What is the latest evidence on pre-existing immunity against #sarscov2 among uninfected individuals?
 SARS-CoV-2 antibodies detected in uninfected individuals. (Science, 6 Nov 2020)
 Population: Particularly prevalent in children and adolescents.
 Antibodies: predominantly IgG class, which mainly targeted the S2 subunit of spike protein.
 Why S2? Why not S1 subunit of spike protein?
The S2 subunit exhibits a higher degree of homology among coronaviruses than S1. Hence, likely the main target of cross-reactive antibodies.
 In other words, SARS-CoV-2-uninfected patient sera cross-reacts with SARS-CoV-2 S2. But, infected (COVID-19 patient) sera additionally recognizes S1.
 Activity: SARS-CoV-2-“uninfected” donor sera exhibited specific neutralizing activity against SARS-CoV-2 and SARS-CoV-2 S pseudotypes.
 Which common cold coronaviruses does it resemble more?
SARS-CoV-2 S exhibits marginally closer homology with the spike proteins of beta-coronaviruses (HCoV-OC43 and HCoV-HKU1), than of alpha -coronaviruses (HCoV-NL63 and HCoV-229E).
 Net effects:
(a) HCoV cross-protective immunity is unlikely to be sterilizing or long-lasting, which is supported by repeated reinfection
(b) Prior immunity induced by one human coronavirus (HCoV) can reduce the transmission of homologous and heterologous HCoVs, and ameliorate the symptoms where transmission is not prevented.