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unless there's some mutation (no evidence yet to suggest this will or won't happen)
there is some evidence of a new mutation from Spain.
A coronavirus variant that originated in Spanish farm workers has spread rapidly through much of Europe since the summer, and now accounts for the majority of new Covid-19 cases in several countries — and more than 80 per cent in the UK.
sohi-
OK, maybe I should have said "significant mutation". Mutations in viruses happen all the time, and there are a whole boat load of different variations of SARS-Cov-2 going round at the moment.
The danger is that it mutates in such a way that the existing immunity becomes less useful (or even useless).
Most of the time a significant mutation makes it less dangerous overall as it does something like lessen the time between infection and onset of symptoms (which makes it less likely to be spread) or reduces the percentage of asymptomatic cases (which, again, makes it less likely to be spread).
Greenbank
120k predicted deaths at 0.25% CFR would mean 48 million people getting it. Combined with the first wave that's pretty much everyone in the country who would have been exposed (except for those who are shielding superbly well).
Doesn't leave much scope for a 3rd or 4th wave unless there's some mutation (no evidence yet to suggest this will or won't happen) or if the acquired immunity isn't enough to ward off a bad subsequent case. (Some people will get various viruses multiple times, but that doesn't mean a significant proportion of the population will.)
Many more people <80yo will die in this wave though as the demand for hospital beds will greatly outstrip supply and so there will be some avoidable deaths. Even if many of those survive there will be a lot more 'long Covid' sufferers after this wave.