Since I’ve been asked about it a bunch today, here’s my take. Herd immunity is a journey not a destination. Asking “if we’ll get there” is just the wrong question, both technically and operationally 1/
Technically, “herd immunity” describes the incremental reduction or risk to everyone of being in a population where an increasing number of people are immune. That journey starts with the first immune person … it’s just not much of an additional push then 2/
The “herd immunity threshold” is not the endpoint of this journey. The HIT is about preventing outbreaks from starting in the first place and is not practically predictive of elimination (due to randomness, heterogeneity, metapopulation effects) 3/
Operationally, predicting the population immunity necessary for elimination during an epidemic is hard bordering on impossible, because the course of the outbreak is responding to what we’re doing ... and will respond to what we do in the future 4/
My field is littered with errant predictions of when we’ll eliminate malaria, polio, measles (that last one was me … and I sat in that big room at the WHO and was dressed down by luminaries in my field) 5/
As I’ve said before here - the herd immunity threshold is better assessed by a historian than predicted by an epidemiologist because the consequences of getting it wrong can lead folks to take their foot off the brakes too early 6/
Rather than focus on an imprecise theoretical target, we should focus on the benefits of vaccines to hasten the journey 7/
Vaccines MIGHT get us to elimination (only one, smallpox, has done that in human history) but WILL get risk down to low enough levels that we don’t have to structure all of society around this virus and we can get back to addressing other issues 8/
We must humanize the benefits of vaccination. Rather than efficacy and herd immunity, we should be talking about protecting ourselves and protecting our communities. Vaccines does this on all fronts - healthy people, healthy populations, healthy economies /x
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