Decline, impact of reopening: Where is the Covid-19 epidemic and what to expect for the start of the school year?

Reopening of restaurants, cultural venues, lifting of the curfew, end of wearing the compulsory mask outdoors ... The Covid-19 epidemic is stalling in France, and is even decreasing significantly faster than the models had projected. last April. Mircea Sofonea, lecturer in epidemiology and evolution of infectious diseases at the University of Montpellier, explains why, and takes stock of the hypotheses for the months to come.

The Conversation France: The epidemic has declined faster than models anticipated in April. What happened ?

Mircea Sofonea: It should be remembered that mechanistic epidemiological models (that is to say, which explicitly rely on the explicit dynamics of transmission) do not produce forecasts, but projections. Formally, they are related to a logical implication: if condition A is fulfilled, then situation B can be expected to arise.

If the working hypotheses are in fact not satisfied, the scenarios produced become obsolete and the simulations must be updated. This problem mainly arises when the signal of the effect of a new measurement is still incomplete in the hospital data, on which our models have been based since the start of the pandemic.

Our first projections relating to the third deconfinement were developed at the end of April. At that date, the effect of the third confinement was limited compared to the first two, since an average of 10 people infected 9, against 8 (or even 7) during the previous two. Our most optimistic scenario was therefore based on the assumption that this trend would not reverse.

The good surprise came a week later, at the beginning of May, with a strengthening of the effect of the confinement. From then on, we were able to update the simulations.


For a month, the epidemic has followed the most optimistic trend in the model, without requiring any adjustment.

TC: Why not have imagined an “optimistic” scenario, with such a drop in the number of reproductions?

MS: The scientific approach is based on an important principle, the principle of parsimony or “Ockham's razor” (named after the Franciscan philosopher William of Ockham who formulated it): “the simplest sufficient hypotheses should be preferred”.

In the absence of solid elements making it possible to quantitatively anticipate a dynamic of the number of reproduction which would not already be included in the model (typically, the effect of vaccination and immunization by infection), the The minimal and methodologically neutral assumption is to extrapolate the dynamics of the epidemic based on the latest data. In April, this extrapolation was itself supported by the analogy with the first two confinements. During the previous two times, the estimated reproduction number had reached its minimum level ten days after the initiation of the measures.

However, the kinetics of the third confinement were different: during the first 10 days, the number of reproduction stagnated between 0,9 and 1, then it dropped sharply to fall below 0,8, before rising slightly in early May. .

(Editor's note: The number of reproduction is an estimate, over the last 14 days, of the average number of individuals infected by an infected person. We speak of the basic reproduction number (or R0) at the start of the epidemic, in the absence of transmission control measures and when the population is fully susceptible to the virus. During the epidemic, this number changes: we speak of the effective or temporal reproduction number (Rt). If it is less than 1, the epidemic regresses, above 1, it progresses.)

If, in retrospect, it now appears clear that the third confinement induced a decline in the number of reproduction slower than that of the first two confinements, it was not possible at the end of April to anticipate it, just as there was no element in favor of a reversal of the trend during containment, a scenario that, for the same reasons, we did not consider.

TC: Is the reliability of the models called into question?

MS: No, but it should be understood that several reasons, methodological and biological, complicate the estimation of the number of reproduction shortly after the introduction or the lifting of a restriction measure.

First, there is a difference between the reality and what can be understood from the measurements. When we implement restrictions (confinement, curfew ...) the number of reproduction drops overnight in everyday life. However, this discontinuity is not seen in the figures collected, quite simply because various biological parameters vary from one person to another (incubation time, onset of contagiousness, appearance of symptoms, etc.).

The discontinuity due to the restrictions could be visible if all these events occurred in a homogeneous way in everyone, assuming that the screening is done exactly at the same time, with results obtained with identical delays. But this is not the case. In fact, there is a smoothing, and we only see the effects after two weeks, indirectly, on daily hospitalizations and admissions to critical care services (more reliable data than screening, a fortiori in the presence of holidays).

Secondly, the methods for calculating the number of reproduction also use smoothing, in particular to overcome the "weekend effect": the sliding average over 7 days makes it possible to distribute the decrease in activity in the detection of cases on the weekend (the laboratories being closed) throughout the week, and therefore no longer to be impacted by irrelevant oscillations. The downside is that this approach buffers the variations that testify to a recent change in trend in the event of the implementation of restrictive measures, for example.

