Now, those folks will say "But look at NYC!" where the caseload has been low and steady. This sustained recovery must be herd protection, they argue.

But based on the simple math behind herd immunity, this is a fantasy h/t Virginia Pitzer from @YaleSPH https://www.nationalgeographic.com/science/2020/10/natural-herd-immunity-mentality-cannot-stop-coronavirus-weak-vaccine-cvd/
Likewise, if Sweden has achieved sufficient herd immunity, then their cases wouldn't be increasing right now at basically the same pace as eight months ago. https://ourworldindata.org/coronavirus-data-explorer?zoomToSelection=true&time=2020-02-22..latest&country=~SWE&region=World&casesMetric=true&interval=smoothed&smoothing=7&pickerMetric=location&pickerSort=asc
Ok, when most people say "herd immunity," they mean the herd immunity threshold (HIT).

It's the percentage of a population that must be immune to stop a disease's spread.

Paul Fine at @LSHTM explained this simple way to think about it, and I went 🤯🤯 https://www.nationalgeographic.com/science/2020/10/natural-herd-immunity-mentality-cannot-stop-coronavirus-weak-vaccine-cvd/
. @kennelliott built this cool interactive so you can see the herd immunity threshold in action.

Achieve it, and enough people in your community will be shielded so that a germ can't sustain an outbreak.

It can also set our desires for a COVID-19 vaccine https://www.nationalgeographic.com/science/2020/10/natural-herd-immunity-mentality-cannot-stop-coronavirus-weak-vaccine-cvd/
Related: @kennelliott is the best!
Now, when you see "weak vaccine" in a headline, you might think "Oh no, Nsikan has gone anti-vax."

I haven't, but the FDA's minimum goal for vaccine approval (50% efficacy) couldn't establish herd immunity—even if everyone took it 😯😲
https://www.nationalgeographic.com/science/2020/10/natural-herd-immunity-mentality-cannot-stop-coronavirus-weak-vaccine-cvd/
Don't misunderstand me. A vaccine with 50% efficacy could still spare hundreds of thousands from hospitalizations, chronic health issues, and death, as @bruce_y_lee explains.

But the sweet spot for saying goodbye to masks/social distancing would be 80%. https://www.nationalgeographic.com/science/2020/10/natural-herd-immunity-mentality-cannot-stop-coronavirus-weak-vaccine-cvd/
Professor @Bruce_y_lee's lab ( @PHICORteam) also provided the data behind @kennelliott's interactive, which I'm sharing for a third time because it's awesome. https://www.nationalgeographic.com/science/2020/10/natural-herd-immunity-mentality-cannot-stop-coronavirus-weak-vaccine-cvd/
Ok, you may have heard of some models claiming the herd immunity threshold could be as low as 40% (or even 10%) through regular ol' infection, due to something called heterogeneity.

But those models are just that, according to their authors.
https://www.nationalgeographic.com/science/2020/10/natural-herd-immunity-mentality-cannot-stop-coronavirus-weak-vaccine-cvd/
For sure, the herd immunity threshold will vary from city to city. It might be higher in some, lower in others, says Jeff Shaman of @ColumbiaMSPH

But we're seeing some signs from rapid outbreaks that the simple equation for COVID's threshold might be 🎯 https://www.nationalgeographic.com/science/2020/10/natural-herd-immunity-mentality-cannot-stop-coronavirus-weak-vaccine-cvd/
One of the report's co-chairs is William Foege. He's best known as a former CDC director.

But when he was 30 years old, and serving as an EIS officer in Nigeria, he helped devise a vaccination strategy that eliminated the final strongholds of smallpox. https://www.nationalgeographic.com/science/2020/10/natural-herd-immunity-mentality-cannot-stop-coronavirus-weak-vaccine-cvd/
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