Since the COVID-19 pandemic broke out a few months ago, there has been a lot of talk about the possible role of supercontagators in the spread of the virus. This term, already used during other epidemics, refers to people who would have a greater capacity to transmit a virus that causes infectious diseases.
In the case of SARS-CoV-2, some episodes of massive contagion have been recorded in great detail in different countries of the world. For example, in early March, a single person infected 52 others during a choir rehearsal in Washington state in the United States. In Spain we have also had documented cases in which a few individuals would have been able to infect a large number of people. It is estimated, in fact, that 20% of those infected would be contributing to 80% of the transmissions. It would be, according to experts, people who for some unknown reason would have been carriers of a high viral load and, either were in the early stages of infection, before showing symptoms, or would have passed the disease asymptomatically . Anyway, beyond these hypotheses, the truth is that today it is not known what are the characteristics, if any, that contribute to a person having a greater capacity to transmit the virus.
A team of researchers from the University of Santiago de Compostela (USC) and the University Hospital of said city (SERGAS) has published the preliminary results of a study that concludes that these supercontagators could be behind a good part of the cases of COVID- 19 in Spain and in the world. Scientists examined more than 4,700 SARS-CoV-2 genomes from different countries and found multiple bottleneck episodes, most likely associated with super-propagating hosts, that would largely explain the COVID-19 pandemic. This is a study based, yes, solely and exclusively on the genome of the viruses, with very little epidemiological and clinical information about the patients – only sex and age -. “Our objective is not to make an assessment of what characteristics a supercontagator would have, in the event that there was indeed a correlation between certain clinical features of the patients and the different virus strains,” explains Antonio Salas, one of the authors of the investigation. “In other words, the intention is not to carry out an epidemiological study in which an almost police investigation is carried out to find out who has been the zero patient, to whom he could infect, and to reconstruct the events as one who reconstructs the scene of a crime.”
However, the work provides very valuable information to help understand what has happened. “One of the pillars of this work, in my opinion, is the elaboration of that kind of genealogical tree that allows us to explore how the virus has moved around the world and to reconstruct the dispersal patterns. It is an analysis impossible to do if you do not start from a good classification of the virus ”, the scientist explains.
Different transmission patterns
“In all epidemics there are different transmission patterns, and one of them is that of the supercontagator,” says Salas. “For this type of contagion to occur, several circumstances have to occur: the first one is to find the same genetic profile that is repeated many times , as this denotes that a single person is capable of transmitting the same virus to many others. Furthermore, this has to take place in a specific geographic space and in a very short period of time, because it makes no sense for a supercontagator to infect today and in two months’ time ”.
This type of contagion pattern is very different, for example, from chain transmission, in which one individual transmits the virus to another, this in turn to another, and so on. “By studying the genetic variability of the virus you can see these differences: generally, around a supercontagator we have a specific lineage with a very high frequency and other lineages that emerge, in turn, from the main one “, explains the researcher. “In the case of a chain of contagion, the pattern changes because consecutive mutations are produced, so that there is no main lineage that stands out above the others.”
Supercontagators or supercontagion events?
As we have already said, it is estimated that in the case of SARS-CoV-2, 20% of those infected could be contributing to 80% of infections, and it is even beginning to speak that this relationship could reach 10 -90. But for this massive contagion to occur, it is not enough to be a carrier of a high viral load, you also have to be, as they say, ‘in the right place and at the right time’.
“There are a series of factors that are amplifying, this means that they greatly increase the probability of transmission: closed environments, large numbers of people around and continuous contact ,” explains Iñaki Comas, researcher at the Institute of Biomedicine of Valencia (CSIC) and member of the Interdisciplinary Global Health / Global Health Platform launched by the CSIC to address the challenges posed by the coronavirus epidemic from the point of view of science. “ We have many examples of super-dispersion events , just go back to what has happened in Germany these days. In the agri-food and meat processing industry there is a high risk of outbreaks: we are talking about many people who work closely in a closed environment where it is cold. Also, there is noise, so you have to shout more … it is the perfect breeding ground. And Germany has not been an exception, in the United States there have been outbreaks in industries of this type that have had to close, now we have the case of outbreaks in Huesca … there are many infected from a single event that brings together the suitable conditions to facilitate transmission ”, explains the researcher.
What are we left with, then? Supercontagators or supercontagion events? At the moment, it seems that there is a mixture of both situations. “Genetic variability can be quantified with numbers”, explains Antonio Salas. “A phylogenetic tree is a topology, a shape, and to that shape you can put numbers that, when they approach a certain value, indicate that there is a person who has been able to infect many others. Our results indicate that behind these transmission patterns the figure of the super-disperser hides, and also this would have had an important weight in the expansion of the epidemic at a global level ”, indicates the expert. “Both things can be mixed in the same image, both supercontagators and large contagion events. We cannot separate exactly how many of these have been the result of contagion from a single person or from several, but in global terms in that image we see it seems that the supercontagator has played an important role ” .
The stigma of the supercontagator
One of the problems, at least ‘at street level’, of talking about super-dispersed people is the stigmatization that this term implies. “We know that about 40% of the population that has passed COVID-19 has done so asymptomatically, so many of those supercontagators could not know that they were transmitting it, no one has gone anywhere to infect Many people. That is why we try to avoid that term and talk about superdispersion events where all the conditions exist that amplify the possibilities of massive contagion ”, explains Iñaki Comas. ” Obviously there is a factor dependent on the person, not everyone transmits the virus, but this capacity is greatly amplified depending on the type of activities carried out .”
The more we know the better
We are facing the most devastating pandemic of the 21st century. Scientists are working hard to advance as quickly as possible in the search for treatments and vaccines to tackle it, and any new information about this still-unknown virus should be welcomed. Every day there are new publications and data, but everything goes very fast and is transferred to society too fast, almost without filtering , generating an avalanche of overinformation about citizens that stuns and confuses.
The figure of the supercontagator is controversial and can indeed generate very damaging stigmas when we talk about it too lightly from the media or in street conversations. Let us not forget the series of racist episodes that have occurred these months against the citizens of Wuhan or, in the same vein, the indiscriminate killings of bats in different parts of the world for making them guilty of the pandemic.
At the scientific level, it is necessary to deepen the knowledge of SARS-CoV-2 and do it from all fronts. “From a practical point of view we can think that there are no great differences between super-dispersion events or super-dispersive people, after all it is a super contagion, period”, Salas reflects. “But I think it is very important to know what the mode of transmission is, to know what are the molecular mechanisms that underlie a super-transmitter. If we get to know them, it will be a very interesting advance that helps us fight the virus . “
And, while scientists advance this much-needed knowledge, what should be clear to citizens is that, since today we do not know what its characteristics are, any of us could be a supercontagator. As Iñaki Comas indicates: “Potentially everyone can transmit, and that is why it is so important to maintain distances, use protection measures and avoid, as far as possible, events in which all the circumstances that could trigger a massive contagion ”.
