One factor I’ve heard discussed is the UK’s very high rate of prior immunity (from both infection and vaccination). About 90% of adults in the UK have COVID antibodies (either from vaccination or infection), meaning the fully susceptible population is very small. Obviously breakthrough cases are possible in people with antibodies, especially if they’ve only got one dose of the AZ vaccine, but R in that population will be much lower. Given the high initial rates of prior immunity, it would be expected that R will drop pretty quickly as COVID “buns though” the remaining susceptible population.
Sure, the UK had high vaccination rates going into this wave, but the emergence of the delta variant (plus loosening of restrictions) greatly increased COVID transmission. So you’d expect a growth in case numbers. But if there aren’t that many fully susceptible people to infect, the case counts wouldn’t peak at a high number before turning around because of population immunity.
However, I want to be clear that I think this is just one factor, not the only thing going on. If you play around with SIR model parameters, you can see that you can’t get quite as steep a drop in cases as there appears to be in the UK data just by inputting some reasonable values for delta. Changes in the number of tests, weather, behavior, etc. are all potentially playing a role as well, but I don’t think we should discount the basic role of immunity.
R fell from something like 1.4 to 0.7 in a week. If we would be talking about a change from 1.2 to 1.1 in a week basic immunity seems like a reasonable explanation. For the change we are seeing it doesn’t seem to be.
Could this be something of statistical a mirage? (and hopefully this is not too poorly expressed or thought out as it’s very much an off the cuff type thought. It’s also really just a slightly different statement of the above explanation.)
I don’t know if this hypothesis comes close to fitting with the reality in the UK but what if one is looking at general, aggregate statistics but the cases are largely in some “unique” sub populations.
If there were pockets where previously few people were infected, were taking their time (or were anti-vaccers) and then delta hit those areas. Previously COVID was spreading slowly in such areas. Now it starts spreading quickly. The aggregate measures suddenly turn up but as soon as those sub-populations start to look like the general population you see a very rapid drop in transmission and in the aggregate numbers.
Putting it a bit differently, is it maybe one of those devils in the details, but not necessarily a factor that we can really do much with other than note it needs to be kept in mind at times?
I agree that this seems to explain it, but it raises a new question: how did the antibody rate get so high? Is it possible that part of Delta’s contagiousness is that it has a lot more carriers who don’t get sick?
I actually don’t think the high level of antibodies should be such a surprise. I updated my original comment to clarify, but much of that is from vaccination, not from natural infection. Between high rates of vaccination plus historical infections, it’s not surprising to me that such a high fraction of adults in the UK have antibodies.
I think I agree. I hadn’t realized the UK vaccination rates were so high. In that case I’ll lean towards the pockets of unvaccinated reaching herd immunity + shorter incubation period hypothesis.
Unfortunately for this explanation, COVID cases in the UK appear to be rising once again. At this point I’m at a loss for potential explanations of what could have caused the rapid rise, fall, and rise again in cases.
One factor I’ve heard discussed is the UK’s very high rate of prior immunity (from both infection and vaccination). About 90% of adults in the UK have COVID antibodies (either from vaccination or infection), meaning the fully susceptible population is very small. Obviously breakthrough cases are possible in people with antibodies, especially if they’ve only got one dose of the AZ vaccine, but R in that population will be much lower. Given the high initial rates of prior immunity, it would be expected that R will drop pretty quickly as COVID “buns though” the remaining susceptible population.
I don’t see how that explains the drastic shift. The UK also had high vaccination rates in the beginning of July.
Sure, the UK had high vaccination rates going into this wave, but the emergence of the delta variant (plus loosening of restrictions) greatly increased COVID transmission. So you’d expect a growth in case numbers. But if there aren’t that many fully susceptible people to infect, the case counts wouldn’t peak at a high number before turning around because of population immunity.
However, I want to be clear that I think this is just one factor, not the only thing going on. If you play around with SIR model parameters, you can see that you can’t get quite as steep a drop in cases as there appears to be in the UK data just by inputting some reasonable values for delta. Changes in the number of tests, weather, behavior, etc. are all potentially playing a role as well, but I don’t think we should discount the basic role of immunity.
R fell from something like 1.4 to 0.7 in a week. If we would be talking about a change from 1.2 to 1.1 in a week basic immunity seems like a reasonable explanation. For the change we are seeing it doesn’t seem to be.
Could this be something of statistical a mirage? (and hopefully this is not too poorly expressed or thought out as it’s very much an off the cuff type thought. It’s also really just a slightly different statement of the above explanation.)
I don’t know if this hypothesis comes close to fitting with the reality in the UK but what if one is looking at general, aggregate statistics but the cases are largely in some “unique” sub populations.
If there were pockets where previously few people were infected, were taking their time (or were anti-vaccers) and then delta hit those areas. Previously COVID was spreading slowly in such areas. Now it starts spreading quickly. The aggregate measures suddenly turn up but as soon as those sub-populations start to look like the general population you see a very rapid drop in transmission and in the aggregate numbers.
Putting it a bit differently, is it maybe one of those devils in the details, but not necessarily a factor that we can really do much with other than note it needs to be kept in mind at times?
I agree that this seems to explain it, but it raises a new question: how did the antibody rate get so high? Is it possible that part of Delta’s contagiousness is that it has a lot more carriers who don’t get sick?
I actually don’t think the high level of antibodies should be such a surprise. I updated my original comment to clarify, but much of that is from vaccination, not from natural infection. Between high rates of vaccination plus historical infections, it’s not surprising to me that such a high fraction of adults in the UK have antibodies.
I think I agree. I hadn’t realized the UK vaccination rates were so high. In that case I’ll lean towards the pockets of unvaccinated reaching herd immunity + shorter incubation period hypothesis.
Unfortunately for this explanation, COVID cases in the UK appear to be rising once again. At this point I’m at a loss for potential explanations of what could have caused the rapid rise, fall, and rise again in cases.