Optimistic hospital data, controlled clusters and an end to the state of health emergency announced for July 10: in France, the Covid-19 epidemic seems to continue its decline with no visible sign of rebound. How to explain it? Is the episode short-lived or impossible to assess? Epidemiologist Antoine Flahault, professor of public health and director of the Institute of World Health at the University of Geneva, already questioned by Release in full crisis on 1er April, attempts to analyze this unprecedented post-containment situation.
In France, the scientific council and its president, Jean-François Delfraissy, believe that the epidemic is currently “under control”. What are the indicators that support this statement?
We must first look at the reproduction rate of the virus, known as “effective R” (ie “Re“, Called” R0“At time zero). A situation of “controlled epidemic” can only be pronounced if this rate has dropped and remains below 1 for a prolonged period (a person contaminates less than one on average). In France, this has been the case for about two weeks. The rate was 0.74 on June 9. A trend that is also found in Spain, Italy, Germany, Switzerland, Austria … and which is only confirmed in Europe, even if there may be small transient rises in the Re, in particular due to counting problems (corrections to statistics made after the launch of a massive test campaign over a given time, for example).
Then, to assess the residual risk associated with the epidemic, the number of new cases of Covid-19 must be taken into account. France reported between 200 and 600 confirmed cases every day this past week. The number of new infections is obviously higher: in Geneva, for example, we estimated that behind each detected case, hide 10 people infected but not identified. For France, if we retain the same level of under-notification on the health watch, the range would be between 2,000 and 6,000. At the time of the peak, more than 5,000 cases were reported per day, probably corresponding to more than 50,000 new infections daily. So, yes, we can say that the epidemic recession has been underway for several weeks, that it is continuous and lasting. But that doesn’t mean that the virus is no longer circulating or that the epidemic has disappeared. France is not yet in the situation of New Zealand, which no longer registers any case and has just declared “Elimination” virus on its territory.
Why doesn’t this dreaded second wave come?
There are places in the world where this second wave seems to appear, but not in Europe today. Saudi Arabia in particular is experiencing a very strong epidemic rebound, with more than 3,000 new cases per day, of an intensity similar to that of the first peak (which had risen to 2,800 new cases per day). The same situation is encountered in Iran and Djibouti. One explanation could be that the residents of these Muslim countries celebrated Eid shortly after their deconfinement on May 24, and that these local and family festive gatherings may have been the factors of the rebound. This is one of the hypotheses put forward by researchers in the field that I was able to question. For Europe, however, there may not be a rebound this summer as we may be “secured” by a stronger seasonal brake in our temperate zones. It is still too early to say definitively for SARS-CoV-2, but it seems that it follows the pattern of many respiratory viruses, such as that of influenza, but also the respiratory syncytial virus (responsible for bronchiolitis) and d other rhinoviruses, which completely or sometimes partially extinguish in summer, and thrive in winter.
From this perspective, we understand why Latin America and South Africa, which are starting their winter, are experiencing epidemic outbreaks. But then why does Sweden, for example, stay on the plateau?
Nothing is yet very clear about the seasonal component. It is true that Sweden, but also Poland have high incidence levels (number of new cases) and clusters that are still large as summer approaches. It may all go away, but it’s a little confusing. What I can simply recall and which one should not forget, is that this summer brake on the respiratory viruses is an epidemiological characteristic which relates only to the temperate zones of the sphere. Intertropical areas are not affected by this seasonal dike: for example, the flu virus produces epidemics there throughout the year. In contrast, in temperate countries in the northern hemisphere, as in the south, there is never a flu epidemic in the summer.
What are the mechanisms underlying this phenomenon?
There is certainly a milder temperature, and generally less humidity, but also more sunshine, UV radiation, changing behaviors… We do not yet know very well what is most determining in the summer brake on these viruses.
Are you surprised by the French post-containment situation?
I’m not “surprised” that we are not witnessing a rebound right now, as I hadn’t predicted anything about it. In my laboratory, we produce Covid-19 forecasts for 209 countries, which never go beyond eight days. In any case, this is good news. Containment was a barrier – known to be transient – to the circulation of the virus. When we lifted it, the risk that the epidemic could start again with intensity was obviously in everyone’s mind. It is therefore a relief to see that the containment implemented throughout Europe seems to have succeeded in breaking the dynamics of epidemic transmission. Moreover, most scientific work today accredits the thesis of a major effectiveness of this measure in the control of this pandemic. What is less clear cut is the assessment of the effectiveness of the containment arrangements. There does not seem to be much added value from strict confinement in comparison with the semi-confinement that we have experienced in Switzerland, Germany or the Nordic countries. House arrest has of course been associated with a very significant reduction in the epidemic, but similar effectiveness has been seen in countries where people have been told, but not forced by law, to stay at home.
Photo Mathias Zwick. Hans Lucas
When can collective immunity act as a bulwark against the virus?
This is again a subject debated by scientists: we do not agree, concerning this new virus, on the value of the immunity threshold that the population must reach to allow it to turn the page definitively of this pandemic. Models from Imperial College London predict a necessary threshold between 60% and 70% of the population. Other models, which have introduced more complex and more precise social interactions into their calculations, predict a much lower threshold of 15%. This would obviously be good news, because we would turn the page much faster in this second scenario. Today, therefore, in the absence of reliable forecasts for the medium and long term, the important thing is to prepare for the least favorable scenarios of protective immunity (60% -70%). As a reminder, if sites have experienced very intense outbreaks, such as Bergamo where we would have achieved collective immunity rates of 57%, our results in the canton of Geneva are closer to 10%, those for the department of Morbihan of 5%, and it is likely that France has not yet reached 10% of collective immunity after the first wave.
What about cross-immunity acquired from a previous seasonal coronavirus infection?
Some authors have recently mentioned this phenomenon of cross-immunity. But for the moment, we don’t have a reliable study that assesses the number of people involved. Above all, there is a real question around this mechanism because the elderly seem not to have benefited from it. During the last influenza A (H1NOT1) from 2009-2010, our seniors were much less likely to suffer from severe forms of this new virus, precisely because they had maintained cross-protection, with neighboring viruses, which they had developed during their young age. No one knows the level of cross-immunity with this new coronavirus. If we are to remain open to new knowledge that will arrive on this subject, I do not think it is prudent to place too much hope on it.
Does the behavior of the population remain a key indicator?
Yes, in the sense that we are still in a “decontamination lock”: the population is now less and less likely to die from the Covid, but certain situations remain more dangerous than others. We haven’t come back to life before. However, it is time to support our relaxation with a real participatory risk reduction strategy. Let’s take a current example: rather than telling people not to demonstrate because it is too dangerous, it is better to inform them of the risks and offer them to reduce them by wearing (correctly) a mask and respecting social distances as much as possible. The police can also be asked to restrict the use of tear gas that causes coughing and tearing. The time is no longer for the logic of prohibition and prohibition. Public health has remained somewhat paternalistic. It should now be more empowering and involve people in their own risk-taking.
Is a “loss of control” of the epidemic a real risk?
The experience of the past six months proves that it would be risky to pretend to know the future. Of course, France and Europe are at very high risk of a resurgence of the epidemic this winter. This summer respite, if it is confirmed, should be seen as a chance given to the public authorities to organize themselves in order to prepare the country for this potential second wave. A second wave that could last much longer than the first, since it would occupy the entire cold season from the end of October until April… The only real question that must animate us is this: are- Are we ready to face this eventuality in the best possible conditions, both in terms of health, social and economic?
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