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Currently, the best protection available to any of us in this third year of COVID-19 is to have a third booster shot of either the Pfizer or Moderna mRNA vaccine. I’ll briefly review the story of how immune responses get underway from late last year, then talk about the boosters.

When an immunologically ‘naïve’ (previously unexposed to virus or vaccine) person is ‘jabbed’ in the upper arm, the non-replicating vaccine mRNA is taken up by specialised (for antigen presentation) dendritic cells (APDCs) where it directs the manufacture of the SARS-CoV-2 spike protein ‘antigen’. As this is happening, the APDCs are travelling in the lymph that drains from the arm to the ‘filter station’ of the regional axillary lymph nodes (ALNs) in the armpit. Here, they ‘hang-out’ to ‘hook up with’ the few, naïve B and T lymphocytes that have receptors (BCRs or TCRs) specific for either the spike protein (the B cells) or for small peptides (p) bound into the tip of class I or class II major histocompatibility complex (MHC) glycoproteins. The TCRs on CD8+ killer and CD4+ helper T cells recognise these pMHCI and pMHCII complexes respectively.