In vaccinated individuals, monocytes and/or natural killer cells exhibit an upregulation of surface markers of activation and synthesis of cytokines, such as IL-1β, IL-6, IFN-γ, and TNF-α, in response to infection compared to non-vaccinated individuals; this helps in the faster clearance of pathogens, including influenza (Arts et al., 2018). Here, IL6 is linked to infection.