SCIENCE – Just under six months ago, on January 23, the city of Wuhan in China was confined to stem the spread of a new virus, Sars-Cov2. This July 7, the pandemic of coronavirus has infected more than 11.4 million people worldwide. The disease it causes, Covid-19, has resulted in the deaths of at least 535,000 people, including more than 200,000 identified in Europe. Figures necessarily underestimated.
Some countries have successfully stopped the spread of the virus. Others, such as United States, see the epidemic resurface. Whatever happens, the pandemic is far from over and the risk of second wave still present.
For more than six months, a large part of scientists around the world have been trying to better understand this new virus. Gradually, study after study, some evidence began to accumulate, allowing us to see a little more clearly. If a consensus seems to be emerging on certain specific points, uncertainty remains for many and it is to a science in action that the world must trust to fight the coronavirus pandemic.
Better understood contagions
To stem the epidemic or prevent it from taking, it is necessary to lower the famous reproduction rate (Rt or Re) which corresponds to the number of people infected on average by a contaminated person. Without any measurement, this rate (R0) is estimated around 3. If it is less than 1, the epidemic does not logically progress. It is one of the main indicators followed by the French and world authorities since the deconfinement.
Indicators to watch for in the second wave:
What we know now is that this Rt is not really representative of the real situation. This is an average that hides significant disparities: some people will spread the virus to dozens of others when the majority will not transmit it at all.
This is why even if the virus is circulating, it is never certain that the epidemic will take hold, there is a share of chance for each contamination. “It is as if you are rolling a die, you cannot know the result”, explains Jean-Stéphane Dhersin, CNRS researcher, mathematician at Sorbonne Paris Nord University, specialist in epidemic modeling. But once the epidemic has really taken hold, everything becomes clearer: “If you roll 1000 dice, you know that the result will be around 3500, that’s the law of large numbers”.
Asymptomatic or presymptomatic contagion
These episodes of super contamination may be partly related to biology (a person could have more viruses, cough more, etc.), but are mainly dependent on timing, the number of people in contact and external conditions.
On the first point, is a site now that a person is much more likely to spread the virus when symptoms appear, but also a few days before and after.
This is why contact tracing is so important for breaking the transmission chains. Because it is possible to transmit the virus without having any apparent symptoms. Without even talking about the proportion of people who are really asymptomatic, which is still debated: the rate varies according to the studies, for example two recent ones mention a third or 40%.
Exterior vs interior
Even if a person is at his peak of infectivity, he still has to spread the disease. Obviously, the more people she meets, for a long time, at close range and without a mask, the more the risk increases.
Another element which today seems quite safe, but which was still unknown a few months ago, is that the risk of contamination is much greater inside, in a poorly ventilated environment than outside. Indeed, most studies show that episodes of super contamination and, more generally, the majority of clusters are formed in closed environments. Obviously, that doesn’t mean there are no risks out there, especially if you’re in a crowded space.
However, the exact ways in which the coronavirus is spread are still unclear. We know that contamination is possible via the droplets expelled when we talk or cough, or when we touch a contaminated surface before touching our face. Researchers have also noticed that there is some form of air contamination. But on this point, the debate is still present.
The WHO recognized, for the first time this Tuesday, July 7, that evidence is emerging on this issue. Therefore we must be open to this possibility and its implications, as well as to the precautions that must be taken, ”said Benedetta Allegranzi, an official with the international organization.
293 researchers had sent an open letter to the organization to claim that there is evidence that small particles of virus, so light that they can float in the air, can infect people. However, of numerous researchers believe that the main mode of contamination remains droplets and that transmission by air remains a minority. In short, the debate is not yet settled.
The question of children between two waters
Since the start of the epidemic, researchers have been amazed at the small number of children affected. It was first believed to be a statistical bias linked to the fact that the youngest are less likely to develop serious forms of Covid-19 and therefore to be underrepresented in hospitals.
