As soon as you have decided not to choose for an uncontrolled outbreak of the virus or for a (maximum) controlled herd immunity built up under a certain maximum IC beds condition, then there remains only one other solution, which is a strategy with the lowest number of new infections. The reason is, that it does not make sense to have intermediate solutions, because you have to put in the same amount of effort to arrive at an effective reproduction number R, no matter the level of new infections. And because that is the case, you better chose the level as low as possible. Additionally, you minimize number of people with Covid-19, low IC beds use, no postponed regular healthcare and cheaper to execute TTI.
As long as
we have no vaccine or medicine apart from social distancing (including mouth
caps, airco filtration, handwashing,1.5 m distance, etc.), we have only one
other method to lower the effective reproduction number R, which is TTI. We can
say that social distancing has been proven to be an effective way of reducing
new infections. It comes, however, with an extreme negative side effect for our
social way we like to behave and for the economy. So, exchanging social distancing
measures with negative side effects for more neutral measures is desirable.
Here TTI comes into play.
There is,
however, a principle difference in the mechanism between TTI and Social
Distancing. The advantage of Social Distancing measures is that it lowers the
speed of virus transmission and comes into effect immediately when you start
the measure, although you will notice the effect later due to the incubation
time. It will also work for a-symptomatic people. TTI on the other hand will
remove infected out of the population of infected people. It will not work for a-symptomatic
persons, simply because you are not aware of them and so cannot track down
their contacts, although they might be one of the traced contacts.
Suppose we limit ourselves to track down the
secondary infected people. For that we need to know all the people who fall
ill daily due to SARS-COV-2. Let f be the fraction symptomatic, let g be the fraction of people who are tested
and q the fraction of found corona contacts that we can put in quarantine, then
the product fgq is a measure for the effectiveness of the TTI method.The factor fgq=1
means that every possible infected secondary contact is traced and put into isolation.
We will do
a simulation and suppose there is social distancing with 41 % transmission reduction of virus diffusion speed to arrive at an effective R of Rt=1 and fgq=0 and measures start to
act after day 5, representing the first noticed case of illness. The incubation time is distributed normal with mean x=6 days and
standard deviation sigma=2 days. Instead of random dispersion we start with an outbreak of 100
infected people to ease comparison. The result of the simulation is shown in
fig. 1.
Why do we have to lower Rt below 1. As long as the number of infected people is high you want to lower that number as quick as possible. The higher Rt the fastest the process in an exponential upward trend. The opposite is also true. The lower Rt the fastest the process in an exponential downward trend. Onces the numbers are low you can mitigate the measures, but still keeping Rt<1.
Continuing our example we add TTI as a mean to lower Rt to Rt~0.6. See fig. 2. Interesting point is that with Rt=.06, we can easlily handle an outbreak of 100 people each month, leaving it a few days unattended, with needed IC capacity under a few beds.
If the mean incubation time is 4 days then the effect is slightly different but still characteristic.
 |
| fig. 1 Simulation of outbreak, 100 infected at day 2, Rt=1 after day 5. Transmission due to social distancing 0.59. |
Why do we have to lower Rt below 1. As long as the number of infected people is high you want to lower that number as quick as possible. The higher Rt the fastest the process in an exponential upward trend. The opposite is also true. The lower Rt the fastest the process in an exponential downward trend. Onces the numbers are low you can mitigate the measures, but still keeping Rt<1.
 |
| fig. 2 Simulation of outbreak, 100 infected at day 2, Rt=1 after day 5. Transmission due to social distancing 0.59 and in addition fgq=0.3 (which is the same as social distancing with transmission 0.59*0.7=0.413). |
Continuing our example we add TTI as a mean to lower Rt to Rt~0.6. See fig. 2. Interesting point is that with Rt=.06, we can easlily handle an outbreak of 100 people each month, leaving it a few days unattended, with needed IC capacity under a few beds.
If the mean incubation time is 4 days then the effect is slightly different but still characteristic.
In case of an agressive virus like SARS-COV-2 with a too high claim on health resources and too long time to built up herd immunity, the only remaining strategy is minimizing the number of infected people as much as possible. It is the best strategy for the economy and public health.
Social distancing and TTI have essentially different working principles. Where social distancing is lowering the risk of getting contaminated, TTI comes into action after you have been contaminated and put your contacts in isolation.
It also motivates why we have to put a lot of effort in making TTI a success, in order to allow mitigating social distancing measures, that negatively affect the economy.
Disclaimer: This article has not been reviewed.
