If there has been one phrase that has been repeated like a mantra throughout the COVID-19 pandemic, it has been: “flatten the curve.” And it is that there was an urgent need to stop the contagion of the virus, so that the number of infected stopped growing exponentially, in order to adequately treat the sick.
Currently, in Spain the curve has not only flattened, but is also in a steep decline. So much so that we can be tempted to believe that the virus is no longer contagious or even that it has disappeared. It is true that right now there are reasons for optimism, but, as I will try to explain throughout this article, there are still more reasons for caution.
Let’s see what factors the transmission of a virus depends on and analyze how they can affect SARS-CoV-2.
The contagion capacity of a virus is not a constant value
In these months we have all heard about the R0 factor, which is nothing more than the average number of infections produced by an infected person, in this case by SARS-CoV-2. If R0 is greater than 1, it implies that the virus can be stably maintained in a population.
However, we all know that epidemics end up disappearing. This happens because when a person contracts an infectious disease and manages to recover, it is normal for them to develop some immunity against the pathogen, which is a barrier to its transmission. As a consequence, the R0 factor decreases and, when it reaches a value less than 1, the epidemic begins to decline.
How many immune people does it take in a population to be able to be calm? The answer that epidemiological studies give us is that about 60%. That number is what gives the long-awaited herd immunity or herd immunity, which means that immune people protect those who are still susceptible.
The results of the second round of the national seroprevalence study indicate that only 5.2% of the Spanish population has been in contact with SARS-CoV-2. Very far from herd immunity, and very far also from that longed-for tranquility.
In a way it is logical that we have not achieved high levels of immunity. After all, we have been locked in our houses for weeks. With the confinement, what we have achieved has been to reduce contacts between people, thus slowing down the transmission of the virus. If it hadn’t been done that way, we probably would have gotten a higher level of immunity, but it would have come at the cost of a much higher death toll.
Fortunately, herd immunity can also be achieved artificially, through vaccination. An alternative that may require great efforts and economic costs, but not human lives.
SARS-CoV-2, summer and behavioral habits
Another factor that determines the transmissibility of viruses is their stability in the environment, when they are outside the cell.
The environmental conditions typical of summer (high temperatures, higher intensity of ultraviolet radiation, higher absolute humidity) are quite harmful for viruses. Especially for those that are surrounded by a lipid envelope like SARS-CoV-2. To this is added that in summer we also spend less time in closed environments, which promote contagion by allowing greater closeness between people in an environment in which viruses can remain longer than outdoors.
It is no coincidence that there are seasonal viruses that cause outbreaks in winter and seem to disappear in summer. That we are at the gates of the summer season could be contributing to the lower rate of SARS-CoV-2 infections in our country. If, in addition, we all wear a mask and maintain a certain social distance, we will have a lot of cattle.
Can you change the virus in a way that makes it less contagious?
Viruses are always mutating, and to a greater extent if their genetic material is made up of RNA, as in the case of SARS-CoV-2. As a consequence, viral populations are huge distributions of mutants on which natural selection acts, favoring those that provide some advantage. And an obvious advantage for a virus is that it can spread better between its hosts.
Therefore, it is not expected that SARS-CoV-2 will become less contagious. What we can expect is that it becomes less aggressive, since high transmissibility is usually associated with milder symptoms, so that the virus can multiply for longer in the same host.
However, to date, there is no study that demonstrates a clear association between genetic changes and decreased aggressiveness of SARS-CoV-2). It has been reported that, on average, current patients have a lower viral load than at the beginning of the pandemic. But it could be for other reasons, such as earlier detection and the relief of health systems that can now better serve the sick.
The example of the flu
If we take as an example another respiratory virus, the influenza virus, which has caused multiple pandemics that we have always overcome, we can venture what we can expect from SARS-CoV-2. Of course, without certainties.
The influenza virus, in the northern hemisphere, practically ceases to circulate in summer and, in addition, a large part of the population has some immunity due to past infections. However, the virus resurfaces every winter. It is true that its great variability, which appears to be greater than that of SARS-CoV-2, favors this fact. But it is still a reality that should make us think that, as long as we do not have a vaccine or an effective treatment, we will have to continue talking about a “new normal”. Because the “old normal” will continue to have many risks.
The bottom line is that we have managed to lower the curve. But this has not happened because the virus is less harmful or because there is herd immunity. It has been thanks to the limitations in our contacts and the changes in our customs. So let’s maintain a cautious optimism that allows us to restart our lives again without being caught off guard again by autumn.
Ester Lázaro Lázaro. Scientific Researcher of Public Research Bodies. Specialized in virus evolution, Astrobiology Center (INTA-CSIC).
This article was originally published on The Conversation. Read the original.
