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There is one more concept to add to the already extensive vocabulary that we have acquired with this pandemic in recent months. For some time now, some scientists have been talking about the importance of the so-called “K factor”, which may more accurately reflect the heterogeneous pattern of covid-19. Indicator K refers to the factor of spread of infection and is based on the principle that 80% of infected people are the result of less than 20% of positive cases. If we take this indicator into account, contact tracking can include a retrospective search to find a “super-disseminator” moment that makes that 20% exist. However, there are those who believe that it may already be too late to follow this path.
One aspect of COVID-19 that has most intrigued scientists has to do with the heterogeneous pattern of this infection that affects people very differently, without being able to clearly explain the reasons for these distinctions. In the dynamics of the infection, some models have resorted to the R indicator, which gives us an average of the infections that an infected person will be causing. However, “the R starts from the principle that we all transmit in the same way and this is not true because we are not all the same in anything or transmitting SARS-CoV-2”, says pulmonologist Filipe Froes, who concludes: “Hence the importance of the K factor “.
The calculations for this indicator are complex, but the underlying idea is to find the source of an outbreak, based on the principle that 20% of cases are responsible for more than 80% of transmission chains.
That is why, in addition to R, scientists use a value called the dispersion factor (k), which describes how much a disease clusters. The lower you are, the more transmission comes from a small number of people. Also in May, news from the magazine Sciences I already mentioned the importance of the K factor. To put it in context, the article states that in research published in the journal Nature in 2005 it was estimated that in other viruses such as the one that appeared in 2003 (SARS) “Over-transmission also played a role – and that it had a K of 0.16 ”. The estimated OK for the coronavirus that caused Middle East respiratory syndrome (MERS) in 2012 is around 0.25 and “in the 1918 flu pandemic, the value was around one, indicating that the groups they played a role. a less important role ”.
“Estimates of K for SARS-CoV-2 vary,” continues the same article from Sciences, adding that initial calculations seemed to indicate that the K for covid-19 would be slightly higher than for SARS (in 2003) and MERS (which appeared in 2012). However, in another article published on the same topic, Adam Kucharski of the London School of Hygiene and Tropical Medicine estimated that the K for covid-19 is as low as 0.1. “Probably about 10% of cases lead to 80% of the spread,” says Adam Kucharski, quoted in the news.
The truth is that the K factor confirms the uneven spread of the virus. Some researchers say that this disparity may be associated with the characteristics of an infected individual (for example, an asymptomatic person, but with a high viral load) or the conditions of an event in which people were more vulnerable. It will probably be the two problems combined.
The truth is that the point is not to find the culprit, but a time when the virus has spread by overdose. And this can only happen with the right conditions for that. Filipe Froes, coordinator of the COVID-19 crisis office of the Order of Physicians (OM) and consultant to the General Health Directorate, also agrees that for a super-spreading event it is not enough to have a person with certain characteristics. The risk only exists when this individual is, probably without knowing it, because many times he is asymptomatic, in a situation that allows contagion. If an infected person is in certain conditions more conducive to the spread of the virus, be it a dinner party, a wedding or a church choir, we can have “super broadcast” episodes there.
The well-known indicator R helps us to show us the reproduction of a pathogen by offering us an average of the contagion installed in the population at a given time (and when this value is greater than 1, it means that the infection is on an upward curve). The K factor gives experts some clues about the spread of the virus, revealing whether it is spreading more or less evenly or is associated with spikes or outbreaks of infection, clearly showing that at any given time a person has infected to many people. others at the same time. The several months of the pandemic have already revealed that this problem often manifests itself in outbreaks, as would have happened, for example, in northern Italy with a large number of cases concentrated in just three regions during the first wave. However, it is not known why.
When a person receives a positive SARS-CoV-2 test, several questions arise. One of them aims to clarify when (and how) that person got infected, then it is also important to trace the contacts he had after that, which are now classified as risk contacts. But what if, instead of contact detection focused on the moments after infection, you invest more in trying to understand where, how and through whom that person was infected? Some scientists argue that this could be an important tool to break the chain of transmission and reduce the number of cases.
From there, if it were possible to identify this critical point of contagion, it would be necessary to identify the moment when several people were infected by the same individual. For some scientists, the identification of these specific events that are shown to have generated a high number of cases can be decisive.
“We are already in the Z factor”
Analysis of this indicator (K factor) suggests that screening could be done against what is currently done. That is, not following the contacts that an infected person will have had after the suspicions (or confirmation of infection) but, instead, looking at the contacts they had before that in order to detect the person who infected them and who can (or not) they have infected many others. And does one form of detection have to override the other? And it will be feasible to think about doing both, especially taking into account the difficulty that the “traditional” formula of searching for postinfection contacts is already meeting. The truth is that some scientists argue that retrospective detection may be more helpful in breaking the chains of transmission.
“People must have various tools and must adapt them to situations. In other words, we do not need to substitute one for the other, but rather to know when to choose to look for this super-designer moment ”, replies Filipe Froes. The decision, therefore, corresponds to the public health teams that, with the data they have on a certain contagion situation, must choose (or not) to carry out this retrospective screening in search of its origin and a possible “super-disseminating” moment. . We do not need to replace one screening with another, the expert argues, adding that screening in outbreak situations can now benefit from the important feature of rapid antigen testing that allows for earlier substantiation and decision-making.
But “now it is difficult,” admits Filipe Froes, who acknowledges that it may be a bit late. “We are victims of the circumstances we create. We should have optimized everything before we got here to avoid the current situation or at least delay it as long as possible. Now we don’t have people to carry out an epidemiological survey. When we had them, we should have done our best to identify the facts and the people who spread them excessively, to prevent them from spreading in the community. Now it is out of control, ”says the expert.
Now we have to get back to the basics. Masks, isolation, confinement, distance and hygiene. “Now we have to use the weapons that we have,” says the expert, who emphasizes that there can be no room for fatigue and “fatigue” in the population. We can’t get tired of this. Not to the point of hurting us so much. And that’s why he concludes in a rapturous way: “The K factor is interesting, but right now we are already in the Z – Z factor at the end of the alphabet.”
