“If the outbreak continues at this rate, stronger measures will be needed to prevent hospitals from becoming overwhelmed.”
While the Omicron variant and its numerous mutations are the focus of attention, the COVID-19 pandemic is on the rise again in Europe, where the majority of new cases worldwide are now being reported.
Mircea T. Sofonea, University of Montpellier and Samuel Alizon, French National Research Institute for Sustainable Development (IRD)
Researchers at the “Infectious Diseases and Vectors: Ecology, Genetics, Evolution, and Control” unit (IRD / CNRS / University of Montpellier), Mircea Sofonea, associate professor of epidemiology and the 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 “rapid,” as government spokesperson Gabriel Attal claimed? What explains it?
Mircea Sofonea: The term “rapid” is a matter of communication: the resurgence of the epidemic has been evident for several weeks. The All Saints’ Day school break acted as a buffer, just as we observed 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 and work year was a pleasant surprise, as the combined effects of vaccination, the health pass, and the continued use of masks made it possible to lead an almost normal life. But with the arrival of winter, the resurgence of the epidemic was quite predictable: studies estimate that weather conditions account for 20% of the variations in the epidemic’s rate of spread, either directly or indirectly.
MS: The question is whether what we’re seeing is a steady trend, or whether we should expect a further surge as we head into winter. We are indeed seeing that the reproduction number of the epidemic has been rising since the end of the All Saints’ Day holiday. 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 the “effective R”; the reproduction number is an estimate of the average number of people infected by a single infected person over the past 7 days, Ed.)
The pattern of each wave is always the same: an initial lull in cases gives way to a resurgence of infections, which then spreads to all age groups. The incidence rate then quickly rises to high levels, with the ever-present risk of strain on hospital systems, 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 does this imply in terms of the epidemic’s dynamics? What scenarios are emerging?
MS: Generally speaking, our estimates differ very little from the official figures; however, this time we are calculating a lower reproduction number based on testing data: at most 1.5. In any case, in both scenarios, this indicates a very rapid spread of the epidemic, with a doubling time for incidence of less than 10 days. Hospital trends are currently slower, as the doubling time for hospitalizations is at least two weeks. However, as you know, the dynamics of hospitalizations consistently lag behind the incidence.
SA: Hospital admissions are unfortunately 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 drop back down to 1, or even below, within a few weeks. We might then have seen a moderate wave of hospitalizations, similar to what happened this summer. But if the reproduction number stabilizes at 1.2 or higher, as seems to be the case, the epidemic wave risks triggering a surge in hospitalizations too large for the healthcare system to handle.
To give you an idea of the scale of the problem, as we explained in our report dated March 17, 2020, in major urban centers, based on these figures, one in four people would contract the infection within two months.
TC: Are the measures announced by the government (a health pass valid for 24 hours after a PCR test instead of 72 hours, changes to school protocols, an earlier booster shot, no vaccinations for children before 2022, etc.) capable of flattening the curve?
SA: No one can answer this question with certainty, because we do not have precise measurements of the effects of each intervention in this new context, let alone their combined impact. Added to this is the possibility of spontaneous behavioral changes, the extent of which is unpredictable.
One thing is certain: if the outbreak continues to spread at this rate over the next two weeks, stronger measures will need to be implemented immediately to prevent hospitals from becoming overwhelmed by the end of December.
TC: Which geographic areas are likely to experience the most tension?
MS: Western France was less affected by previous waves, so post-infection immunity—which, incidentally, boosts the post-vaccination immune response—is lower there. However, vaccination coverage there is also higher than the national average. So even if clusters continue to emerge and temporarily increase the incidence rate in areas that have been relatively unaffected so far, these two effects will tend to offset each other.
As we enter the winter season, we expect population density to once again play a significant role: the reproduction number of the epidemic has consistently been higher in major cities than in sparsely populated areas. At the national level, therefore, the risk appears to primarily affect large cities.
*TC: One thing is clear: vaccination alone will not be enough to control the outbreak, 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 how contagious the Delta variant is and how effective the vaccines are, 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 mask-wearing and preventive measures, 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 systems in public spaces, are virtually nonexistent today due to a lack of sufficient investment. Finally, there are now also promising treatment options available, thanks to the introduction of new therapies. Unfortunately, it seems unrealistic to expect these to be implemented in the coming weeks.
MS: When it comes to public health impacts, it’s also important to keep in mind that high prevalence eventually offsets vaccine efficacy (which is not perfect). In this context, unvaccinated people are at risk of becoming infected, including from vaccinated individuals.
TC: In such a situation, can we determine what measures would be most effective in keeping the outbreak under control?
SA: It is difficult to assess the impact of a specific measure, as they are often implemented in conjunction with one another. Comparisons between countries can be helpful, but one must take into account significant local variations. Finally, the concept of effectiveness in public health is multifaceted, as it must encompass not only hospital care but also a broader health perspective, as well as the social consequences of these measures.
If we focus solely on public health, a study on the subject has shown that, among non-pharmaceutical interventions, bans on small gatherings and school closures stand out as the most effective in reducing hospitalizations. It is worth noting that, according to the same study, the dissemination of knowledge and public education also have an impact.
MS: The problem is that the number of factors influencing the dynamics of the epidemic increases as time goes on: a decline in post-infection and vaccine-induced immunity over the months, the administration of the third dose, varying degrees of adherence to preventive measures and the health pass… Unfortunately, French epidemiologists lack the data (as well as the support and funding) to determine exactly how these various factors influence the epidemic. Yet some of them need to be updated. This is the case, for example, with the distribution of the time interval between successive infections in a Delta variant transmission chain.
Ideally, systematic field studies should be conducted to precisely characterize the contexts of transmission and quantify how each measure affects risk, taking vaccination status into account. A rigorous analysis based on this type of data would help identify the most acceptable and effective combinations of measures for controlling the spread of the epidemic.
TC: How does France compare to its European neighbors? Can we learn anything from their situation?
MS: In our country, unlike what happened last fall, the outbreak is “lagging behind” compared to the situation in some of our neighboring countries. 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 heavily affected by previous waves. Furthermore, strict public health measures have been maintained there (Italy, in particular, has also implemented a health pass).
The situation is the opposite in Eastern European countries, particularly in Romania, Bulgaria, and Hungary. The initial 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 high transmissibility offset this structural buffer. Against a backdrop of low vaccination rates and limited post-infection immunity, the situation has deteriorated sharply, and the cumulative relative mortality rate has now surpassed that of Western Europe.
The situation in the United Kingdom is unique. The outbreak resurged at the end of the summer, following the end of mask mandates and the absence of a vaccine passport. However, the outbreak is neither surging nor subsiding completely: the reproduction number fluctuates, regularly exceeding 1 before dropping again. Around this threshold, every shift in the trend becomes difficult to explain, but this is to be expected on the path toward endemicity.
Mircea T. Sofonea, Associate Professor of Epidemiology and the Evolution of Infectious Diseases, MIVEGEC Laboratory, University of Montpellier and Samuel Alizon, Research Director Research CNRS, French National Research Institute for Sustainable Development (IRD)
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