Have we had this yet?
Roger Edward Adlard - (shared from Facebook)
I have heard that a lot of people are confused by the statement that the new variant of the coronavirus is ‘70 % more transmissible’ than the original variant.
I thought I would give a brief (ok not THAT brief) explanation as to what this means for those that can’t get their heads round it - apologies to friends and colleagues who I’m teaching to suck eggs!
It also helps explain why coronavirus is not just a simple flu and why we are seeing far fewer cases of flu this year and yet the coronavirus is still spreading.
All viruses have different rates and means of transmission - whist HIV is blood or body fluid born, some can be transmitted purely by touch (eg human papilloma virus) by contact with mucus membranes like the mouth (eg herpes simplex virus) whilst colds and flu (and coronavirus) are droplet born and are therefore present in coughs, sneezes or even just the air that we exhale when talking or breathing.
Even between those viruses that are droplet born there can be differences in how transmissable they are. This depends on factors around how the virus is constructed and will influence, for instance, how much they are excreted from say the mucus membrane cells in your nose and mouth and to how long they can exist outside of the body before breaking down or ‘dying’. It also depends on how long a person is ‘contagious’ with virus - if they are producing the virus for just a day there is clearly a lot less chance of catching it than a virus that is expressed in droplets for days on end.
Consider two different groups at the extremes of virus build. In one group the viruses are heavily excreted by cells with a high density of the virus in droplets in say a sneeze and can last for a long time on surfaces or in the air and at a wide range of temperatures. In the other group they are barely excreted at all, with droplets containing very little virus, and they are almost instantaneously broken down outside of the cell from which they are made. This makes viruses in the former group highly transmissable and those in the latter group barely transmissable at all
It’s difficult to measure this transmissibility experimentally so the data is calculated mathematically by looking at the number of people within a studied population that are expressing the virus and how this changes in a daily basis. This will give you the famous ‘R’ number we keep hearing on the news.
An R=1 means that one person, say with a particular strain of flu virus, is likely to infect one other person with that virus whilst they are contagious with it. If R=3 they are likely to give it to 3 people during the course of their illness and so on.
The R number for seasonal flu under normal circumstances is about 1.3 though some years the flu is worse than others. The R number for Spannish flu was about 1.8.
Any R number above 1 means the spread is dramatic and logarithmic. This is beccause, for instance with an R number of 5:
Week 1 - 1 person with the virus gives it to
Week 2 - 5 persons with the virus, gives it to
Week 3 - 25 persons with the virus, gives it to
Week 4 - 125 persons with the virus.
So one person has spread it to 125 by 4 weeks in this scenario.
For seasonal flu you normally become contagious and able to spread the flu virus between 1 and 4 days and almost immediately become symptomatic (only 20-30% are asymptomatic carriers). For coronavirus it is between 3 and 10 days - a lot longer period in which to spread the virus and moreover, it is thought up to 80% maybe asymptomatic during this time whilst with seasonal flu, it’s normally quite obvious that you are ill and you don’t feel like leaving the house!
For coronavirus, before mask and social distancing was introduced its R number was between 3 and 5. When mask and social distancing was introduced its R number dropped to 1.4 and during the first lockdown to 0.8. Clear evidence that the measures taken reduced the spread.
So a summary or R numbers in various scenarios:
Seasonal Flu with no measures. R=1.3
Seasonal Flu with social distancing R=0.6
Normal variant Coronavirus R=3-5
Normal variant Coronavirus with social distancing. R=.1.4
Normal variant Coronavirus with lockdown R=0.8
By dropping the R number to 1.4 by social distancing and mask wearing - that’s a reduction in transmissibility by over 50%. This reduction has also caused a similar reduction in transmissibility of seasonal flu but as its R number is only 1.3 without measures taken, the social distancing and mask wearing has dropped its R number to less than 0.6, so few people are catching the flu and being below the magic R=1 number it’s spreading far less than would be normally the case.
The new variant is ‘70% more transmissable’.
This means if the R number of the original variant is 1 then its R number would be 1.7. So extrapolating the data from the R numbers in for the normal variant, the new variant will have R numbers as follows:
Variant Coronavirus with no measures R=5.1-8.5
Variant Coronavirus with social distancing etc R=2.38
Variant Coronavirus with lockdown R=1.38
Remember, any R number above 1 means the number of people getting the virus is increasing and so it is easy to see why this new variant and its high transmissibility maybe a problem even if people behave similarly to how they did in the last lockdown!