As you've probably heard, we're getting close to having #CovidVaccines authorized in western countries (and they have been in the UK). It's tantalizing to think that life will be "normal" again soon, but there's a lot of factors to consider—a thread 1/x https://qz.com/1941409/how-many-people-need-to-be-vaccinated-for-a-return-to-normal/

2/x For simplicity's sake, let's assume "normal" means people have been vaccinated to the point of herd immunity, everything is open, travel is fine (but expensive, like the good ol' days).

3/x Okay: Herd immunity means that enough people are sufficiently immune to an infection that it can't be transmitted to others. How many vaccinated people will that take? Well, it depends on two factors:

4/x 1) How well does the vaccine actually work in practice? We've only seen press releases from various companies about the vaccine's EFFICACY in clinical trials. Real life is messier than clinical trials! People may not get doses on time, which could give folks less protection.

(4.5/x -- for background on efficacy in clinical trials vs real-world effectiveness, see here: https://qz.com/1930285/what-does-90-percent-effective-mean-for-pfizers-covid-19-vaccine/)

5/x Next, we gotta ask: Does the vaccine actually stop transmission? We don't know. It should! Bc it should make it hard/impossible for SARS-CoV-2 to replicate, making it less likely to shed. But trials have only examined if people got Covid-19 so far.

6/x Assuming each person infects between 2 - 3, more people (R0 for all you nerds out there), and assuming effectiveness is similar to demonstrated efficacy, we could assume ~60 - 70% of the population needs to be vaccinated for herd immunity, @Sms3Steven told me

7/x BUT that vaccine needs to be equitably distributed! You can't have your 70% of a population all in one place!! And that's a whole other can of worms to open; we know there won't be equitable access to Covid-19 vaccines across countries/the globe.

(7.5/x

See: https://qz.com/1907302/us-china-and-russia-opt-out-of-covid-19-vaccine-global-access-plan/

https://qz.com/1937072/the-countries-that-have-already-ordered-enough-covid-19-vaccines/

https://qz.com/1903327/gates-hoarding-covid-19-vaccines-will-extend-the-pandemic/ )

8/x And then there's the fact that herd immunity is a nice idea and all, but theory isn't the same as real-world transmission. THOSE metrics, like community rates, new daily cases, and hospitalizations, should be the ones to inform public health decisions...

9/x ...Like loosening mask mandates, reopening schools/bars, and allowing mass gatherings. Community rates SHOULD decrease as vaccines get rolled out, and we should be able to learn about their actual effectiveness—but we can't assume on vaccination rates alone.

10/10 So anyway, that's a lot more unknowns than we'd like. But at least we've got metrics that we can watch moving forward: https://qz.com/1941409/how-many-people-need-to-be-vaccinated-for-a-return-to-normal/