Thread by @WesPegden

A short thread on why I think it is important to engage seriously with analysis of herd immunity strategies regardless of whether you think we should be pursuing them right now. (Short version: understanding them may help prevent the most COVID deaths in the long term). 1/7

The first thing to emphasize is that not all herd immunity is the same. The pandemic flu of 1918—where death rates varied among U.S. cities by a factor of 3 depending on how they implemented mitigations, even though all reached herd immunity—is a historical demonstration. 2/7

For COVID in particular, the orders-of-magnitude variation in the mortality rate by age (anti-correlated with transmission rates) makes it plausible that we can have an order-of-magnitude effect on mortality and morbidity even within the context of reaching herd immunity. 3/7

But wait. Isn't it better to prevent all of those deaths, through containment measures like test+trace?

Definitely! If we have high confidence they will work. But if there is a realistic chance containment will falter after e.g., a year, and transmission rates increase... 4/7

... in chaotic ways, then we would risk reaching herd immunity "by accident", which, again, could be an order of magnitude worse than reaching it with targeted strategies.

The current high compliance with mitigations is a precious resource for affecting the herd! 5/7

Actual responses will depend on a mix of ideas. Immunity phenomena give MORE promise to the utility of measures like test+trace: At first glance, Singapore's experience with test+trace has been disheartening, despite apparent structural advantages in climate, gov, geography...6/7

But if we take seriously the idea that heterogeneity can make immunity matter at thresholds significantly lower than suggested by simplistic homogeneous models, it should increase our confidence that test+trace can be more effective in places with higher levels of immunity. 7/7