A recently published study in Cell Press is the first I'm aware of that confirms the hypothesis that B and T lymphocytes from people who had a confirmed previous common cold coronavirus infection have a significant reaction against SARS-CoV-2.
The theorized 60 - 70% of people who needed to be infected might need to be revised to a much lower number, perhaps no more than 10 - 20% globally (although with significant heterogeneous distribution, that is to say, some populations will be more vulnerable than others).
This might also explain why it's burning so slowly in some places and so rapidly in others, since it will directly affect the virus' reproductive number.
Edit - it also means if only anyone had a SARS vaccine ready, we wouldn't be going through any of this, since the two are effectively the same virus for all of your immune system's intents and purposes. In a way, SARS completely vanishing, instead of popping up here and there like Ebola and prompting vaccine efforts, was a bad thing in retrospect.
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