Excellent article on the implications of "T-cell immunity". Important thing to note for the #COVIDIOT types claiming it means Herd Immunity is closer is that any existing "T cell immunity" is *already baked in to the figures*. I'll explain quickly below. https://twitter.com/profshanecrotty/status/1313580981341712386

1. How well a virus spreads is measured by what's called the Basic Reproduction Number, or R0. This is estimated by observing the rate of infections in a population, and then tweaked as more data comes in.

2. Later you may calculate Rt or Re which reflects how this has changed over time due to more people being infected, people being vaccinated, the effect of "non-pharmaceutical interventions" (NPI) such as social distancing and masks.

3. R0 for Sars-COV-2 is estimated at abt 2-3, which means *on average* each infected person infects 2-3 people. "clustering" means that average isn't actually a good measure, because instead of each person infecting 2-3 it may be 1 person infect 6-9 and 2 others infecting nobody.

4. If a population (say China or Italy, where are first data came in) has an existing level of T-cell immunity, that will *already be incorporated* in the calculations estimating R0

That figure of R0 is then used to calculate the level of infections needed to reach herd immunity

5. Suddenly discovering cross-reactivity T-cell immunity may be occurring *doesn't change* this calculation, which is based on *observed data*. We now just understand the data better, like why children may be less affected.

6. Now it may be you have fairly discrete populations that don't mingle much and you want to calculate R separately for them - maybe elderly vs children, and the children have more t-cell immunity. In this case R would be lower for children, and higher for the elderly.

7. The basic formula for the herd immunity threshold is HIT=1-1/R0. R0, remember, is just the initial value of R, at time 0 before people are starting to get immune from having been sick and recovered and before NPI.

If you have a higher R0 then you have a higher HIT. Math!

8. So a group with less T-cell immunity will have a higher R and thus HIT will be higher than the generally estimated 60-70%.

You can't just say "well we discovered 40% of people are immune, so we'll subtract that from the 60-70%".

You need to recalculate.

9. So, no having T-cell immunity does not change the Herd Immunity Threshold. Any pre-existing immunity is built in to the calculation from the start.

/end