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The most likely duration of immunity to a respiratory virus like SARS CoV-2 is multiple years.

Why do I say that? We actually have the data for a virus that swept through parts of the world seventeen years ago called SARS, and remember SARS CoV-2 is 80% similar to SARS, so I think thats the best comparison that anyone can provide.

The evidence is clear: These very clever cellular immunologists studied all the people they could get hold of who had survived SARS 17 years ago.
They took a blood sample, and they tested whether they responded or not to the original SARS and they all did; they all had perfectly normal, robust T cell memory. They were actually also protected against SARS CoV-2, because theyre so similar; its cross immunity.

So, I would say the best data that exists is that immunity should be robust for at least 17 years. I think its entirely possible that it is lifelong.

The style of the responses of these peoples T cells were the same as if youve been vaccinated and then you come back years later to see if that immunity has been retained.

So I think the evidence is really strong that the duration of immunity will be multiple years, and possibly lifelong.

In other words, previous exposure to SARS that is, a variant similar to SARS CoV-2 bestowed SARS CoV-2 immunity.