The way that COVID-19 cases spread is not always the same.
Not all those infected are equally contagious, but most of the transmissions seem linked to specific events and supercontagators.
Most infections come from a few people, while many other infected never transmit the disease.
This is both good news and bad news.
On the one hand, it means that if these events are controlled, contagions can be reduced without completely blocking a country.
But it also requires exhaustive tracking work to connect the positives that are detected with the events where they were infected.
In Spain, the truth is that the vast majority of cases have not been linked to any known outbreak.
During the first wave, there was much talk about R, or reproductive number, which measures how many infections occur on average for each case.
This number evokes a typical and homogeneous pattern for transmission: if an infected person infects three people, those three infected will infect another three and so on.
This represents, more or less, the behavior of the flu.
But there is evidence that COVID infections do not work like that, but are governed by what happens in groups of people.
Research such as that of the London School of Hygiene & Tropical Medicine or a recent study published in Nature indicates that a few events (such as a choir or a poorly ventilated meeting) and a few sick people (perhaps people with a high viral load or with a lot of social life ) are responsible for a large part of the infections.
It is estimated that 80% of transmissions are produced by 10% -20% of cases.
If we imagine five infected, two would not infect anyone, another two would infect perhaps one person each, and the fifth would infect eight.
To decide whether the rate of an infection is one type or another, scientists use another parameter, the dispersion factor K. The lower it is, the more transmissions occur in large groups of people.
The K of covid can be as small as 0.1, according to some studies, lower even than SARS (0.16) or MERS (0.25) and much lower than influenza (1).
The debate on transmission is not closed, but there are more and more studies and evidence that emphasize the importance of
, which is how the groups of people where contagions originate are defined.
This would have consequences on how to combat the epidemic: we should know better the outbreaks and propose another way to track the cases, two things that Spain is not doing.
We should know the
to act surgically.
If we discover that there are places and circumstances where outbreaks are likely, we can impose measures to prevent them.
Japan is one of the countries that has focused its strategy on the detailed identification of each
It has had ups and downs in its curve, but the worst 'peak' of confirmed cases in Japan is similar to the most benign moment of the epidemic in Spain.
Deaths per million inhabitants are there 50 times less.
In the first wave, European countries "have looked at the forest and have been distracted by the trees,"
a member of the team advising the Japanese government
In Japan they focused on identifying and avoiding
, which turned out to be mostly closed places where many people gathered.
It was the same strategy of South Korea: there 66% of the cases at that time are linked to an
, another 10% are individual infections and only the remaining 24% are unidentified.
In Spain it is the other way around: our tracking has only been able to link 12% of the cases to specific events.
The problem with not having a good trace is that it makes it impossible to know where the outbreaks are occurring.
"With the tracking capabilities of many autonomous communities, the most that can be done is to confirm that there is transmission in homes without being able to quantify the relative importance of each source of transmission," says Miguel Hernán, professor of Epidemiology at the University of Harvard.
He believes that political will has been lacking, for not understanding that public health and the economy go hand in hand: "A good tracking system is one of the best economic investments, to know which activities are more or less safe and to make very selective closures."
Should we track backwards?
Going to the origin of the outbreak is a strategy to cut the chains of contagion.
Right now the tracking works forward: if I test positive, my contacts are searched for people who could spread the days around my symptoms.
But it is probable - as we are seeing - that I have not infected anyone.
How can tracking change for a disease that is transmitted in
One idea is to do it the other way around: go find where I got it.
Because it will probably be a super contagion event.
"The best thing would be to preventively isolate those who form the
and do a more detailed and backward tracking," explains Yamir Moreno, from the University of Zaragoza, who has analyzed the impact of events on the spread of covid-19 in Spain .
"It would allow you to see ramifications of the chains of contagion that you would not see doing traditional tracking," he adds.
With conventional screening, the infected person the scanners study has little chance of having transmitted the disease.
But the person who infected him will probably belong to the 10% or 20% responsible for 80% of the infections.
Around it there may be more infections.
"Trying to find the source of the infection gives a better chance of identifying a
" explains Mirjam Kretzschmar, professor of infectious disease dynamics at Utrecht University.
"In this backtracking you must identify the contacts that a confirmed case has had up to two weeks before testing positive."
The difficulty with that is that it requires a lot of speed.
His team has estimated that if more than three days pass from the onset of symptoms to testing and isolation, the screening will bear little fruit.
Especially if that tracking is backwards: the others infected by the supercontagator will be more difficult to locate and will have had many opportunities to transmit.
Doing this 'backwards' tracking would be a double challenge in Spain.
Right now it seems that neither the conventional nor is being done: the median of contacts identified for each known case has fallen to only two people, according to official Health data.
In other words, often neither the cohabitants are covered.
"The alert system was not prepared for so much volume of cases," says Ángel Garay Moya, a doctor and epidemiologist specialized in Castellón.
The Public Health services regret a permanent lack of resources.
In addition, the tracing has often been entrusted to Primary Care physicians, also saturated.
In these circumstances, there are voices that almost rule out backtracking, which in addition to being more demanding, is less proven: “It is possible to handle it with few cases, like the 60 newspapers at the beginning of the second wave, but not with community transmission, when more than 200 enter, ”says Garay Moya.
Miguel Hernán also regrets that there is no such capacity.
"It's a shame, because tracing the transmission chains to their source is, if done quickly, one of our best weapons."
Before the tracking system collapsed, also in Spain, linking cases to specific outbreaks has revealed some patterns.
This summer, in the Valencian Community, a series of connected outbreaks caused 100 positives in the Colombian population there.
Their analysis has made it possible to identify that all were related to the celebrations, in dance halls and private parties, of the national holiday of this country.
The parameter K is behind some unknowns.
Cluster transmission helps explain why the virus hit some places earlier.
In part it could be a coincidence.
Population density, customs or climate are factors that are often mentioned.
But several studies (this one by Adam Kurcharski and others, this one from the University of Bern in Switzerland or this one in Nature) point to massive contagion events.
Many countries received imported cases, which were almost always sparks that were put out without being transmitted.
Until one or more of those sparks ignited a group, perhaps due to bad luck: an infected person who attends a funeral, a market, a call center, a residence or a party indoors.
Having one of these events or not having it may depend on the first development of an epidemic.
This dynamic could explain, for example, that in Italy cases soared earlier than in Germany or that the first cases in Spain or France did not cause an outbreak.
The good news.
That a virus is spread by clusters has a positive side: if a few events are responsible for many infections, an epidemic could be controlled without having to act everywhere, at least in theory.
If there are many situations that involve little risk, and we are aware of them, we could recover patches of normal life without putting ourselves in danger.