But their impact has been moderate compared to the global havoc triggered by the coronavirus that is behind the Covid-19 pandemic. In just a few months, it has triggered closings in dozens of countries and the disease continues to spread.
It is an extraordinary achievement for a ball bristling with genetic material covered with fatty chemicals called lipids, and which measures 80 billionths of a meter in diameter. Humanity was shot dead by a very humble assailant.
On the other hand, our knowledge of Sars-CoV-2, the virus that causes Covid-19, is also remarkable. It was an organism unknown to science five months ago. Today it is being studied on an unprecedented scale. Vaccine projects are proliferating, trials of antiviral drugs have been launched, and new diagnostic tests are emerging.
The questions are therefore simple: what have we learned in the past five months and how could this knowledge put an end to this pandemic?
Where did it come from and how did it first infect humans?
The Sars-CoV-2 virus almost certainly comes from bats, which have developed fierce immune responses to viruses, researchers have discovered. These defenses cause viruses to replicate more quickly so that they can pass the bat’s immune defenses. In turn, this turns the bat into a reservoir of rapidly reproducing and highly transmissible viruses. Then, when these bat viruses travel to other mammals, creatures that do not have a quick-response immune system, the viruses spread quickly in their new hosts. Most of the evidence suggests that Sars-CoV-2 started to infect humans via an intermediate species, such as pangolins.
“This virus probably passed from a bat to another animal, and this other animal was probably close to a human, possibly in a market,” said virologist Prof Edward Holmes of the University of Sydney. “And so if this wild animal has a virus, it is detected on a bat and we interact with it, there is a good chance that the virus will then spread to the person handling the animal. Then that person will go home and pass it on to someone else and we have an epidemic. “
Transmission of Sars-CoV-2 occurs when droplets of water containing the virus are expelled from an infected person by coughing or sneezing.
How does the virus spread and how does it affect people?
Particles infected with viruses are inhaled by others and come into contact with cells lining the throat and larynx. These cells have a large number of receptors – called Ace-2 receptors – on their surfaces. (Cellular receptors play a key role in the passage of chemicals into cells and in triggering signals between cells.) “This virus has a surface protein that is primed to lock onto this receptor and slide its RNA into the cell, “says virologist Prof Jonathan Ball of the University of Nottingham.
Once inside, this RNA inserts itself into the cell’s own replication machine and makes several copies of the virus. These broke out of the cell and the infection spreads. Antibodies generated by the body’s immune system ultimately target the virus and, in most cases, stop its progression.
“A Covid-19 infection is usually mild, and it’s really the secret of the virus’s success,” adds Ball. “Many people do not even notice that they have an infection and therefore go around their work, houses and supermarkets infecting others. “
In contrast, Sars – which is also caused by a coronavirus – makes patients much sicker and kills about one in 10 of those infected. In most cases, these patients are hospitalized and this prevents them from infecting others – by cutting the chain of transmission. Milder Covid-19 avoids this problem.
Why does the virus sometimes cause death?
Sometimes, however, the virus can cause serious problems. This happens when it goes down into the respiratory tract and infects the lungs, which are even richer in cells with Ace-2 receptors. Many of these cells are destroyed and the lungs are cluttered with pieces of broken cells. In these cases, patients will need intensive care treatment.
Even worse, in some cases, a person’s immune system becomes overloaded, drawing cells to the lungs to attack the virus, causing inflammation. This process can get out of control, more immune cells flow in, and the inflammation gets worse. This is known as a cytokine storm. (In Greek, “cyto” means cell and “kino” means movement.) In some cases, this can kill the patient.
It is not clear why cytokine storms occur in some patients, but not the vast majority. One possibility is that some people have versions of the Ace-2 receptors that are slightly more vulnerable to coronavirus attacks than those of most people.
Are we protected for life if we are infected?
Doctors examining patients recovering from Covid-19 infection find fairly high levels of neutralizing antibodies in their blood. These antibodies are produced by the immune system and they coat an invading virus at specific points, blocking its ability to enter cells.
“It is clear that immune responses are mounting against Covid-19 in infected people,” said virologist Mike Skinner of Imperial College London. “And the antibodies created by this response will provide protection against future infections – but we should note that this protection is unlikely to be lifelong. “
Instead, most virologists believe that immunity from Covid-19 will only last a year or two. “It matches other coronaviruses that infect humans,” said Skinner. “This means that even if most people end up being exposed to the virus, it is likely to become endemic – which means that we could see seasonal peaks of infection from this disease.” We will have reached a stable state with regard to Covid-19. “
The virus will be with us for a while, in short. But could he change his virulence? Some researchers have suggested that it could become less deadly. Others have argued that it could mutate to become more deadly. Skinner is doubtful. “We have to look at this pandemic from the point of view of the virus,” he says. “It spreads very well around the world. Things are going well. The change brings him no benefit. “
Ultimately, it will be the development and deployment of an effective vaccine that will free us from the threat of Covid-19, says Skinner.
When will we get a vaccine?
On April 9, the newspaper Nature reported that 78 immunization projects have been launched worldwide – and another 37 are under development. Among the projects underway, a vaccination program is currently in phase 1 at the University of Oxford, two others in American biotechnology companies and three others in Chinese scientific groups. Many other vaccine developers are planning to start human testing this year.
This remarkable response raises hopes that a Covid-19 vaccine could be developed in a relatively short time. However, vaccines require large-scale safety and efficacy studies. Thousands of people would receive the vaccine itself or a placebo to determine if the former was effective in preventing infection with the virus they would have encountered naturally. It is inevitably a long process.
As a result, some scientists have proposed a way to speed up the process – by deliberately exposing volunteers to the virus to determine the effectiveness of a vaccine. “This approach is not without risks but has the potential to speed up testing of candidate vaccines by several months,” said Nir Eyal, professor of bioethics at Rutgers University.
Volunteers should be young and in good health, he said, “Their health would also be closely monitored and they would have access to intensive care and all available drugs.” The result could be a vaccine that would save millions of lives by being ready to use in a much shorter time than that which has undergone standard phase three trials.
But deliberately infecting people – especially volunteers who would receive a placebo vaccine as part of the trial – is controversial. “We will have to think about it very carefully,” said Professor Adam Finn of the University of Bristol. “Young people could jump at the opportunity to participate in such a trial, but it is a virus that kills the strange young man. We do not yet know why. However, phase three trials are still a long way off, so we have time to carefully consider the idea. “
- This article was modified on April 12, 2020. The original version incorrectly described the Covid-19 virus as measuring “one 80 billionth of a meter”, when it should have said “80 billionth of a meter”. A quote from Mike Skinner, indicating whether the virulence of Covid-19 could change, has also been corrected.
Due to the unprecedented and continuing nature of the coronavirus epidemic, this article is regularly updated to ensure that it best reflects the current situation. All major corrections to this version or to previous versions of the article will continue to be noted in accordance with Guardian editorial policy.