Background
Trained immunity is a functional reprogramming of innate immune cells that bestows them with a form of non-specific memory; allowing a heightened response to subsequent, unrelated insults. Adenoviral vectors have been shown to induce trained immunity in mice. As the AstraZeneca COVID-19 vaccine uses an adenoviral vector, this study aimed to assess the capacity of the AstraZeneca vaccine to induce trained immunity in vivo in human monocytes.

Methods
Monocytes were isolated from the blood of healthy volunteers before receiving the vaccine and at day 14, 56 and 90 post vaccination. Using multiparameter flow cytometry, the expression of cell surface markers associated with trained immunity in monocytes was determined. Metabolic reprogramming in resting monocytes was analysed by examining changes in the gene expression of key metabolic enzymes over time. Monocytes were restimulated in vitro with unrelated stimuli such as LPS, Pam3Csk4 and Mycobacterium tuberculosis (Mtb). Cytokine production in response to stimulation pre- and post- vaccination was determined by multiplex cytokine.

Results & Conclusion
Monocytes show marked features of trained immunity at up to 90 post vaccination compared with the pre-vaccination control. _Ex vivo_monocytes exhibit enhanced expression of HLA-DR, CD40 and CD80/86 up to 3 months post vaccination. Moreover, monocytes display signs of metabolic reprogramming with increased expression of glycolytic enzymes up to 2 months post-vaccination. When restimulated in vitro, monocytes had altered cytokine production compared to pre-vaccine controls, consistent with the induction of trained immunity.
Vaccine strategies that combine the induction of trained immunity with the generation of traditional adaptive immune memory are postulated to have enhanced efficacy and may promote non-specific protection against a range of pathogens.

**Background**
Trained immunity is a functional reprogramming of innate immune cells that bestows them with a form of non-specific memory; allowing a heightened response to subsequent, unrelated insults. Adenoviral vectors have been shown to induce trained immunity in mice. As the AstraZeneca COVID-19 vaccine uses an adenoviral vector, this study aimed to assess the capacity of the AstraZeneca vaccine to induce trained immunity _in vivo_ in human monocytes.

**Methods**
Monocytes were isolated from the blood of healthy volunteers before receiving the vaccine and at day 14, 56 and 90 post vaccination. Using multiparameter flow cytometry, the expression of cell surface markers associated with trained immunity in monocytes was determined. Metabolic reprogramming in resting monocytes was analysed by examining changes in the gene expression of key metabolic enzymes over time. Monocytes were restimulated _in vitro_ with unrelated stimuli such as LPS, Pam3Csk4 and _Mycobacterium tuberculosis_ (Mtb). Cytokine production in response to stimulation pre- and post- vaccination was determined by multiplex cytokine.

**Results & Conclusion**
Monocytes show marked features of trained immunity at up to 90 post vaccination compared with the pre-vaccination control. _Ex vivo_monocytes exhibit enhanced expression of HLA-DR, CD40 and CD80/86 up to 3 months post vaccination. Moreover, monocytes display signs of metabolic reprogramming with increased expression of glycolytic enzymes up to 2 months post-vaccination. When restimulated _in vitro_, monocytes had altered cytokine production compared to pre-vaccine controls, consistent with the induction of trained immunity.
Vaccine strategies that combine the induction of trained immunity with the generation of traditional adaptive immune memory are postulated to have enhanced efficacy and may promote non-specific protection against a range of pathogens.

An adenoviral vector vaccine against SARS-CoV2 elicits trained immunity in humans

Exploring the intricacies of host-pathogen communication is vital in order to understand why
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