But we can make some comments. We can look, for example, at what has happened in other similar situations. In the last century there were three influenza pandemics. The one in 1918 was the most deadly. It developed in three waves: in the spring of 1918, in the autumn of the same year and in the winter of 1919. The really virulent and deadly was the second wave, in which 64% of the deaths occurred. In fact, the first wave was the least strong, it was only manager 10% of the deaths of that pandemic. In the second wave, changes in the genome of the virus have been documented that could explain why it was more virulent. In 1957, a new influenza virus appeared that originated the "Asian flu", which also occurred in three epidemic waves: the first in the spring-summer of 1957 and with a relatively high incidence leave, the second in early 1958 and the third in the winter between 1958 and 1959. Mortality was higher in the second two waves. Ten years later, in 1968, a new influenza virus caused the so-called "Hong Kong flu" whose spread was slower and more irregular, starting in autumn-winter in the northern hemisphere and followed by a second wave the following winter with a higher incidence. The last influenza pandemic, the so-called "influenza A" of 2009-2010, finally did not have such a high incidence and ended up having the effect of a seasonal flu. In fact, this virus ended up adapting to humans and being one of the strains that circulate and predominate every year since then. As we can see, the second and third most lethal waves have occurred with the influenza virus before.

In the case of SARS-CoV2, the appearance of new epidemic waves will depend on the virus itself, on its capacity for variation and adaptation to the new host, the human being; on our immunity, on whether we are really immunized and protected against it; and on our capacity to transmit and control it.

The virus
Can the virus mutate and become more virulent as happened with the 1918 flu virus? We don't know. But unlike influenza, SARS-CoV2 is not the champion of variability. The flu virus also has an RNA genome, but it is eight small fragments that can mix with other types of avian or swine flu viruses, giving rise to new reassortments. Its capacity for mutation and recombination is much greater, which is why influenza vaccines have to be changed every year and pandemic viruses are more frequently produced. Since SARS-CoV2 began, the genomes of several thousand isolates have been sequenced and compared and, of course, the virus mutates! All viruses mutate, but so far, as we expected, this one seems to be a much more stable virus than the influenza virus. Perhaps it is because this coronavirus has a protein (nsp14-ExoN) that acts as an enzyme capable of repairing errors that may occur during genome replication. Therefore, although in this case the definition of the virus as a "cloud of mutants" is still valid, SARS-CoV2 does not appear to be accumulating mutations that affect its virulence.

In addition, on other occasions it has been proven that viruses "jump" from one animal species to another, as in this case, over time they adapt to the new host and decrease their virulence. In other words, it is not always the case that a virus mutates to become more virulent, but generally the opposite. In any case, we will have to continue to monitor it closely.

The immunity of group
Are we already immunized against this virus? To prevent the spread of an epidemic, the chain of transmission of the virus must be cut off. issue This is achieved when there is a sufficient number of individuals (at least more than 60%) who are protected against infection, act as a barrier and prevent the virus from reaching those who could still be infected. This is called herd immunity, group or collective immunity, and is achieved when people have passed the disease or when they are vaccinated. But against this virus we still do not have a vaccine. Is there immunity of group against this virus? It seems that there is not. In the preliminary study on seroprevalence of SARS-CoV2 coronavirus infection in Spain, one of the most important conclusions is that the national prevalence is 5%: some communities had prevalences below 2%, while others exceeded 10%. These data were obtained by detecting anti-SARS-CoV2 IgG antibodies using the immunochromatography technique, the rapid tests. In final what they indicate is that at most, in some areas, no more than 10% of the population has had contact with the virus. We are very far from the 60% or more needed to achieve immunity from group.

But all this is much more complex than it seems. We still do not know if having antibodies against SARS-CoV2, that is, having tested positive in serological tests, really ensures that you are immunized against the virus. We do not know, for sure, how long those antibodies last and if they are neutralizing, if they block the virus and protect you from a second infection. We also do not have data of cellular immunity, that other part of our defense system that does not depend on antibodies but on cells and that is very important to overcome viral infections.