But the multiple studies and tests many confirmed that they were less likely to be affected, at least for the youngest (less than 10 years). And yet, other studies claim that in a home, they are as likely as a parent to be infected if someone is infected in the home. Or that children have as much viral load (the number of viruses found in a nasal sample) as an adult.
As pediatrician Alasdair Munro recalls on Twitter, although transmission to school seems rare, cases have nonetheless been recorded. And remember that if children seem to have less risk of transmitting the coronavirus, it can happen. Continuing to accumulate evidence on this subject will allow us to refine the measures in the event of a second wave, in particular the closure (or not) of schools.
Still fluctuating mortality rate
As with any epidemic, scientists want to know the death rate. Coupled with the rate of reproduction, this is what makes it possible to predict the potential impact of a virus on our societies.
However, this rate is very difficult to calculate, as we explained at the start of the epidemic. In the idea, it is however simple: it is enough to divide the number of dead by the total number of infected people (alive or dead), to multiply by 100 and voila. But in reality, each of these figures can vary enormously. Asymptomatic, lack of screening, poor attribution of deaths, efficiency of the health system, delays of several weeks between contamination and death … These biases can vary this rate from 0.1 to 30% depending on the studies .
However, with six months of hindsight, the modellers and epidemiologists are starting to see more clearly. On July 2, WHO chief scientist, pediatrician Soumya Swaminathan, recalled that most studies estimate this rate today to be around 0.6%. We are talking here about the risk of dying for each infected person, so taking asymptomatic people into account. If this rate seems low, it still represents tens of millions of deaths worldwide if nothing was done to stem the spread of the virus.
Distant and uncertain immunity
But we are very far from this scenario, because the majority of the world population has not been contaminated by the coronavirus Sars-Cov2. Since the start of the pandemic, researchers have performed serological tests on many populations to find out after the fact how many people have been infected. And each time, the result showed that the Covid-19 had only affected a small fraction of people.
A study in Switzerland, carried out in 5 waves to follow developments, found a rate of 11% in one of the cantons most affected by the epidemic. The results of recent work have also been published in The Lancet July 6. In Spain, researchers surveyed 61,000 people. Only 5% were tested positive.
This is good news … but it means that we are far from the famous “collective immunity”. Especially, even for a contaminated person, we don’t know exactly how the acquired antibodies protect it. Nor how long. On this subject, some work has shown protection over several weeks, but it will take time and research to find out whether immunity remains effective over time.
Why nothing is yet certain about immunity:
Finally, we should not rely too much on cross-immunity. In theory, it is the idea that the fact that our body has already learned to fight against other coronaviruses would allow it to face an attempted infection with Sars-cov2. If some in vitro studies have shown interesting results, it is far too early to draw optimistic conclusions for the real world.
Researchers from the Institut Pasteur, the AP-HP and Inserm have precisely analyzed this track. After analyzing antibodies to common coronaviruses in hundreds of children (positive and negative for Sars-Cov2), they found no no significant difference.
Treatments and vaccines
This possibility of natural immunity aside, how to end the disease? Because current measures seem to make it possible to control an epidemic, but the risk of a second wave is always present. What has changed in six months is that we are starting to have a (very small) idea of how to treat the Sars-Cov2 coronavirus.
After long and exhausting debates on the usefulness of such and such a drug, the first serious clinical trials (randomized, double-blind, etc.) began to give initial results. Yes l’hydroxychloroquine and the association lopinavir/ritonavir seem ineffective, the anti-inflammatory dexamethasone seems to reduce mortality for severe cases in intensive care. Other tracks, like the remdesivir, are explored and have shown encouraging results, which should be validated. Anyway, it will still take several months to validate the effectiveness of the different treatments, but also to create new drugs (or combinations of old) in order to have a higher efficiency rate. The battle is far from won.
And for a vaccine, which today seems to be one of the main final exit routes from Covid-19, things are even more complicated. In general, developing a vaccine takes time, at least a year at least. In addition, we have so far never succeeded in developing a vaccine against a coronavirus.
Why we have never succeeded in creating a vaccine against a coronavirus: