COVID-19; Do Travel Bans Make Sense?

In (CNN) Coronavirus cases rise to more than 64,000 globally, Anderson Cooper asks Sanjay Gupta whether travel bans make sense. Gupta’s reply seems to be in the negative, with a little vagueness thrown in.

Let’s nail it down. In the short term, travel bans have benefit. The benefit will decrease with time. It will be obvious when the utility of travel bans expires.

If people were like plants, did not travel, and could infect only their nearest neighbors,  an epidemic could be modeled as a pure diffusion process. Drip a drop of ink or food dye in a glass or water. Watch it spread out in the liquid, betraying the invisible currents of water in the glass. This is diffusion. The plume from a smoky fire is another form of diffusion, combined with convection.

The purest diffusion is equalization of temperature, between something which is hot, and something which is cold. But it’s almost always combined with something else. The cooling of your cup of coffee is a combination of diffusion, and air currents.  The currents are analogous to  travelers.

Now pour a fresh glass of water, add a drop of ink, and stir or shake the glass. This adds “travelers”, bulk movement of the water. The ink quickly spreads out through the water in the glass, until it all has the same tint. The ink in the glass will never concentrate again. That’s the past history of the ink, just as the current concentration of COVID-19 around Wuhan will be replaced by general prevalence.

The spread of COVID-19 is by a combination of modes:

  • Diffusion, as described above, by which a cluster grows locally.
  • Non local processes, which resemble stirring of the glass: an infected, asymptomatic  traveler, the “super spreader”.
  • Establishment of new clusters, which spread again by the above, diffusion and travelers.

Diffusion can be modified by sanitation.  Travel bans target the non-local modes. Eventually, COVID-19 will be equally present everywhere, subject only to local factors that have not yet been identified.

Travel bans buy time. The amount of time is impossible to predict.  At some unknown time, asymptomatic “super spreaders” will bypass bans. At that point, travel bans will become useless, but not before.  When is a roll of the dice. Politics will doubtless prolong the bans beyond usefulness, but that is future.

What do we do in the meantime? With a  fully functioning economy, we should be preparing, training, stockpiling, and researching. This is a time to screen small molecule drugs. For inspiration, (Science Daily) Dozens of non-oncology drugs can kill cancer cells. Quoting,

Researchers tested approximately 4,518 drug compounds on 578 human cancer cell lines and found nearly 50 that have previously unrecognized anti-cancer activity. These drugs have been used to treat conditions such as diabetes, inflammation, alcoholism, and even arthritis in dogs…

This kind of information, even in the negative, is vital to stave off quack remedies. From the debatable lore of medieval plagues,

Ring-a-round the rosie,
A pocket full of posies,
Ashes! Ashes!
We all fall down.[3]






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