There is increasing to reference to this as the desirable thing we need that lockdown is preventing. I am not well-versed in this, and am happy to be corrected. Okay, not happy, exactly, but willing. Is our faith in this justified? Is this true, or some sort of magical incantation? I know that some amount of herd immunity happens when a contagious illness is present in a population, but isn't it often...limited? In colonial New England, when a hantavirus from Europe struck, 90% of the natives dies, but only 30% of the English. I guess that's better, but that doesn't mean that it's good thing and we just move on? That's the best strategy we can aim for? Is herd immunity what got us through smallpox? Because Africa had some herd immunity to malaria while the Europeans trying to penetrate the jungle just died, does that mean all was well for the Africans? Polio? We have MMR vaccines for children precisely because herd immunity over centuries didn't actually solve the problem. It's better than nothing, I suppose, but...?
The assertion that we just have to hurry along because we want to get to herd immunity sooner, even though we don't know how much "immunity" that means strikes me as ill-founded. I suspect that the fact that the word "immunity" is right in the phrase allows us to slip into thinking it means "very manageable disease presence." I don't think we know that. There is a common argument arising that "well, we don't really have any choice. Whatever we do is just going come out the same in the end until we just gird up our loins and allow Herd Immunity to happen."
Sounds great. Is it actually true? I understand that it will be true in a limited sense, but doesn't the variance matter?
You know the model right? A virus will tend to spread if the number of new cases per existing case R is greater than one, and will die out if it is less than 1. If R0 is the rate of transmission in a naive (nobody immune) population, then to get R less than one the immune fraction has to be at least 1 - 1/R0. Foe example, if R0 is 3, then the immune fraction has to be at least 2/3 to keep the virus from continuing to spread.
ReplyDeleteOf course, the idea that R0 is purely a characteristic of the virus and the same everywhere is an oversimplification. But the central insight that a virus will spread if the number of new cases per existing case is more than one has to be correct. So there are only two ways to end an epidemic: get R below 1 and keep it below 1, or absolutely completely separate the contagious from the uninfected. The second choice is pretty much impossible when you've got a million contagious yet asymptomatic carriers walking around.
Sub-saharan Africans have resistance, but not immunity, to malaria. Malaria kills there too; not as frequently as it kills non-African populations in the same situation, but frequently enough to be a scourge.
ReplyDeleteWe can't know much about the herd immunity theory until we've established antibody persistence. The common cold (another strain of coronavirus) gives you immunity for 3-4 months, then it fades. If this acts like that, we won't even start to know until another few months go by, as I think the oldest cases are just hitting the 4 month mark now.
ReplyDeleteIf it turns out everyone who got sick this spring can get sick again this fall (and might be in worse shape for it), this won't work.
I think *true* herd immunity requires greater than 95% immunity. The problem with C19 is that we still don't know if it will turn out like measles, with an outer coat that doesn't change much so one vaccine and you're protected, or if it's going to be like influenza and change its outer spikes so frequently that you need a new vaccine every year and even then you have only limited immunity. In the latter case I don't think that there can be real herd immunity. I suspect that once we start pouring antiviral medications and vaccines on it, that will force it to evolve and we will create the influenza-like scenario.
ReplyDeleteOn the plus side, this is focusing attention and funding on attempts to discover universal antiviral medications and vaccines that target virus parts that don't change as easily.