1. People have a lot of questions about the concept of herd immunity and what it means for COVID-19. What is it, can we get there, what does it get us if do, etc. In this thread, I'll try to answer some of these.

2. Q: What *is* herd immunity?

A: When a large fraction of a population is immune to a disease, a *new* outbreak of that disease can no longer take off and spread. The herd immunity threshold is the point at which enough people are immune for this to be the case.

3. Q: How does it work?

A: An outbreak grows from one case to many when each person who is infected transmits disease to more than one other person on average.

Conversely, if each person transmits to fewer than one person on average, an outbreak will quickly fizzle out.

4. Imagine a disease where each person transmits to two other people when no one is immune (everyone is susceptible). Now vaccinate 75% of that population so that only 25% are susceptible. In this case, each infected person transmits to two people, but both may be immune.

5. On average each infected person will then transmit disease to 2 × 0.25 = 0.5 susceptible others. At this rate, the infection quickly fizzles out.

That's the idea behind herd immunity. Vaccinate enough people that on average each case generates fewer than one subsequent case.

6. Q: Some people are immune because they were vaccinated; others because they had the disease and recovered. Do both count toward herd immunity?

Yes. What you are trying to do is get the number of immune people up to the "herd immunity threshold".

7. While it's safer to get there by vaccination, "natural" and vaccine-induced immunity both have the effect of reducing the number of susceptible people in the population.

8. Q: Roughly what it is the herd immunity threshold for COVID-19?

A: That depends on the basic reproduction number R0 that everyone talks about. R0 is the number of people infected, on average, by a single contagious person in a wholly susceptible population.

9. As a rule of thumb, the herd immunity threshold (HIT) is reached when at least a fraction (1-1/R0) of the population are immune. So if R0 is 2, we need at least 1/2 of the population immune. If R0 is 3, we need at least 2/3 immune. And so forth.

10. There are various refinements on this rule of thumb depending on exactly how people in the population contact one another, and other details.

For COVID-19, R0 is probably around 2.5 to 3.0, though it may be higher for some of the new variants of concern such as B.1.1.7.

11. This puts the herd immunity threshold somewhere in the range of 60-70% for the earlier SARS-CoV-2 strains. It could be 80% or even higher for some of the new highly transmissible variants including B.1.1.7 and the likes.

12. Q: So have we reached herd immunity yet?

A: If we are talking about herd immunity in a population where no one takes any precautions, probably not. Right now cases are declining in some places, but this is in part because of closures, mask-wearing, social distancing, etc.

13. Q: Can we reach herd immunity at all for COVID?

A: In principle, yes. If we can reach a point where 70-80% of the population is immune, we have a good chance to achieve herd immunity.

14. Q: Once we reach herd immunity, can we relax all control measures and go back to life-as-in-2019?

A: Not quite. Herd immunity will be necessary for living like 2019 (if we want to do so without COVID around, anyway). But it's not sufficient.

15. Before we can relax all our controls we also want to have a low number active cases. Recall that herd immunity is the point at which a new outbreak cannot start from scratch. It is not the point that an ongoing outbreak is over. I address that here : https://twitter.com/CT_Bergstrom/status/1378741102476337155

16. If we continue control measures once we reach herd immunity, the number of cases will rapid decline and we can soon relax. If we relax as soon as we reach herd immunity, cases will only very slowly decline and many unnecessary infections will occur.

17. Q: What about escape variants? Won't they prevent us from reaching herd immunity?

A: I'm optimistic that we'll be ok on this front. I haven't seen any compelling evidence that the current variants of concern (P1/2, B.1.351, etc) escape the US-authorized vaccines.

18. While sera from vaccinated individuals do exhibit reduced activity against some of these strains, my guess is that the immune response remains sufficiently strong even against these variants to prevent disease.

19. Even if escape variants do arise, we can readily incorporate these variants into booster shots. There is no reason why with concerted effort, we shouldn't be able to stay out ahead of any evolving escape variants with a proactive program of vaccine boosters.

20. Q: Immunity from COVID infection or vaccines may not last all that long? Will waning immunity prevent us from reaching herd immunity?

A: Of course we don't know yet, because we haven't had enough time to find out.

21. That said, my guess is that we'll get at least a couple of years of immunity from natural infection, and at least 3-5 years from vaccination. If so, that generates a modest schedule of booster shots not that different from the tetanus vaccine.

22. Q: You said "in principle" we can reach 70-80% immune. Why did you qualify that?

A: Three reasons. The first two, escape variants and waning immunity, I've already addressed. The third, and I think the big one, is vaccine hesitancy.

23. Without vaccine hesitancy, we'd be in really good shape. We have Pfizer and Moderna with <90% efficacy and J&J somewhere in the 66-75% range. Any reasonable mixture of these, applied across the entire population, would confer more than 80% immunity and would do the trick.

24. With vaccine hesitancy, it could be close here in the US.

I'm hoping that much of the hesitancy we see is really more like what @DrMelissaClarke describes as vaccine deliberation. A lot of people feel their communities have not been so well served by US healthcare.

25. Understandably, they would like more information *that they consider trustworthy* before deciding what to do. As the vaccines prove themselves safe and effective, and as we improve health equity, those deliberating about what to do may decide to take the vaccine.

26. Q: If we don't reach herd immunity, what would it look like?

A: I suspect we'd see some seasonality, much as we do for influenza. Minimal cases in the summer, followed by a winter wave. With vaccination, most people will be protected from serious illness at the very least.

27. Ongoing boosters or "updates" would remain essential, though. Notice also that even if we reach herd immunity in the US, disease is a global problem. Without aggressive vaccination programs worldwide, we'll face continual reintroductions into the US.

28. Unmitigated spread worldwide also offers abundant opportunity for escape variants to arise. Ultimately the best way to ensure COVID safety in the US will be to take care of the US — and the same time recognize and address the global nature of the problem.

29. Q: If we have a safe, readily available vaccine, who cares whether we reach herd immunity or not? If people don't want the vaccine, that's their problem.

A: Recall that not everyone can safely take the vaccine, and not everyone has an immune system that will respond to it.

30. One very important function of herd immunity for diseases such as measles is that by vaccinating the majority, we are able to protect a minority for whom vaccines are not safe or effective. For many diseases this includes *all* young infants.

31: Q: I asked a question in the replies and you didn't answer. Do you hate me?

A: Unlikely. I probably just don't know the answer, and want to defer to those who do.