"If the epidemic continues at this rate, stronger measures will be needed to avoid hospital tensions."

While the Omicron variant and its multiple mutations are the focus of much attention, the Covid-19 epidemic is on the rise again in Europe, where the majority of new infections worldwide now occur.

Mircea T. Sofonea, University of Montpellier and Samuel Alizon, Institute of Research for Development (IRD)

Researchers at the "Infectious Diseases and Vectors: Ecology, Genetics, Evolution and Control" unit (IRD / CNRS / University of Montpellier), Mircea Sofonea, lecturer in epidemiology and evolution of infectious diseases, and Samuel Alizon, research director specializing in the spread of infectious diseases, decipher the French situation and review the measures announced by the government.

The Conversation: The number of people infected with HIV is on the rise again in France. Is this rise "meteoric", as government spokesman Gabriel Attal claimed? How can this be explained?

Mircea Sofonea: The term "dazzling" is a matter of communication: the resumption of the epidemic has been attested for several weeks now. The All Saints' Day school vacations acted as a buffer, as had been the case in previous waves. As soon as schools reopened, the number of contaminations picked up again, and the epidemic has been on the rise since the beginning of October.

Samuel Alizon: The return to school and work was a pleasant surprise, as the combined effects of vaccination, the health pass and the continued wearing of masks enabled us to lead an almost normal life. But with the onset of winter, the epidemic's resurgence was fairly predictable: studies estimate that climatic conditions explain, directly or indirectly, 20% of the variations in the epidemic's rate of spread.

MS: The question is whether what we're observing is a cruising speed, or whether we should fear a further acceleration as winter sets in. Indeed, we've noticed that the number of reproductions of the epidemic has been rising since the end of the All Saints' vacation. It was around 1.1 in October. Today, it averages 1.3 according to hospitalizations and 1.5 according to screenings. In other words, 10 people infected last week infected an average of 15 others.

(also known as "effective R", the number of reproductions is an estimate, over the last 7 days, of the average number of individuals contaminated by an infected person, editor's note)

The course of the wave is always the same: an initially low level of circulation gives way to a resurgence of contamination, which then spreads to all age groups. Incidence then rapidly rises to high levels, with the risk of hospital tensions, at least locally, looming on the horizon.

TC: What do your models predict in this respect? At the press conference on November 25, Jérôme Salomon announced that the current effective R of 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 number of reproductions based on screening: at most 1.5. In both cases, however, the epidemic is progressing very rapidly, with incidence doubling in less than 10 days. Hospital dynamics are slower for the time being, with hospital admissions doubling by at least 2 weeks. However, as you know, hospitalization kinetics systematically lag behind incidence.

SA: Unfortunately, hospital dynamics are on the rise. As long as the number of reproductions was around 1.1, as it was at the beginning of November, we could have expected it to fall back to 1, or even below, within a few 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 could result in a wave of hospitalizations too large to be absorbed by the health services.

To give an order of magnitude, as we explained in our March 17, 2020 report, in large urban centers, with these values, in two months one person in 4 would contract the infection.

TC: Are the measures announced by the government (health pass valid for 24 hours after a PCR test instead of 72 hours, change of protocol in schools, advanced booster vaccination, no vaccination of children before 2022, etc.) capable of limiting the height of the curve?

SA: No one can answer this question with any certainty, as we have no precise quantification of the effects of each intervention in this new context, let alone their synergy. Added to this is the possibility of a spontaneous change in behavior, the magnitude of which is unpredictable.

One thing is certain: if the epidemic continues to progress at this rate in two weeks' time, new and stronger measures will have to be put in place immediately to prevent hospitals from finding themselves under strain at the end of December.

TC: What could be the geographical areas of greatest tension?

MS: Western France has been less affected by previous waves, so post-infectious immunity - which in turn reinforces the post-vaccination immune response - is lower there. However, vaccination coverage is also higher than the metropolitan average. So, while clusters will continue to emerge and temporarily increase incidence rates in previously unexposed areas, these two effects will tend to offset each other.

