I spent a bit of time over the weekend reading research literature on hybrid immunity. Specifically for…

I spent a bit of time over the weekend reading research literature on hybrid immunity. Specifically for Covid, but also any other reference to it. A very contentious topic, and reading the literature you could come away being convinced it's an unproblematic thing. /1

Reading Christina Pagel's assessment for the UK over their Christmas period, you could believe that population immunity is building, most of the metrics suggest a great improvement on previous years. And maybe that is what's happening. /2 https://t.co/G3t72GIboG

As a layperson looking in, to see very smart people on both sides of this debate offering detailed arguments, it's extremely difficult to make sense of it. Purely on logic grounds, there's the obvious (and likely deliberate) conflation of individual and population immunity. /3

Not my area of expertise, but in my understanding this is where there seems to be some sleight of hand going on. It seems uncontroversial to say that an individual's immunity can build over time, through a combination of infection and vaccination. /4

It's another thing to claim that this creates a population-level effect, any sort of herd immunity, where the virus encounters enough hybridly immune individuals to slow its progress. All often discussed , I know, but often glossed over as well. /5

Because an individual's immunity trajectory can be so varied, through the timing of any infections and vaccinations, and the need to get some sort of coordination in time of this immunity across a population to create a herd effect, it's hard to see how any of this lines up. /6

How you get herd immunity, using hybrid immunity. Unless you spend a long time (generations?) vaccinating and infecting everybody, so that maybe a threshold of immune protection is reached. Except we're not vaccinating everybody, and those we are aren't doing it regularly. /7

It's not even clear whether hybrid immunity as a strategy adopted by multiple countries is intended as a herd immunity strategy as well. The public narratives often suggest that, but it's more implied than explicit. And yes, lots of memes to say hybrid immunity doesn't exist. /8

Though the literature clearly suggests otherwise. Suggesting that individual hybrid immunity leads to herd protection seems more the sticking point. Or maybe governments don't even care about herd protection, they think individual hybrid protection is the best we can do. /9

But again, it's not really clear, and probably deliberately not. It seems as many have said more like an attempt to fob people off with vague associations with ideas of herd immunity, to make them feel like this will eventually free the community from constant infection. /10

But again, not my area of expertise. How do we explain what seem like clear descending rates of severe illness and death at the population level, in places like the UK and even here in Oz? We've already killed off most of the dry tinder? 'Reaping'? /11

Leaving a constant sea of infection that will never resolve, but which may kill fewer people? People who were likely not going to get seriously ill or die anyway, in the acute phase of Covid, so what appears like growing immunity may just be fewer 'vulnerable' victims? /12

Though in an ageing population that pool of vulnerability should actually be growing. My own thinking about this literature over the weekend made me think about the things we can't know. For example, we might say we have some degree of population/herd immunity to influenza. /13

But how long did it take us to achieve that, if it's true? How many died and were maimed getting there? Public health statistics don't go anywhere near far enough back to answer that, surely. Other experts say herd immunity is only ever a vaccination outcome. /14

I think mostly because of the timing considerations I mentioned above, getting widespread immunity at roughly the same time, to slow and prevent spread. That seems uncontroversially logical to me. And influenza clearly still has epidemic and pandemic outbreaks. /15

So any population-level immunity will only be useful provided the pathogen hasn't mutated significantly. When it does, you're back to square one. I still don't know what it all means, but doubt that any immunity strategy that involves 'natural' infection makes sense. /16

Because of the completely contingent nature of those infections. We do seem to have locked in a strategy of permanent infection and illness, with our individual hybrid immunity our only defence against it. /17

And because many aren't getting vaccinated, it really is difficult to see how this is hybrid anything, anyway. Just more political spin to excuse doing nothing. A pandemic is still defined in terms of number of infections, by my reading. Not by what they do to people. /end