High Latency, High Gain

In automation systems, circuits that have high latency (and high gain) are the most difficult to control. Assuming you're running a closed-loop control algorithm, you apply an input signal and check for changes in the output. If the reaction is instantaneous, it is easy to apply an input that results in a required output. But if the reaction is delayed, it all becomes messy and you must be very careful with manipulating the input.

There are many real life examples of such "systems".

A simple one involves drinking alcohol. If you only control your input by judging the output, you will overdose very quickly. This is where usually other external inputs, like experience, come in, suggesting slowing down.

Many years go I had a car, which was phenomenal on paper and terrible to drive. It had a very powerful turbo-charged engine and an automatic gearbox. The engine had a long turbo-lag and the gearbox did not have any predictive logic, only reacting to the engine output. And it was impossible to gently accelerate with this car. Cruising on a high gear, you started pressing the pedal. With very little effect, so you pressed more, still nothing, you pressed even more. At which point the turbo charger finally was kicking in, increasing the output way more than expected, at which point the gearbox was waking up reducing to a low gear and the car was becoming a bullet. Just because the reaction (acceleration) was way delayed to the pedal input. Later on it was a joy to switch to an equally powerful car, but with almost no turbo lag and a manual gearbox.

COVID-19 is a high latency, high gain system. The latency, which is the key problem, comes from the asymptomatic transmission. The virus is incubated and spread by a person that is not aware of being infected. Similar diseases in the past were different. SARS was causing almost immediate sickness, so infected people stayed at home. With COVOD-19, infected people spread the virus around, not anticipating of being ill.

This is why it is different and so difficult to control. The global relaxation of lockdowns we have now, which is possible due to relative stabilization of the pandemic, is the effect of the March / April lockdowns. And it is impossible to judge quickly what will be the impact of the relaxation of social rules today. And when we see a rise in "cases", it will be too late, the whole multi-week lockdown cycle will have to repeat.