Posted by Samsaranda on 02/04/2020 10:07:32:
… The reality is that just one virus particle is enough to infect a person and the virus particles are approximately 20 nanometres in size, very difficult to dodge.
…
Dave W
Although it's possible to be infected by a single virus, it's very unlikely. Much as the probability of throwing six sixes in a row with dice is 1 to 46656 against. Don't bet the farm on odds like that!
The basic reproductive ratio of a disease ( R₀ ) is the number of infections directly started by 1 individual in a population where everybody is susceptible. Covid-19 is dangerous because everybody is susceptible and a small percentage of those catching it die.
The R₀ of Covid-19 is fairly low, in the range 1.4 to 3.9 Other common diseases have higher R₀: measles 12 to 15, ChickenPox 10 to 12, Whooping Cough 5.5. Covid-19 is less infectious than the common cold, but somewhat worse than ordinary seasonal Flu. Given that, say, 2.5 people will be infected by each Coronavirus victim, the probability of being infected by a single virus is almost zero.
Susceptibility (S) is another ratio. It's the proportion of the population who are vulnerable. S is normally close to average age divided by life expectancy, but Covid-19 puts everybody at risk, making S = 1.
R₀ and S suggest a way of controlling epidemics. By keeping Susceptible people apart, the disease cannot achieve it's natural R₀. If actual R₀ is forced down to less than one by reducing S, the epidemic will stop. Another way is to reduce S by vaccination. Given time S increases naturally as recovered patients cannot be infected again.
One model is based on R₀ is called SIR. It's main parameters are, at a given time:
- S is the number of people not infected yet,
- I is the number who are infected, and
- R is the number removed, no longer susceptible because they're recovered immune or deceased.
The basic maths of modelling an epidemic is fairly simple to those in the know, but it's difficult to get the assumptions right. For example, how effective is the lock-down, given some people must work, others will behave irresponsibly, or fail to understand the guidance applies to them too!
Epidemics are about probability, not certainties. It's not about you or me, it's about us. Individuals don't matter much, it's how the flock behave. So the modellers run many simulations, which the politicians pick from to decide what works best. Unfortunately, there's an element of chance in it. An assumption might be wrong, or manufacturers and services might be unable to deliver counter-measures in time. Anyone care to guess what the world demand for protective masks is at the moment; hundreds of millions or more. That number won't appear instantly by magic – as Churchill remarked of supply problems during WW2: 'Year 1, nothing; Year 2, a trickle, Year 3, a flood'.
I find probabilities difficult to grasp. If only I'd paid attention at school. Aged 14 I thought maths was irrelevant to real-life. Silly me, wrong again!
Fortunately what we need to do is fairly simple. Don't panic buy, avoid contact with others, self-isolate if ill, and wash hands often with soap and water, especially when handling food. Probably best to avoid home remedies, Old Wives, common sense solutions, conspiracy theories, and anything printed in the Daily Boobies!
Dave
Edit: pesky smileys!
Edited By SillyOldDuffer on 02/04/2020 14:42:07