الفهرس 
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Abstract
Since December 2019, the world has been facing COVID-19 pandemic which has led to a global public health threat. The emergency use listing was guaranteed to approve COVID-19 vaccines including mRNA vaccines as Pfizer-BioNTech and Moderna, viral Vector vaccines as AstraZeneca and inactivated virus vaccines as Sinovac and Sinopharm. COVID-19 vaccination programs have targeted more than 60% of the global population, but little is known about the effects of these vaccines on different parts of the immune system, including innate immune cells.
MDSCs are a heterogeneous group of innate immune cells of myeloid origin. They are a double edged weapon in viral infections by being harmful to the host by immunosuppression. It is also beneficial by preventing inflammation and tissue damage.
Natural killer cells (NK) cells are cytotoxic lymphocytes that play an essential role in the innate immune response through the destruction of stressed, infected, or cancerous cells. Defective NK cell function has been linked to many infectious diseases.
The current study was conducted on 60 subjects who were classified into 15 healthy unvaccinated volunteers, 15 Sinopharm vaccinated volunteers, 15 AstraZeneca vaccinated volunteers, 15 Pfizer-BioNTech vaccinated volunteers.
Blood samples were taken from volunteers in different tubes, a tube was sent to the laboratory for CBC, PBMNCs isolation of other blood samples was done in less than 12 hours via density gradient centrifugation using ficoll-Hypaque. This was followed by flowcytometric analysis of MDSCs (HLA-DR-, CD33+ and CD11b+)
Then culture was done by propagating the PBMNCs in complete culture media (RPMI) for about 24 hours. K562 cells were then co-cultered with PBMNCs for 48 hours after which supernatants were collected and kept in -20oC, then LDH cytotoxicity assay was done after collection of the whole samples.
The present study revealed a statistically significant difference in MDSCs values between the four groups where both Sinopharm and Pfizer-BioNTech vaccinated groups showed a significant increase in MDSCs compared to control group. In addition, Sinopharm vaccinated participants showed a significant increase in MDSCs absolute count compared to AstraZeneca vaccinated candidates.
Moreover, our results showed that the mean of MDSCs was significantly increased in AstraZeneca vaccinated individuals who received 2 vaccine doses with an interval time > 6 months between last vaccination date and sampling.
Regarding NK cytotoxicity index, Pfizer-BioNTech vaccinated group showed the highest cytotoxicity index levels. Furthermore, the mean cytotoxicity index was significantly increased in vaccinated individuals who received 3 vaccine doses with an interval time ≤6months between last vaccine dose and sampling.