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This 21 day period is based on preliminary evidence available. But model outcomes vary widely depending on the characteristics of a pathogen and the affected population.
21-day quarantines have been considered earlier elaborately in the context of Ebola and the calculations are based on the estimated incubation period of the virus in a human host. The 21-day quarantine value is derived from interpretations of outbreak data, past and present.
According to epidemiologists, the logic is that we have arrived at an incubation period of 14 days. Give another week for the residual infection to die out, to be entirely safe, and you arrive at 21 days. This being a new coronavirus, they have estimated that the median incubation period (the time between the entry of virus to the onset of symptoms/ disease) falls within this period. Several groups had published roughly similar estimates of the number of new infections caused by each infected person when no control measures are taken—a parameter epidemiologists call R0. “This approximate consensus early in the pandemic gave modelers a chance to warn of this new pathogen’s epidemic and pandemic potential less than 3 weeks after the first Disease Outbreak News report was released by the World Health Organization about the outbreak .
This must have been taken into account by the Indian Government to take this 21 day period lockdown decision. The lockdown or quarantine also creates some breathing space — to convince people of the seriousness of the situation and build positive public opinion, and allows hospitals to prepare themselves for the next phase of operations, if needed. "Flattening the curve" (1) needs this space so that medical systems can cope with the demand.
Models can produce vastly different pictures that vary from region to region. Policymakers in developed countries have relied too heavily on COVID-19 models. But these models are not always accurate. There’s also a lot that models don’t capture. They cannot anticipate, for instance, the development of a faster, easier test to identify and isolate infected people or an effective antiviral that reduces the need for hospital beds. That’s the nature of modeling: We put in what we know. But there will be several unknown factors that dictate things.
“Response fatigue” is one thing we will have to consider if we have to do it for a long time. People really get very tired of longer ones.
Long lockdowns to slow a disease can also have catastrophic economic impacts that may themselves affect public health. “It’s a balancing act between protecting health, protecting the economy, and protecting people’s well-being and emotional health.”
Different countries are going for different period lockdowns, depending on the situation they are in.
It is tough to take these decisions. You are doomed if you take them, you are doomed if you don't take them. But in cases of life and death, you take them anyway based on your perception of the situation and hope that they would give positive outcomes.
But do you have any choice when death is staring you in the face? NO.
So all countries who have been affected someway are taking them anyway.