saw this study about how little protection prior infection gives against reinfection with Omicron.
https://www.imperial.ac.uk/news/232698/omicron-largely-evades-immunity-from-past/
Any thoughts Spud?
Yes I think it's to be expected. Imagine for a moment that I'm writing an exam essay on why naturally acquired immunity appears not to be permanent. With my limited knowledge and using the models explained by Geert, I can at least have a go.
In theory, and perhaps historically, the third wave of a pandemic in which minimal intervention measures are implemented would be characterized in its latter stage by more infectious variants that can overcome the innate antibodies in the fittest of the population who so far have only been asymptomatically infected, leading to disease in those people.
I imagine that further global waves of infection would in that scenario be prevented by the fact that the whole process would happen in a much more condensed time frame than what we have seen with Covid-19. In that scenario: while symptomatic disease is increasingly occurring in people of increasing fitness, there is a build-up of long-lasting immunity. As we know, this immunity is depleted after a year or two, but it's there long enough to see in 'herd immunity'. The reason we have had a prolonged pandemic is because the lockdowns have prolonged the periods between waves, and mass vaccination has (I think) put extra evolutionary pressure on the virus.
Concerning the latter, I imagine the fitness cost of becoming more infectious would under non-vaccination circumstances prevent these variants from becoming dominant. But with mass vaccination, a more infectious variant is the only one that can be transmitted from one vaccinated person to another vaccinated person.
Natural selection theory would suggest that just because a more infectious variant can survive in an environment of high immune pressure, it doesn't follow that it would survive better without that pressure. Dominance of more infectious variants would occur when the immune pressure increases due to lock down and vaccination, but without that increase, maybe less infectious strains would have continued to be dominant.
Maybe there is some fitness cost of increased infectiousness such that a more infectious virus wouldn't be as efficient as less infectious strains in the absence of the high immune pressure generated on top of natural immunity by vaccines?
Edit: I meant to say that this would explain why naturally acquired antigen-specific immunity to coronaviruses lasts for the length of time it does. Historically the high levels of antibodies were only needed for long enough to ensure herd immunity.
Edit 2: if a voc emerges in a region where there are high levels of suboptimal immune pressure, the 'fitneas cost' phenomenon might prevent spread outside that region where there is less immune pressure.