Comment by mlyle | Hacker News Reader

Comment by mlyle | Hacker News Reader

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70% is the kind of "default" number. It's based on 1-1/R0. If R0 is 3, then you need 66.7% to no longer be susceptible before cases start to decay. If the vaccine is 95% effective, then you need about 70% vaccinated to get 66.7% no longer susceptible.

But: 1-1/R0 assumes universal, uniform susceptibility and uniform contact graph, which aren't true-- so it's pessimistic. And some people have caught the virus already and are no longer susceptible through that means.

Many computational models put it at about `25% of people infected will result in herd immunity with normal behavior, and the US is presently at about 16%. Unfortunately, getting the "right" additional 9% of people will require vaccinating much more than 9% of the population (we're not as good at selecting the most susceptible and most involved in spreading as the virus itself), but substantially less than 70%.

Further, R0 is based on baseline behavior. I suspect a decent chunk of us will be much more careful for some time, which effectively changes the required decrease in susceptibility to cause a case count decay.

> Latest COVID-19 infection totals come to about 16 million. https://www.nytimes.com/interactive/2020/us/coronavirus-us-c...

Current COVID-19 infection total estimate per CDC is about 53 million infections so far. https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/burd...

Sure, there may be 53M infections, but how many of those untested positives will go on to get a vaccine? This will actually hinder herd immunity, wasting limited vaccines.

> Sure, there may be 53M infections, but how many of those untested positives will go on to get a vaccine? This will actually hinder herd immunity, wasting limited vaccines.

Oh, ideally you wouldn't vaccinate people who are already immune, but saying it hinders herd immunity is a bit silly.

I find it's always helpful to do thought experiments at the limit.

Do you think reaching herd immunity requires more doses A) in a population that is 99% of the way to herd immunity, but that 25% of doses will be wasted because they'll be given to people who are already immune, or B) in a population that is 1% of the way to herd immunity, but only 4% of doses will be wasted in this way?

> Oh, ideally you wouldn't vaccinate people who are already immune, but saying it hinders herd immunity is a bit silly.

If you only have 100M doses and you waste half on undetected positives, that definitely hinders achieving herd immunity. Unless you are saying uncontrolled spread helps develop herd immunity. It does, but it isn't a desirable method.

> but it isn't a desirable method.

Nobody said it was a desirable method. But I did assert that past infections does reduce the doses you need to give to reach herd immunity, which is a point you seemed to argue with.

We'll likely be at >70M natural infections before vaccination efforts hit full steam in mid-January. This is pretty close to the lower end of where models predict herd immunity effects (not ~60% because of non-uniform contact graphs and susceptibility). You don't need a whole lot of vaccination tacked on top of this to start to tamp things down, even though A) some of the vaccinations will be wasted, and B) the vaccinations will not be as "on-target" to break important links in the graph as infection itself is.

> But I did assert that past infections does reduce the doses you need to give to reach herd immunity, which is a point you seemed to argue with.

Sorry for the misunderstanding. I wasn't arguing that past infections change the needed doses, I was arguing that undiagnosed positives waste limited doses. Hope that's clarified.

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