“If the epidemic continues at this rate, stronger measures will be needed to avoid pressure on hospitals.”
While the Omicron variant and its multiple mutations are the focus of attention, the Covid-19 epidemic is on the rise again in Europe, which now accounts for the majority of new infections recorded worldwide.
Mircea T. Sofonea, University of Montpellier and Samuel Alizon, Research Institute for Development (IRD)
Researchers from the "Infectious Diseases and Vectors: Ecology, Genetics, Evolution, and Control" unit (IRD/CNRS/ University of Montpellier), Mircea Sofonea, associate professor of epidemiology and evolution of infectious diseases, and Samuel Alizon, research director specializing in the spread of infectious diseases, analyze the situation in France and review the measures announced by the government.
The Conversation: The number of infections is on the rise again in our country. Is this increase "staggering," as government spokesperson Gabriel Attal claimed? How can it be explained?
Mircea Sofonea: The term "rapid" refers to communication: the resurgence of the epidemic has been evident for several weeks. The All Saints' Day school holidays acted as a buffer, as was the case during previous waves. When schools reopened, infections began to rise again and the epidemic has been growing since the beginning of October.
Samuel Alizon: The start of the school year and return to work was a pleasant surprise, as the combined effects of vaccination, health passes, and continued mask wearing made it possible to lead an almost normal life. But with the arrival of winter, the resurgence of the epidemic was fairly predictable: studies estimate that climatic conditions directly or indirectly account for 20% of the variations in the speed of the epidemic's spread.
MS: The question is whether what we are seeing is a steady rate, or whether we should fear a further acceleration as winter sets in. We have seen that the reproduction number of the epidemic has been growing since the end of the All Saints' Day holidays. It was around 1.1 in October. Today, it averages 1.3 based on hospitalizations and 1.5 based on testing. In other words, 10 people infected last week infected an average of 15 others.
(also known as "effective R," the reproduction number is an estimate, over the last 7 days, of the average number of individuals infected by one infected person, editor's note)
The wave always unfolds in the same way: initially low circulation gives way to a resurgence of infections, which then spreads to all age groups. The incidence then quickly reaches high levels, with the ever-present risk of pressure on hospitals, at least locally.
TC: What do your models predict in this regard? At the press conference on November 25, Jérôme Salomon announced that the current effective R value for the epidemic in France was 1.6: what implications does this have in terms of dynamics? What scenarios are emerging?
MS: As a general rule, our estimates differ very little from the official figures, but this time we are measuring a lower reproduction number based on testing: 1.5 at most. In any case, in both cases, this indicates a very rapid spread of the epidemic, with the incidence doubling in less than 10 days. The hospital dynamic is slower at the moment, since the doubling time for hospitalizations is at least two weeks. However, as you know, the kinetics of hospitalizations systematically lag behind the incidence.
SA: Unfortunately, hospital admissions are on the rise. As long as the reproduction number remained around 1.1, as it was in early November, we could have expected it to fall to 1 or even below in a matter of weeks. We could then have seen a wave of moderate hospitalizations, similar to what happened this summer. But if the reproduction number stabilizes at 1.2 or above, as seems to be the case, the epidemic wave is likely to result in a wave of hospitalizations too large to be absorbed by the health services.
To give an idea of the scale, as we explained in our report of March 17, 2020, in large urban centers, with these values, one in four people would become infected within two months.
TC: Are the measures announced by the government (health pass valid for 24 hours after a PCR test instead of 72 hours, changes to protocols in schools, early booster shots, no vaccination of children before 2022, etc.) capable of flattening the curve?
SA: No one can answer that question with certainty, because we don't have precise quantification of the effects of each intervention in this new context, let alone their synergy. Added to this is a possible spontaneous change in behavior, the extent of which is unpredictable.
One thing is certain: if the epidemic continues to spread at this rate in two weeks' time, new, stronger measures will need to be introduced immediately to prevent hospitals from becoming overwhelmed by the end of December.
TC: Which geographical areas could see the greatest tension?
