Recent preprint reporting that 24/24 (100%) people form Singapore infected by SARS-1 in 2003 have pre-existing T-cell immunity against #SARSCoV2, but more surprisingly 9/18 (50%) with no exposure to SARS-1 also possess T-cells targeting #SARSCoV2.
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https://tinyurl.com/yauc42as 

One take home message is that infection with coronaviruses induces strong and long-lasting T-cell (cross-)immunity. T-cell immunity is likely a far more important for our immune response to #SARSCoV2 infection than antibodies, in line with other recent reports.
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What remains unresolved is which virus caused T-cell immunity in the people with no prior exposure to SARS-1 in 2003. We know of seven coronaviruses infecting humans: #SARSCoV2, SARS-1, MERS and four causing 'common colds' (OC43, HKU1, 229E and NL63).
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Intriguingly, none of the known viruses in circulation in humans looks like a good candidate for the T-cell immunity to #SARSCoV2 in those with no prior exposure to SARS-1. This might suggest that other yet unknown coronaviruses could have been in circulation in humans.
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