Thread by @MarciniakLab

We’ve heard lots about R0 (R naught) recently.

R0 is the reproduction number or the average number of people an infected person will infect.

1/n

Why does it matter?

The spread of a contagions disease depends on R0.

R0 equals 1, the disease persists without increasing or degreasing in number.

2/n

If R0 is less than 1, the disease dies out because each week there are fewer cases than the week before.

If R0 is bigger than 1, a disease spreads at an increasingly fast rate.

3/n

After infection with a virus, we develop immunity. This means that as time goes by, fewer people are ‘available’ for the virus to infect.

4/n

So actually, for a virus to spread the R0 isn’t everything. The proportion of the population that is susceptible matters to. This value (S) falls over time as people become immune.

5/n

So, for an epidemic to spread

R0 x S must be bigger than 1

Let’s say the proportion of the population who become immune is called q. This value is equal to (1-S), I.e. proportion of the population not susceptible. So, S=1-q

6/n

This means, for an epidemic to spread

R0 x (1-q) > 1

Therefore

(1-q) > 1/R0

q < 1 - 1/R0

[7th tweet/n]

The R0 for CORVID-19 is about 2.5

This means it will continue to spread while q (proportion of population that is immune) is

q < 1 - 1/2.5, which is 60%

[8th tweet/n]

Once more than 60% of the population is immune, the virus can’t spread (because R0 x (1-q) becomes 2.5 x (1 - 0.6) < 1

That’s herd immunity.

[thread & epidemic ends]