For a disease caused by an infectious agent to occur, it is not enough that such an agent exists, but rather that it must be transmitted and affect a person who is susceptible to suffering from the disease caused by that infection.
In other words, there are three links in the so-called epidemiological chain.
- First, the infectious agent must be able to multiply in sufficient quantities to cause disease.
- Second, the infectious agent must be transmitted from the source to the subject susceptible to illness.
- Third, the recipient or susceptible subject must have an accessible route of entry and, once the infectious agent has accessed the subject, the subject may (or may not) become ill.
From a practical point of view, the coronavirus has a target tissue. That is, a tissue in which the virus feels comfortable and in which it multiplies rapidly. This tissue is the airway, mainly the nasopharyngeal cavity, nose, mouth, throat and pharynx.
From this tissue, it invades the lung cavity and is eliminated by respiration. In this sense and since it is an infectious agent that we did not know until now, all human beings are susceptible subjects.
What we know about transmission routes
The main transmission mechanism, recent studies have determined, is respiration. When we breathe, speak, cough or sneeze, we expel thousands of droplets containing the virus.
These droplets can be more or less large. If they are large, they fall to the ground, surfaces, furniture, knobs, doorknobs, etc., where they can remain for some time (depending on the type of surface that is contaminated). If the droplets are small, microscopic, they remain in the form of an aerosol and can remain suspended in the air for some time, even several hours.
Infected people, whether or not they have symptoms of illness, are shedding viruses in greater or lesser amounts with each expiration. If we speak loudly, shout, sing, and especially if we sneeze or cough, the number of particles is very large. In each of these particles there are viruses with the capacity to infect after being inhaled by a susceptible subject.
Droplets (called Pflügge) that are larger than 100 microns fall by gravity and can be projected about two meters from the emitter. As they fall on surfaces, theoretically, by touching a contaminated surface and then touching our faces, infection can occur, although the probability of contagion through these fomites is small.
Why then do we use hydroalcoholic gel?
According to the Centers for Infectious Disease Control (CDC), SARS-CoV-2 can be transmitted between people, when a healthy person touches surfaces on which a patient has recently coughed or sneezed and then directly touches the mouth, the nose or eyes.
However, recent studies indicate that transmission through this route is low compared to direct contact, droplet transmission, or airborne transmission.
In most situations, cleaning surfaces with soap or detergent is sufficient to reduce risk, although the CDC recommends disinfecting surfaces in community settings where a case of COVID-19 is suspected in the last 24 hours. .
In outdoor environments it is highly unlikely that there is a risk of transmission by fomites, but it is necessary to maintain an interpersonal distance of around two meters and to continue using the mask correctly whenever you are with other people.
And the masks?
On the other hand, droplets of less than 100 microns form aerosols that remain suspended in the air for a time that ranges from minutes to hours. When the droplets are between 15 and 20 microns, up to 100 microns reach the upper respiratory tract (nose, mouth, throat, pharynx).
If they are less than 15 microns, they can reach the trachea and bronchi. If they are even smaller, less than 5 microns, they reach the pulmonary alveoli, as described in different publications.
The droplets that are produced when we breathe or speak are very small and come from our lungs. In general, it is estimated that about 500 aerosols are produced for every liter of air that is exhaled. On the other hand, when coughing or sneezing, the number of particles is greater than 20,000, they are also very small, but in this case they come from the upper respiratory tract, where the virus is multiplying at high speed in subjects infected.
All this indicates that there are viruses in aerosols in sufficient quantity to cause infection, especially in closed or poorly ventilated spaces. In addition, the greater the number of individuals in an enclosed space, the greater amount of virus circulating in aerosol. In these places the proximity to these droplets also influences.
Continue prevention until data indicates otherwise
That is, the main way in which people become infected with SARS-CoV-2 is through exposure to respiratory droplets that carry infectious viruses, according to the most recent data. Infected but not sick people are also capable of transmitting the virus by producing aerosols.
Ultimately, we cannot relax. For all that has been explained, the best way to avoid infection and protect ourselves against covid 19 is, on the one hand, to use a mask correctly whenever we are in contact with others and anywhere. Even outdoors if distance between people cannot be maintained, which is often difficult to achieve.
We must also avoid closed spaces, ventilate vigorously, and wash our hands frequently. Although the risk of contagion through surfaces is low, frequent hand washing and the use of hydroalcoholic gel are useful to avoid transmission by fomites, according to the CDC. That the probability of contagion is very low does not mean that it is impossible.
Likewise, we must minimize meetings with other people, ensuring that they take place in ventilated spaces, with the least possible number of attendees, with the shortest possible duration and with separation between people of at least two meters. In other words, we cannot relax prevention measures until the data and the evolution of vaccines tell us otherwise.
We should act as if anyone we come in contact with is potentially infected. In this way, we will greatly reduce the risk of contagion and, therefore, the possibility of becoming infected and ill.
María Elisa Calle Purón, Epidemiology, Preventive Medicine and Public Health, Complutense University of Madrid
This article was originally published on The Conversation. Read the original.
