Thread by @adamjkucharski

Suppose we have a SARS-CoV-2 variant that is inherently more transmissible, and another that is more likely to reinfect people who've previously developed immunity. Which will spread more easily? A thread... 1/

We know we can measure transmission using R, but it helps to break R down into four components - duration, opportunities, transmission probability and susceptibility - or 'DOTS' for short. As below describes, R = D x O x T x S. 2/

https://twitter.com/AdamJKucharski/status/1320797430124863488?s=20

For example, if have a variant (call it V1) that is inherently better at transmitting during social interactions, it would mean an increase in 'T'. If it was 50% more likely to transmit per contact, we'd replace 'T' with '1.5 x T'... 3/

What about a variant (call it V2) that eludes previous immunity to some extent? Here it helps to rejig the equation so rather than proportion susceptible (S) we define in terms of proportion immune P (which is approximately equal to 1-S). So we have R = D x O x T x (1-P). 4/

If new variant means existing immunity isn't as effective at reducing infectiousness, it will reduce value of P, and hence increase R. Suppose our new variant means previous immunity now only reduces infectiousness by 30% – i.e. we replace 'P' with '0.3 x P'. 5/

So in summary, we have variant V1 with R = D x O x (1.5 x T) x (1-P) and variant V2 with R = D x O x T x (1 - 0.3 x P). We can work out relative advantage of one vs other by looking at ratio of R, i.e. [D x O x (1.5 x T) x (1-P)]/[D x O x T x (1 - 0.3 x P)]. 6/

"Hold on", you say, "that's far too much maths and I'm going to stop reading now." But don't stop just yet, because a lot of the stuff in that equation cancels out, which leaves us with ratio of 1.5 x (1-P)/(1 - 0.3 x P). This is what our simple example looks like graphically: 7/

In summary, we can break R down into its components to understand how different variants might have an advantage. When population immunity low, higher inherent transmissibility more important, but as immunity grows, so does advantage of variants that can evade this immunity. 8/8