MS: Western France was less affected by previous waves, so post-infectious immunity—which also boosts the post-vaccination immune response—is lower there. However, vaccination coverage is also higher than the metropolitan average. So even if clusters continue to appear and temporarily increase the incidence rate in areas that have been relatively unexposed until now, these two effects will tend to offset each other.
With the onset of winter, we expect population density to once again play a significant role: the reproduction number of the epidemic has always been higher in metropolitan areas than in sparsely populated areas. At the national level, the risk therefore appears to be greatest in large cities.
*TC: One thing is clear: vaccination alone will not be enough to control the epidemic, even though 89% of people over the age of 18 have now received two doses (and a number of people have received three)... *
SA: Given the contagiousness of the Delta variant and the level of vaccine effectiveness, this comes as no surprise. We have known since June that vaccination alone cannot contain the epidemic and that it must therefore be combined with other types of interventions. Some, such as wearing masks and social distancing, are already in place but could be better enforced. There is also room for improvement in contact tracing and testing.
Other solutions, such as installing air purification devices in public places, are virtually non-existent today due to a lack of sufficient investment. Finally, there are now promising therapeutic options with the arrival of new treatments. Unfortunately, it seems unrealistic to expect these to be implemented in the coming weeks.
MS: In terms of health impact, it is also important to bear in mind that high prevalence ultimately offsets vaccine efficacy (which is not perfect). In this context, unvaccinated individuals are at risk of becoming infected, including by vaccinated individuals.
TC: In such a situation, can we determine what measures would be most effective in keeping the epidemic under control?
SA: Estimating the effect of a particular measure is tricky, because they are often implemented in concert. Comparisons between countries can help, but strong local specificities must be taken into account. Finally, the concept of effectiveness has many facets in public health, because it must include not only the hospital side, but also a more global health dimension, as well as the social consequences of the measures.
If we limit ourselves to the health sector, among the non-pharmaceutical interventions that most effectively reduce the number of hospitalizations, a study on the subject showed that banning small gatherings and closing schools stand out. It should be noted that the dissemination of knowledge and public education also have an impact, according to the same study.
MS: The problem is that the number of parameters influencing the dynamics of the epidemic increases as time goes by: post-infectious and vaccine immunity declines over the months, the third dose is administered, compliance with protective measures and health passes varies... Unfortunately, French epidemiologists lack the data (as well as support and funding) to determine how these various parameters precisely influence the epidemic. However, some of them need to be updated. This is the case, for example, with the distribution of the time elapsed between successive infections in a Delta variant transmission chain.
Ideally, systematic field surveys should be conducted to accurately characterize the contexts of contamination and quantify the modulation of risk by each measure, cross-referenced with vaccination status. A rigorous analysis based on this type of data would identify the most widely accepted and effective combinations of measures for controlling the epidemic dynamic.
TC: How does France compare to its European neighbors? Does their situation provide us with any insights?
MS: In our country, unlike what happened last fall, the epidemic is "behind" compared to what it is in some of our neighbors. This is also the case in Italy and Spain: the incidence rate there is lower than in France. This is not surprising: in these countries, not only is the vaccination rate high, but their populations have also been severely affected by previous waves. In addition, restrictive health measures have been maintained (Italy, in particular, has also introduced a health pass).
The situation is reversed in Eastern European countries, particularly Romania, Bulgaria, and Hungary. The first waves were less severe than in Western Europe, partly due to lower population density. But the arrival of the Delta variant changed the situation, as its contagiousness offset this structural brake. With low vaccination rates and limited post-infectious immunity, the situation has deteriorated significantly, and the cumulative relative mortality rate has now exceeded that of Western Europe.
The case of the United Kingdom is unique. The epidemic resumed at the end of the summer, following the lifting of mask mandates and the absence of health passes. However, the epidemic is neither taking off nor completely regressing: the reproduction number fluctuates, regularly exceeding 1 before decreasing again. Around this threshold, each change in regime becomes difficult to explain, but this is to be expected on the path to endemicity.
Mircea T. Sofonea, Associate Professor of Epidemiology and Evolution of Infectious Diseases, MIVEGEC Laboratory, University of Montpellier and Samuel Alizon, Director of Research CNRS, Research Institute for Development (IRD)
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