC, D. dimer, ferritin, CRP, ESR, LDH, and IL-6 were significantly higher among infected group compared to vaccinated group. Total lymphocytes count, and CD4+ and CD8+ subsets of T cells were significantly lower among infected group compared to vaccinated group (p< 0.001). Conclusions: Lymphocyte subsets changes is different in covid patients compared to post-vaccination, indicating different mechanisms of alteration. Lymphocyte counts could be monitored in COVID-19 patients to guide proper healthcare and vaccination programs. Keywords: Covid-19; Vaccination; Lymphocyte; CD4+; CD8+