What we know months later about how COVID-19 is transmitted: with droplets, through surfaces and in "clusters"

There are still many unknowns about SARS-CoV-2, a virus whose nature we are slowly revealing. Luckily, today we have many more clear things about its transmission than it did months ago: channels, times, routes and other characteristics are getting to know each other, slowly clearing the doubts that have worried us for a long time. What do we know about it? This is what we have learned so far.

The virus is spread by droplets

At this point it is vox populi that the virus is transmitted by micro droplets. That is, the tiny drops that we expel when we breathe out or speak. These appear as a consequence of breathing, as part of the water vapor or saliva that we have in our mouths. These droplets are so tiny, up to 5 microns, that they can be invisible to the naked eye and depending on the circumstances. If these drops come in contact with any of our mucosa (eyes, mouth or nose), then the virus can infect us.

We also know that these drops can fall on objects where the viral load (the amount of virus) will remain active and, if we touch it, and then touch a mucosa, we can become infected. The virus, as far as is known at the moment, does not infect by contact with the skin.

For now, SARS-CoV-2 does not appear to be an aerial virus.

A question that is known from the beginning, and that has been confirmed recently. Since the start of the pandemic, the virus has been suspected of not being transmitted through the air. This means that it would not be an aerial virus as it is measles or chicken pox. This reduces their already high contagion capacity and has as a consequence several important issues for their control and prevention: the droplets can reach one or two meters from the vector person, but not beyond. However, there is some evidence that leads us to suspect the ability to survive in the environment for a certain time or even to travel several meters in the air. For this reason, and although there is no evidence that it spreads through the air, some guides to good protection practices recommend taking precautions against the possibility of airborne transmission.

What do we know about the droplets that remain in the air?

Although it is not transmitted through the air, a CSIC report last April warned of the possibility that the droplets remain in the air for hours. This transmitted the same information as another previous report published in The New England Journal of Medicine. This would cause contaminated and dangerous environments by bringing these droplets into contact with everyone, as they pass.

However, the scientific evidence to date insists that these microdrops disappear after a short time outdoors and in heat, reducing the amount of viral load. The WHO has reiterated that the possibility that there is an infection in the open air by droplets in suspension is very low.

Saliva is the main liquid in transmission

At the moment, it is known for sure that the virus is found in the saliva and mucus of the lungs. In other words, it appears associated with complete safety to the respiratory tract. We do not know for sure if the virus can be present in other fluids, although it is not expected to be present in the most common or "accessible" fluids, such as sweat.

How long does the virus stay and on what surfaces?

As we said before, the coronavirus appears associated with certain surfaces and can remain active for hours on them. What surfaces are we referring to? According to studies to date, the SARS-CoV-2 can stay up to 72 hours in stainless steel and plastic, 24 hours on cardboard and four hours on copper (like that of coins). This depends on the particular circumstances of the surface, especially humidity. The drier the environment and / or the surface, the less likelihood of contagion.

We can transmit the virus from the beginning and up to 10 days after the infection begins

The window in which a person becomes a vector of the disease, that is, the time during which he can transmit it, is greater than that of other coronavirus-associated diseases, for example. SARS-CoV-2, we now know, can be transmitted from very early times in infection, when the virus has only just begun to reproduce. Unlike other diseases, this extends the infective period, which can reach up to 10 days, being the first key moments because there are no symptoms.

On the other hand, there are also indications that depending on when a person is infected, the consequences can be diverse. In this way, infections when the viral load is lower, that is, during the first days, are usually milder in symptoms and subsequent consequences.

Vectors and supervectors: the virus is transmitted in "clusters"

Another of the most recent evidences points out that contagions, far from occurring in a sustained way, occur by small waves or clusters. As we already explained, as the spread patterns of the disease are analyzed, this issue becomes more evident. This shows the existence of "supervectors" or "superdispersers". Thus, people capable of infecting many others (tens or even hundreds) are known.

Thus, some people infect many people while others do not infect anyone or almost no one. Adam Kucharski, from the London School of Hygiene and Tropical Medicine, pointed out that approximately 10% of those infected would be responsible for at least 80% of new infections. This role of supervectors is one of the most interesting unknowns in terms of the epidemiology of the disease and the hypothesis that supports them is gaining strength.

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