Thursday, September 09, 2021 :::
Some miscellaneous thoughts related to herd immunity, especially as it relates to covid.
- Until at least 18 months ago, the term "herd immunity" was pretty much always used in the context of vaccination, wasn't it? "We need enough people vaccinated against measles to get to herd immunity." More recently you have to attentive to context, and, where context is lacking, try to figure out whether the speaker means something else; a lot of people now seem to refer to "herd immunity" as opposed to vaccination, i.e. enough people get infection and natural acquired immunity that we get to herd immunity that way. Of course, both contribute, though in a highly vaccinated regime as we've had for measles in the last fifty years, especially the last quarter of the twentieth century, there's little natural acquaired immunity because the disease can't really spread, which of course is the point.
- At some point last year some folks were advocating for just trying to seal off the most vulnerable populations and letting covid rip through the less vulnerable populations, and this was much maligned; probably too much. The likelihood that a vaccine would become available seemed to me like a crucial part of the counterargument; in particular, suppose there would be no vaccine possible, and assume away evolutionary dynamics --- I have a response to them, but verbalizing it requires more work than I want to do here. If that's the case, then, once elimination has been ruled out as a possibility, if we're ever going back to normal, it's going to be through herd immunity; if the basic reproduction number is 3, 2/3 of the population will get the disease, and controls just postpone things somewhat. Now, if hospital capacity constraints are an issue, you want to delay things enough to respect that, but ultimately, if 2/3 of the population is going to get infected, to the extent that you can choose which 2/3 that is, it seems that you want to choose the 2/3 who are least "vulnerable" and get them infected --- preferably as soon as possible, so that you can get back to "normal" quickly and because your ability to isolate the vulnerable people is going to be limited, and probably better the less the time you have to sustain it.
- The term "endemic" seems a bit fuzzy; at least in theory, I conceive of it as the end phase in which the fraction of the population that has been infected is more or less stationary. Typically this means that, if that fraction is about 80%, the rate of newly infected people is about 80% of the birth rate. Even with an endemic disease, though, you don't expect exact stationarity; you'll get little outbreaks in various places, and there will be seasonal fluctuations, so on a broad scale at least the fraction probably bobbles up and down a bit. As a practical issue, though, things are even a bit harder, because we can't really measure the fraction of actual infections, let alone tell whether a plateau or slight dip is herd immunity or a pause before the next leg higher. A lot of people seem to use the term in a way that is basically sociological; a disease is endemic when a society has basically decided to treat it the way that that society treats endemic diseases. There perhaps should be a different definition as well, but the sociological one may ultimately be just as important.
::: posted by dWj at 10:55 AM