I don't usually tweet threads, but I wanted to give 8 reasons why we should consider shifting language away from "herd immunity threshold :: Rt<1".

1. Rt changes every day, and humans have a strong tendency to push Rt toward 1.0. If case counts are rising, pressure builds to restrict activity more. If they are falling, people want to live their lives. The idea of a "constant" Rt is a fantasy.

2. The target Rt depends on the current epidemic level. If you have 1 case per 10 million, an Rt of 1.01 will be fine. If you're FL, GA, Mexico, etc., and not aiming for Rt of ~0.8, a lot of people are going to die in the coming months. The goal is epidemic control, not Rt<1.

3. The use of the term "threshold" suggests that something magical happens at Rt<1.0. But going from 1.25 to 1.05 is a major achievement, as is going from 0.95 to 0.75. No matter what your current Rt, lower is better.

4. Population immunity is only one of many factors that affects Rt - and not the most important. Policies, individual behaviors, social components all currently play a bigger role. Focusing on herd immunity causes people to lose focus on today's big picture.

5. The more population immunity, the better. If you have 5% population immunity, this will reduce your Rt by 5% (on average), even if you aren't at a "herd immunity threshold." If 20% of people are immune, Rt will be 20% lower. Every little bit does count.

6. People tend to think about herd immunity for COVID in terms of natural infection, not vaccines. On some level, even talking about reaching a "herd immunity threshold" gives the indication that we should be trying to achieve this (via natural infection).

7. There is no one "herd immunity threshold." Since every population is different (in structure, social interaction network, activity restriction, etc), each population will need a different level of immunity to achieve the same epidemic trajectory.

8. We are still far, far away from reaching any sort of global herd immunity threshold. (Maybe in certain locales, but people move around.) If we talk about this as something that can be achieved in the coming months (without a vaccine), we delude ourselves and our listeners.

Bottom line: Population immunity plays a (relatively small) role in the epidemic trajectory. But IMHO, we should stop talking about reaching a herd immunity threshold or achieving Rt<1, and instead focus on what it will take to control each local pandemic.
Thx for reading.