It is now established that SARS-CoV-2 pre-existing immune reactivity exists to some degree in the general population.

Nat Rev Immunol. 2020 08 ;20(8):457-458. PMID: 32636479

Abstract Title: 

Pre-existing immunity to SARS-CoV-2: the knowns and unknowns.

Abstract: 

T cell reactivity against SARS-CoV-2 was observed in unexposed people; however, the source and clinical relevance of the reactivity remains unknown. It is speculated that this reflects T cell memory to circulating ‘common cold’ coronaviruses. It will be important to define specificities of these T cells and assess their association with COVID-19 disease severity and vaccine responses.

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