Corona crisis: viruses come, viruses go

Unlike the Sars pathogen, the new corona virus will not be able to be eliminated from the human population. However, it can be assumed that his aggressiveness will decrease over time.

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The new Chinese coronavirus Sars-CoV-2 is the undisputed star of the hour. Thanks to modern information technologies, its global spread can be followed from the sofa and in real time. Concentrated media attention may be interpreted by many as a signal that the epidemic, which has long since reached Switzerland, is a singular or at least very special event. But it is by no means. Newly emerged or “emerging” pathogens such as the new corona virus are a fixed factor in the history of human development. The key factor here is the contact between humans and animals.

Man and animal

In the Stone Age, when Homo sapiens roamed the country as a hunter-gatherer in small groups, new pathogens had a hard time establishing themselves in the human population. That changed suddenly with the settling down and the beginning of the cattle breeding around 12,000 years ago. The close contact to farm animals and other people in settlements and cities, but also the penetration into ever new ecosystems favored the transfer of pathogens from the animal kingdom to humans. For example, researchers assume that today’s measles virus began its “career” as a pathogen in humans in antiquity, as a descendant of the cattle plague virus.

The causative agents of Ebola, AIDS or influenza are also “emerging viruses”. In the 20th century alone, four flu pandemics swept across the globe. The new viruses from the animal kingdom also include the Sars and Mers pathogens, two corona viruses that made headlines in 2003 and 2012. Four other corona viruses, on the other hand, should only be known to specialists. Discovered in the 1960s, these pathogens still circulate among the population as cold viruses. They cause up to twenty percent of flu-like illnesses.

Isabella Eckerle, director of the Center for New Viral Diseases at the University of Geneva, says that these corona viruses may also have caused major outbreaks. But that could have been hundreds of years ago. The cold coronaviruses lead to immune protection in the infected. According to Eckerle, this protection does not last a lifetime, unlike the measles virus, but it does last for several years. Then the person can be infected again with the same corona virus.

The virologist also sees a similar scenario for the new coronavirus Sars-CoV-2. The first data showed that this pathogen also stimulates the formation of specific antibodies. After the current outbreak, there should therefore be some immunity in the population. Experts are convinced that there will hardly be a more severe outbreak later than today. However, because there are always people without immune protection, smaller outbreaks are likely to continue, as with the other corona viruses.

So if we can expect the first outbreak to be the hardest, the question arises: how long will it last? According to the infectiologist Pietro Vernazza from the St. Gallen Cantonal Hospital, this could take a long time. Because it is assumed that forty to seventy percent of the world’s population would be infected with the new virus. It could therefore take two to three years for the epidemic to end.

Experts agree that future Sars-CoV-2 outbreaks will not be comparable to the seasonal flu that occurs worldwide every year. Because although flu and corona viruses are both so-called RNA viruses, they differ in essential points. According to the virologist Eckerle, the common cold corona viruses have been genetically relatively stable for decades. This is very different with the influenza viruses. Constant mutations keep creating new seasonal virus strains, against which many people have no immunity. In addition, the genome of type A influenza viruses is housed in segments that are very easy to mix in the presence of another virus strain.

Such a reassortment, i.e. the redistribution of genetic information, led to a new flu pandemic strain in 2009. A pig in Mexico may have served as a “shaking cup” – that’s why people talk about swine flu. A virus that had a triple origin developed in the infected animal: one came from a swine flu virus, another from a human influenza virus and a third from an avian flu virus.

Even without this ability to reassort, coronaviruses can also exchange their genetic material. Such recombinations take place, for example, in bats, from which most of the human-pathogenic coronaviruses are believed to originate. However, the virologist Eckerle states that such a recombination of two corona viruses has never been observed in humans.

Elimination and repression

Viruses can not only reappear, they can also disappear from the human population. In the case of the Sars virus, the containment measures taken in 2003 may have led to elimination. The fact that this success cannot be repeated with the new corona virus is mainly due to its easier portability. This property is probably due to the fact that, in contrast to the Sars virus, Sars-CoV-2 can multiply not only in the lungs but also very well in the nasopharynx.

On this point, the new virus is similar to the common cold coronavirus, which also has advantages. Although it facilitates the transmission of the virus, it could also explain its comparatively low aggressiveness. Its lethality, i.e. fatality, is estimated by leading experts for countries with an intact health system to be less than one percent. For the Sars virus, this figure was ten percent. The fact that children and young adults practically never get seriously ill and die after being infected with Sars-CoV-2 speaks against a very aggressive pathogen.

Doesn’t this knowledge mean that the infection can be “released” to the young and that only the older can be protected from infection? The infectiologist Pietro Vernazza points out that what makes sense from the idea is not so easy to implement in practice. Because with more infections among the young, the risk for the older and ill people in society increases. The contained containment measures should therefore also be understood as an act of solidarity between the generations.

There are always new viruses that are replaced by others after a certain time. The flu also provides illustrative lessons for this. After the devastating outbreak of the Spanish flu in 1918/19, the causative H1N1 influenza virus circulated in the population for decades, with an estimated fifty million deaths. It wasn’t until 1957 that the strain was replaced by another flu virus. From 1977, a new H1N1 virus circulated, which was replaced in 2009 by the pandemic H1N1 virus (swine flu). This pathogen is the most frequently isolated influenza virus in Switzerland even in the current flu season.

Could Sars-CoV-2 be driven out of human circulation by other viruses? The experts consider this to be unlikely. Rather, they assume that we will probably just have another cold virus. For Isabella Eckerle, it cannot be ruled out that the pathogen will still lose virulence when it adapts to the human host. Because from the point of view of the virus, it is advantageous not to kill its host.

The comparatively low genetic diversity in coronaviruses could also be of benefit in vaccine development. Unlike seasonal flu, where a combination of multiple influenza viruses must be packed into the vaccine each year, the new coronavirus could have a single virus strain in the new coronavirus. However, it remains to be seen whether there will ever be a vaccine. Because the current huge interest in the new virus should abruptly decrease as the epidemic subsides – until the next epidemic, which will come as safely as the next seasonal flu wave.