It is true that in the case of other coronaviruses, having passed the infection and having generated antibodies, these last a few months or years and seem to have a certain protective effect, but this may also depend on the person (not all people have the same effect). It is also true that there are some trials with plasma from patients cured of the coronavirus that are blocking the virus and have a beneficial effect in infected persons, which would demonstrate that these antibodies are protective. In trials with macaques infected with the virus, it has been shown that their antibodies do protect them against a second infection. But this has been done in macaques. It has also been suggested that having had previous contact with other coronaviruses, the ones that cause common colds and flus, might have some protective effect against SARS-CoV2. This has so far only been demonstrated in in vitro assays, but could explain the large number of asymptomatic individuals. At final, the immunity of group... Remains a mystery!

Three possible scenarios
Taking all this into account, some (1) have proposed three possible models.

1) A second much more intense wave in winter 2020 followed by smaller waves throughout 2021. This scenario would be similar to flu pandemics, but we already know that this coronavirus is not a flu, it does not have to behave the same way. This scenario may require a return to some subject of more or less intense containment measures during the autumn-winter to avoid again the collapse of the health system.

2) Several epidemic waves over a period of one or two years. This first epidemic peak that we have just experienced would be followed by repetitive waves that would occur consistently for a couple of years until disappearing sometime in 2021-22. The frequency and intensity of these outbreaks would depend on each country's control measures.

3) Small outbreaks without a clear patron saint of new epidemic waves. This first wave would be followed by small outbreaks that would gradually die down, also depending on the control and containment measures of each country. This scenario would not require a return to such drastic containment measures, although the issue of cases and deaths could continue for some time.

In any case, it seems that we cannot rule out that the SARS-CoV2 virus will continue to circulate among us for some time, perhaps synchronizing with the winter season and decreasing in severity. Even if there are no new pandemic waves, including a new respiratory virus that can have very serious consequences for a significant group of the population, in the list of dozens of respiratory viruses that visit us every year, is not good news. Every flu season the emergency rooms of many hospitals are saturated, adding a new virus is already a problem.

Controlling and preventing resurgences: getting ahead of the virus
The virus has not disappeared. I am inclined, it is more a hope than a certainty, for the third scenario: small outbreaks without a clear patron saint of new epidemic waves that will gradually die out. But let us not forget that this may continue to leave deaths along the way. This is what is happening in other countries, which had already finished their first wave before us, such as South Korea. In Spain, there have also been resurgences in some cities during the beginning of the de-escalation. In most cases, they have been related to population agglomerations (parties or family meals). But we cannot be confined forever, nor can we sterilize all environments: "it is impossible ... and moreover it cannot be done". Two actions are essential to reduce the frequency and intensity of these outbreaks. 

On the part of citizens: to avoid contagion. We already know how the virus is transmitted and that, fortunately, it is easy to inactivate it. Contagions are more frequent in closed or crowded environments. Let's not forget: many people, very board and moving around is the best for the virus. Avoid crowds, distance between people, use of masks, frequent hand hygiene, cleaning and disinfection (in that order), follow health recommendations. This is what must be demanded of the citizen, we cannot relax.

On the part of the health authorities: tracking the virus. We cannot continue to follow the virus as we have been doing up to now, we must take the lead. We must put in place a system capable of detecting an infected person at the slightest symptom, being able to trace and obtain information on their contacts, follow them up clinically and carry out PCR and serological tests and, if necessary, isolate them. Detect and isolate an outbreak. This requires staff, equipment and diagnostic systems. And we must be prepared so that the health system does not collapse again. This is what needs to be addressed right now, to which all resources need to be devoted, not mass testing of the entire population, to take a "snapshot" of the status. Decisions must be made for health reasons, not political ones. This is what we must demand of our governments, they cannot relax either.

If you have been in contact close proximity to someone who has had symptoms of COVID-19, within 2 meters for more than 15 min, you should isolate yourself for 14 days, and you should demand that the health authorities test you and the person with symptoms.

There may be a second or more waves, ... or there may not. Now we have put out the fire, but we have not extinguished it, there are embers left that can fuel the fire. The relaxation of the confinement measures is not because we have defeated the virus, it is because we also have to save our livelihood. A very long confinement can also cause deaths. We are not going to kill the virus, we can dodge it. We can mitigate its effects. What has happened cannot happen again, this time we have to protect the weakest. And that depends on citizens and governments.

(1) COVID-19: The CIDRAP Viewpoint. Part 1: "The future of the COVID-19 pandemic: lessons learned from pandemic influenza" (April 30, 2020)

This article was originally published in The Conversation. Read the original.