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14-11-2021, 08:50 PM
https://www.facebook.com/permalink.php?s...1110331714&id=100005335308340
Quote:Ho Ching
What is the biggest difference between the old variants and the new Delta variant?
It’s the R0.
This is the basic rate of reproduction - this medical jargon measures on average how many others that an infected covid case will infect.
For last year, the R0 for the old variants was about 2.5. This is about twice as infectious as the seasonal flu.
Even the more infectious Alpha around the beginning of the year in UK had an R0 of around 3~3.5 - this is more infectious but not as much as the Delta.
This year, the Delta has an R0 that is now generally accepted as being around 8.
What does this mean?
Let’s say we use R0 of 3, to cover the old extinct variants, as well as the newer but now extinct Alpha variant.
The average infection tree over 5 generations would be:
1 => 3 => 9 => 27 => 81
If the average transmission time between generations is 4 days, based on a median incubation of 3-5 days - this means we have a total of 121 infections over 20 days or 3 weeks.
This means we handle an average of 6 cases per day over 20 days, just from 1 infected case.
Even if we are bit slow to detect or contact trace, we have a bit of time to catch up.
But things change dramatically if infectiousness goes up.
Take Delta at R0 of 8, and a faster transmission time between generations of infection at 3 days, nased on a shorter median incubation of 2-4 days.
So we will have over 5 generations, an infection tree that looks like :
1 => 8 => 64 => 512 = 4,096
This means a total of 4,681 infected over 15 days or 2 weeks.
That would be an average of 312 cases per day that we need to detect and isolate over 2 weeks.
50 times more case load on average to deal with.
Quite a lot more than the old workload, as well as the much much faster speed of response needed too.
This was why we were trying to encourage fast testing with ART kits, and fast turnaround of even PCR test results.
Even if the ART could be less than perfect, with a sensitivity of say 80%, this means we can detect and isolate faster for 80% of the infectious cases, and shrink our problem space down to 20%, right?
If not, we would easily have a runaway situation if we simply take 3 days for a PCR result, or longer as we have a slow response falling further and further behind the runaway infection wavefronts.
Then what next for another 6 days?
We will have :
4,096 => 32,768 => 262,144
This makes a total of 299,008 in 21 days if left to transmit unfettered without vaccination shields or umbrellas in place.
Ooooph!
That would indeed be a runaway situation just from 1 case not detected and isolated, but allowed to transmit freely over 3 weeks.
This speed of transmission at R0 of 8, is also precisly why it is even more critical to act fast and early when we have small signals that show a doubling of small numbers.
For example, we may have 7 cases over 1 week, followed by a seemingly manageable 14 cases the following week.
However, this is a weekly growth ratio of 2. This is a doubling over a week, and an important sign that we are seeing the start of an explosive growth to come.
So we MUST NOT neglect such small but critical signals of troubles to come.
This is the power of the explosive growth of the highly infectious Delta.
Imagine if we have had Delta before we have high vaccination phalanx in place for our population.
This is the kind of nightmare scenario for the future pandemics if we are not ready.
If SARS was a kindergarten class, then last year’s Covid variants are primary 1 class lessons slowly evolving into the primary 3 syllabus.
Delta would be a PSLE exam that we are taking now.
Don’t underestimate the power of exponential or multiplying growth.
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