Numpty >>
sunnuntai maaliskuu 22 - 00:31, Muokattu
sunnuntai maaliskuu 22 - 00:48 I want to try and address the question about the estimated number of people that will eventually be infected.
When will the virus stop spreading?
Isolation and 'social distancing' is simply a delaying tactic which, if effective, will allow the number of cases to temporarily decrease, but it will still be circulating at a low level in the community and then it will bounce back when any controls get lifted and the population resume their normal behaviour.
It won't be stopped until a sufficient percentage of the population have immunity. Either
- through vaccination or
- those that have been infected and then recovered.
How many will need to have immunity for any disease to prevent it spreading?
This is based on the reproduction number, R, which is the average number of secondary cases each case generates. If R is less than 1 then the number of cases decline and if R is greater than 1 then it continues to spread. When R is substantially less than 1 then some infections will still occur but it declines quickly.
In the case of COVID-19, Imperial College made an assumption based on the early growth-rate of the epidemic in Wuhan that R was 2.4 and on recovery from infection individuals are assumed to be immune.
R is not a constant, but will vary with each country and community based on people's behaviour and social mixing.
If, and it's a big if, the correct reproduction number, R, is 2.4 when there is no immunity, then when 60% of the population are immune the reproduction number drops to 0.96 and when 70% are immune then the number becomes 0.72. At this point it will decrease fairly rapidly with occasional local hot-spots.
So somewhere in the region of 70-80% immunity will be needed before the epidemic is stopped.
This means that eventually around 70-80% of the population will either be infected or have been vaccinated.
The only other option would be for a country and/or community to change their way of life permanently so that in that community R becomes a much smaller number than 2.4. That seems pretty unlikely.