Saturday, 14 March 2020

COVID-19 Herd Immunity Sim

Why Herd Immunity Doesn't work for Covid-19

This is a simulation of the epidemic for the sake of working out how much herd immunity is needed to protect the population.
The answer is around 90+% (98% in this sim). Herd immunity is mistaken.


Herd immunity is designed for vaccines.
In normal epidemics, herd immunity works because the contagion distance between someone who contracts the disease and the next person who isn’t immune is large due to the number of people already immune (depending on the % immunity and infection rate). But here, no one has immunity, so the disease would distribute itself throughout the population and the contagion distance remains small even after nearly everyone has become immune. So in this case, herd immunity is incorrect, despite what you may have heard from our PM saying he’s followed the advice of the chief scientist.

How To Use

The easiest way is simply to press the StartSim button. It simulates a population of 160,000 where each person potentially infects 0.8 people in their vicinity each day until they get isolated (in the example in day 5). The default vicinity is 256 (+/-16 in each direction) and if by chance an infected person tries to infect an already infected person, it has no effect.

You can see in realtime how an infection spreads across the population. Initially it appears to be exponential, but fairly quickly isolation cuts in and people who have it tend to come into contact with those who already have it, so they don't infect so many new people.
So, the epidemic then spreads at the boundary.
As it does so, some people - a few - remain uninfected, by chance and as the disease progresses at the boundary, the mass of people now immune effective protect the very few who have never been infected. But this is a very low figure, because the whole population has no prior immunity and is not vaccinated.

You can play around with the infection rate (<=1) and mobility to simulate a ghastly pandemic that seems to spring from everywhere all at once, but ultimately it has little effect on the herd immunity as it effectively doesn't exist.

Remember, this is only a simulation to demonstrate the lack of herd immunity. It does not really project actual figures for infection, nor are the rates correct, nor the population, nor the mobility, nor the potential impact of warmer weather on the virus.


Pop: Contagion/day:
Isolate on day: Mobility:

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