As we enter the winter season, we expect population density to once again play a major role: the number of times the epidemic has been reproduced has always been higher in metropolises than in sparsely populated areas. At national level, the risk is therefore likely to be concentrated in large cities.

*TC: One thing is clear: vaccination will not be enough to control the epidemic, even if today 89% of people over 18 have received two doses (and a certain number of people, three)... *

SA: Given the contagiousness of the Delta variant and the level of efficacy of vaccines, this is no surprise. We've known since June that vaccination alone cannot contain the epidemic, and that it must therefore be combined with other types of intervention. Some of these, such as mask wearing and barrier gestures, are already in place, but could be better applied. There is also room for improvement in terms of contact follow-up and screening.

Other solutions, such as the installation of air purification systems in public places, are virtually non-existent today, due to a lack of investment. Finally, there are also promising therapeutic options, with the arrival of new treatments. Unfortunately, it seems unrealistic to expect them to be implemented within the next few weeks.

MS: In terms of health impact, we also need to bear in mind that high prevalence ends up compensating for vaccine efficacy (which is not perfect). In this context, unvaccinated people run the risk of being contaminated, even by vaccinated people.

TC: In such a situation, what would be the most effective measures to keep the epidemic under control?

SA: Estimating the effect of a particular measure is tricky, as they are often implemented in tandem. Cross-country comparisons can help, but strong local specificities must be taken into account. Finally, the notion of effectiveness has several facets in public health, since it is necessary to include not only the hospital side, but also a more global health dimension, as well as the social consequences of measures.

If we limit ourselves to the health level, among the non-pharmaceutical interventions that most limit 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 popular 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: decline in post-infectious and vaccine immunity over the months, administration of the third dose, more or less strict compliance with barrier gestures, the health pass... Unfortunately, French epidemiologists lack the data (as well as the support and funding) to be able to determine how precisely these various parameters influence the epidemic. Yet some of them need to be updated. This is the case, for example, with the distribution of the time elapsed between successive contaminations in a Delta variant transmission chain.

Ideally, systematic field surveys should be carried out to precisely characterize contamination contexts, and quantify the risk modulation of each measure, cross-referenced with vaccination status. A rigorous analysis based on this type of data would enable us to identify the best-accepted and most effective combinations of measures for controlling epidemic dynamics.

TC: How does France compare with its European neighbors? Does their situation provide us with any information?

MS: In our country, contrary to what happened last autumn, the epidemic is "behind schedule" compared to some of our neighbors. This is also the case in Italy and Spain, where the incidence rate is lower than in France. This is not surprising: in these countries, not only is the level of vaccination high, but their populations were also strongly affected by previous waves. In addition, restrictive sanitary measures have been maintained (Italy, for example, has also introduced a health pass).

The situation is the opposite in Eastern Europe, particularly Romania, Bulgaria and Hungary. The first waves were less severe than in Western Europe, thanks in part to lower housing density. But the arrival of the Delta variant changed the situation, as its contagiousness counterbalanced this structural brake. Against a backdrop of low vaccination rates and limited post-infectious immunity, the situation deteriorated sharply, and relative cumulative mortality now exceeds that of the western part of the continent.

The UK is a special case. The epidemic resumed at the end of the summer, following the end of mask wearing and the absence of the health pass. However, the epidemic neither took off nor regressed completely: reproduction numbers oscillated, regularly crossing the 1 threshold before dropping again. Around this threshold, each regime change becomes difficult to explain, but that's to be expected on the road to endemicity.

Mircea T. Sofonea, Senior Lecturer in Epidemiology and Evolution of Infectious Diseases, MIVEGEC Laboratory, University of Montpellier and Samuel Alizon, Director of Research at CNRS, Institute of Research for Development (IRD)

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.