Thread by @DrLeanaWen

Many Qs about why it is that new #covid19 variants that are more transmissible will cause exponentially more infections (i.e. why something that is 70% more transmissible doesn't just cause 70% more infections & deaths). 

A thread with an illustrative example (1/5):

Let's say that R0 (virus reproduction number) is 1.1

In the first round of infection, number of people infected = 1 (original person) + 1.1 =2.1

Next round = (2.1-1)*1.1 + 2.1 = 3.3
Third round = (3.3-2.1)*1.1 + 3.3 = 4.6

After 9 rounds, we get to 15.9.
(2/5)

Now, let's say a new variant is 70% more transmissible--say the R0 for the new strain is 1.1*1.7.

You still start out with 1. Then:

First round = (1)*1.1* 1.7 + 1 = 2.9
Second round = (2.9-1)*1.1*1.7 + 2.9 = 6.4

After 9 rounds, we get to a whopping 600 infected.
(3/5)

Screenshot of my calculations and formula here.
(4/5)

This explains why countries where the new more contagious variants have become dominant faced such extraordinary surges.

Even if the lethality remains the same (note: B117 may be more lethal), a much greater rate of infections will lead to much higher number of deaths.
(5/5)