Coronavirus will be over in summer… hopefully

There is an ongoing polemic regarding this issue and the World Health Organization lately said that the subject was worth investigating. Here are my two cents.

Most of my colds and flues came from nowhere, with no culprit to point my finger to. I did not kiss nor embrace a sick person and I tried to keep safe distance from sick people at work, or simply suggested to them that they should go home. So why did I get sick? Where did the virus come from? And why are colds and flues so rare during summer and spread like fire during winter?

Viruses are very small: the diameter of the coronavirus is around 0.12 microns [1]. For comparison the width of human hair is around 100 microns, that is, about 1,000 times larger: we cannot see the virus, it is too small. When a sick person coughs or sneezes the virus does not travel alone: it gets a ride attached to a water droplet. This explains why the N95 masks [2] are useful. The diameter of a droplet when a person sneezes is between 10 to 100 microns. The N95 masks would not be able to stop the virus if it traveled alone. The virus is too small to be filtered by the mask. However, the N95 mask can stop the droplets to which the virus is attached to, and this is good enough.

So, why did I get sick even if I stayed away from sick people? When a sick person coughs or sneezes most of the droplets fall on all kind of surfaces, directly or indirectly. The sick person does not have to even sneeze: when he/she wipes his nose or reflexively touches his mouth he transfers a large amount of droplets (with the viruses attached to them) to his hand. When he touches then the surface of a table it transfers the droplets to the surface. Minutes or hours later a healthy person touches this surface and transfers these droplets to his hand. Then – as one does many times during the day – he touches his mouth or nose. And the virus finds a new nutritious home where it can grow and multiply. And this is why one usually gets a cold or the flu even if no sick person was near him anytime.

So, why I optimistically hope that the coronavirus will fade away with the arrival of summer?

The droplets with the viruses attached to them do not function only as the carriers of the virus: they keep the viruses alive.  Although water is not nutritious for a virus to grow and multiply, it is good enough to maintain the vitality of the virus for quite a long time. Viruses – like humans – need water to keep going for days [3].

Where does summer come in this picture? During summer the temperatures rise. If, in addition, the humidity is low, droplets deposited on surfaces will evaporate quite fast and without water the virus dies. In other words, whereas in winter the virus can stay alive on the surfaces for days, in summer it dies quite fast, perhaps in a matter of minutes or within an hour. Hence, during the summer times healthy people touching the initially contaminated surfaces will remain healthy: there are no viruses alive to carry the illness.

The above looks to me a reasonable model that accounts for the fast propagation of common contagious illnesses like the cold and the flu and their seasonal disappearance during the summers. Direct interaction alone between sick and healthy people cannot explain all the cases of virus transmission from sick to healthy people and the seasonal disappearance of the flu. Surfaces that are an “active” propagation mechanism during winter and become “inactive” during summer can explain this phenomenon.

Mathematically speaking, as the surfaces become “inactive” during summer, they reduce the probability of spread of the virus within the healthy population below the minimum value needed to maintain or increase the population of sick people. In other words: the sickness, as a characteristic of the population as a whole, fades away, although here and there individual sick cases may still appear, mainly as a result of the direct interaction between sick and healthy people.

The above model explains the fading of the common cold and the common flu during the summers. If the mechanisms for coronavirus propagation are similar to the ones for the common cold and the flu, then it would make sense to concentrate the main effort in the fight against the coronavirus in a constant and thorough disinfection of each and every commonly hand-touched shared public surface (to get “the summer effect”).

References and Notes

[1] Encyclopedia Britannica, “Coronavirus”, March 6, 2020

[2] US Food & Drug Administration, “N95 Respirators and Surgical Masks (Face Masks)”, March 11, 2020. An N95 face mask is a loose-fitting, disposable device. On the other hand, an N95 respirator is designed to achieve a very close facial fit and very efficient filtration of airborne particles. The ‘N95’ designation means that the device blocks at least 95 percent of very small (0.3 micron) test particles. If properly fitted, the filtration capabilities of N95 respirators exceed those of face masks.

[3] If you do not like this simplified picture of a virus as a living creature, perhaps you may find more acceptable the argument that the protective protein coat that encloses the virus DNA just deteriorates and is destroyed when the virus is not in a watery environment anymore.

About the Author
Jaime Kardontchik has a PhD in Physics from the Technion, Israel Institute of Technology. He lives in the Silicon Valley, California.
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