For all those who like to dismiss papers when they are either not yet peer-reviewed or published on journals that are not exactly mainstream, here is a paper published in Nature:

SARS-CoV-2-specific T cell immunity in cases of COVID-19 and SARS, and uninfected controls

Fig. 4

In short:

  1. Covid-19 infection obviously creates T-cell immunity which long lasting and will protect people who recovered from ever getting Covid-19 again
  2. People who caught SARS-CoV-1 (the first SARS) also have long lasting T-cell immunity (still working 17 years later) that works against Covid-19 (SARS-CoV-2)
  3. Roughly 50% of the tested subjects who never caught either SARS viruses also had T-cell immunity that worked against Covid-19 due to a high degree of homology to the sequences of some proteins common to both SARS-CoV-2 and the common cold coronaviruses.