Thread by ArisKatzourakis on Thread Reader App – Thread Reader App
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[MEDIUM THREAD] On the topic of covid becoming an endemic virus. You hear this a lot; sometimes in response to critiques against strategies of elimination, sometimes just as a statement in its on right. Statements such as “Covid will inevitally become endemic.” 1/n
The problem is, most of the time, I am not convinced people quite understand what endemic means. An endemic virus is a virus where infection levels are maintained at a baseline level without external inputs. Chickenpox is endemic; HIV is not, as it is still spreading. 2/n
Achieving an endemic steady state requires a very particular set of conditions, namely that R(0) X the proportion of susceptible individuals in the population, is equal to 1. This would lead to a steady state, a fixed number of infections, which neither grow nor fall. 3/n
There are reasons to think might not happen for covid, but it is not theoretically impossible under particular sets of conditions. More importantly, if an endemic state is achieved, at what level will it settle? The difference between a high and a low endemicity is vast. 4/n
Just saying ’endemic’ is more or less information free as a statement. It says nothing about the level of prevalence of the virurs, the level of disease at the endemic steady state, or time time (could it be ten years?) taken to reach this hypothetical endpoint. 5/n
Low endemicity is in many ways, actually akin to a policy of elimination. Keeping numbers low enough that serious disease never occurs, would be a fantastic outcome, and in some ways indistinguishable from elimination. A high endemicity? Not so much. 6/n
Would we settle for a constant steady state of high levels of mortality and morbitity? The statement endemic is rather information free in this respect. Furthermore, would endemicity level be homogenous throughout the world? 7/n
Very unlikely. So, if countries settle on different levels of endemicity, through global travel, we will necessarily continue to see epidemic waves. More importantly, ideas around endemicity completely fail to take into account the generation of strains. 8/n
If we properly conceptualise the variants of concern as strains, with distinct prevalences and characteristics (mortality, spread, resistance), around the world, the prospect of some sort of endemic stable state becomes even more remote. 9/n
Whether endemicity, and some sort of stable outcome is theoretically possible, stability is incredibly unlikely for many years to come without suppression, and will carry a tremendous cost. 10/n
Today’s ‘endemicity endpoint’ is not dissimilar to yesterday’s ‘herd immunity as strategy’, if we include endemicity with high disease prevalence. If we can achieve stability, equivalent to attaining an R(t) of 1...11/n
..then we surely can achieve stability at a value just lower than that, without much additional cost, leading to elimination, or at the very least an incredibly low endemicity. 12/n
It would be useful to understand that the two paths ahead, are either suppression on a massive scale, globally, leading to either low endemicity everywhere, or potentially elimination on the one hand, and on the other hand, a heterogenous, fluid, dynamic situation.. 13/n
..with generation of new strains with unpredictable characteristics, likely eventually including vaccine escape, with distinct prevalence across the globe, and waves of epidemics for many years to come. 14/n
This is the future if we do not go for maximum suppression, not some stable endemic state, at least not in timescales that are relevant to public health outcomes. 15/n