Thread by @JamesWard73

A few people have asked how we managed to have such a stable period in England during August, with cases apparently stuck around 25k per day, and R pretty close to 1. After all, case rates in an epidemic tend to go up and down in waves, not stay at roughly the same level. 1/10

One possible answer is that the growing immunity from vaccines and infections (which would normally be expected to pull R and cases down) was being offset by waning of the existing stock of immunity, causing case rates to stabilise in a sort of bumpy plateau. 2/10

If that is the case, then August could be seen as an early glimpse of what endemicity might look like, although I don’t think we’re near to finding a long-term equilibrium, and there are several good reasons why August isn’t a good predictor of what that will look like. 3/10

But let’s explore the numbers for a second. We know there were around 25k cases per day, and if we make the rough assumption that we find around half of actual infections, that implies around 50k actual infections per day. (feel free to insert your own assumptions here). 4/10

Hence we can estimate there were ~1.5 million infections in August, or 2.5% of the population of England. There were also about 5 million vaccinations in the month– but that’s a mix of first and second doses, neither of which gives 100% protection vs. infection, and some… 5/10

…of those people will already have had decent levels of immunity via prior infection. My model estimates that those 5 million doses are equivalent to about a million people being fully immunised, or 2% of the population. Adding infections, that makes 4-5% newly immune. 6/10

Now if R was stable during August, and we assume (NB big assumption here, quite probably wrong) that other factors such as behavioural caution and weather didn’t change much, that new 4-5% of immunity must have been roughly balanced by a 4-5% loss of immunity via waning. 7/10

My model thinks there was about 70% total immunity at the start of August, so that 4-5% loss is actually a 6-7% waning rate per month. Which is remarkably consistent with various studies (eg PHE khub.net/documents/1359…) suggesting a waning rate of about that level. 8/10

Please note it’s likely that behaviour etc. did change during August, so this calculation is unreliable. But I think it’s interesting that the waning rate implied by PHE is pretty much exactly what you need to explain the behaviour of case rates in England during August. 9/10

Coincidence? Quite possibly. There are hints in other studies (eg nejm.org/doi/full/10.10… ) that the real waning rate is about half that quoted above, and we can’t really tell from this data. Only time, and more data, will tell. 10/end.