Seven months after the onset of the SARS-CoV-2 epidemic (which has since become a pandemic) uncertainty remains about the rules for spreading the virus. What’s the meaning of “R0” and how should we behave? COVID-19 is an infection that has exposed the extraordinary complexity of our society, where the actions of the individual can affect the entire population.

As defined by the Istituto Superiore di Sanità (the Italian National Institute of Health or “ISS”), R0 (pronounced “R naught”), represents the average number of secondary infections produced by each infected individual in a susceptible population (i.e. those who have never come into direct contact with the pathogen). Now, this definition fits — but not perfectly — with the case of SARS-CoV-2, because after seven months of pandemic, it’s rather perplexing that there are entire groups who have not (yet?) come into contact with the Coronavirus. However, for the sake of convenience, we’ll retain the definition that helps us to measure the potential transmissibility of an infectious disease.

R0 is logically a very important parameter to measure as it is a function of:

  • The probability of transmission from a single contact between an infected person and a susceptible person
  • The number of contacts of the infected person
  • The duration of the infectivity

Therefore, by reducing at least one of these three parameters, we can control R0 and reduce the area of infection — and effectively slow down the spread of SARS-CoV-2.

This clearly explains why, during this past Summer, thanks to the gatherings mainly in places with “movida“, a drastic increase in the number of cases of infection has been observed. Since we don’t all live in isolation, these same people have returned to their usual social hangouts and have functioned as triggers in new territories, returning the infection back to growth in basically all regions of Italy.

Opportunities and probabilities of transmission

We now come to the rules of contagion. There are basically four main components that form the basis for the spread of a virus, and that have led an infected person to spread the infection to others:

  • How long a person has been infected
  • The opportunity for transmission during this period
  • The probability of transmission (i.e. exposure) during each contact
  • The probability that a person who comes into contact with an infected person is susceptible to the infection

Which of these parameters can we act on?

Certainly on the second and third, because we can actively decide on how many people we wish to meet up with; even if we don’t know whether we’re infected, and above all, we can decide which measures to take in order to limit the risk of infection — both to others and to ourselves. We know the safety measures very well; they’re written on signs at the entrances of all businesses and are well explained in the media. They’re easy to adopt without necessarily having to overdo it, and thus isolate oneself completely from the rest of the community.

We also know who those most at risk are; the elderly, immunodepressed, people with chronic diseases or multi-pathologies. The treatments are also well developed by now and allow for a lessening of the intensity of therapies.

As for how long a person can continue to infect, it’s not yet completely clear, as the times are quite variable, and we can achieve certainty only after verifying the viral load of the infected person.

Finally, the probability that a person who comes into contact with an infected person is susceptible to that infection is another very interesting and equally complex subject. Starting with children, several studies have demonstrated that they’re less susceptible to SARS-CoV-2. There are several indications that genetics also plays an important role in the transmission of infection, and there are even differences in susceptibility to infection between men and women.

Nothing can be taken for granted when discussing infections from microorganisms; SARS-CoV-2 represents the ultimate expression of this and has spread this newfound awareness, even among non-experts.

This post is also available in: Italiano


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