It is therefore necessary to continue modeling research efforts. It will be necessary in particular to refine the transmission patterns and, above all, to improve the inference from weak signals. Likewise, the acquisition and cross-referencing of complementary data sets is a real challenge. In the UK, for example, school epidemiological surveillance, chain of contact tracking, random population screening and sequencing provide valuable sources for improving model reliability.

Still, parsimonious models still have a role to play, even a year and a half after the start of the pandemic. Indeed, if they are not the most precise in the short term, they make it possible to easily explore all the possibilities in the medium term, a time frame which is of particular interest to decision-makers.

In this, they are suitable for informing anticipatory strategies, in particular in the context of an epidemic outbreak where a delay can translate exponentially into a health impact. Even if it means re-evaluating the schedule every two weeks, as the estimates consolidate.

TC: In retrospect, do we have any idea of ​​the reasons that can explain this faster decrease than during previous confinements?

MS: Today, we still lack the perspective (and time) to explain it causally. However, as we move away from the events, we can use other statistical methods dedicated to the study of the distant past, to study more precisely the course of the epidemic and assess the contribution of different factors. A work which, for the moment, cannot be part of the urgency of the requests.

One of the hypotheses is that the closure of schools made a major contribution to the effect of the confinement (for the first, the closure was general and for the second, the school holidays had already started). However, children are less symptomatic, it takes more time to see the effects of such a measure.

The stagnation in the number of reproduction observed during the first week of the third confinement could be explained by the fact that the chains of transmission were maintained in adults (in the workplace in particular). On the other hand, then the school holidays started, the chains of transmission initiated in the schools no longer existed at that time, and the number of reproductions fell sharply, since many adults were also on leave.

TC: This result again raises the question of the role of schools ...

MS: Yes, and all the more so since the “vaccine” effect has been integrated in a favorable way.

However, we cannot really say that there was a more significant “school effect” during this confinement than during the first, because the conditions were different: presence of the alpha variant (formerly known as “British”, more contagious than the first. the historical strain especially in the youngest), vaccination of those most at risk of complications.

In addition, other factors may have had an impact: the parks were open, the weather was better than in the fall (but the milder weather alone cannot explain the decline). But in prospective modeling, it remains difficult to integrate a parameter such as the weather forecast into the projections, even though forecasts beyond one week are uncertain.

TC: Where are we today? Can we see an effect of reopening?

MS: When schools reopened on April 26 (and secondary half-gauge the following week), then terraces on May 19, we saw a slight increase in the number of reproduction, which went back to between 0,8 and 0,9 , to return to stabilize around 0,8 currently.

We can therefore observe an effect of the return to school and more modestly of the reopening of the terraces, but nothing that is likely to restart the epidemic, which remains on the decline, even if it is a little slower than at the beginning of May. This suggests that the terraces could have been reopened earlier, especially in areas where the incidence was already lower. In general, a territorialisation of the lifting of restrictions (and not only of their implementation) makes it possible to generate data capable of informing decision-making for the less spared territories.

In addition, vaccination plays a key role, which is shown by the various scenarios: even if the reproduction number rose to slightly above 1, the advance of vaccination could cause it to stabilize or decline rapidly.

For the rest, it is advisable to remain vigilant vis-à-vis indoor gatherings with limited air renewal. The reopening of performance halls and indoor restaurant halls are still too recent to assess their effect on the epidemic.

TC: Wearing a mask outdoors is no longer compulsory. What to think of it?

It should be remembered that the reopening of the terraces took place in a context of recognition of the contribution of the airway (by aerosol) in the transmission of SARS-CoV-2 and the dynamics of the epidemic.

Recent literature suggests, however, that the risk of external transmission is very low. It can nevertheless persist in situations of prolonged proximity without drafts, if a person is exposed for several minutes to the aerosol cloud produced by a contagious person without the latter having had time to dissipate.

TC: What can we imagine in the fall? Which variants can we expect to see circulating, in which part of the population?

MS: Again, the goal of mechanistic models is not to predict how many hospitalizations there will be in a given number of days, but rather to know, for example, what level of relaxation one can afford. without fear of hospital saturation or what is the death potential of a fourth and final wave.

Currently, the alpha variant is in the majority (even if the beta variant of South African origin now seems to be spreading faster than it in Île-de-France and Hauts-de-France, perhaps because it would escape more to natural immunity, see our latest work published in the journal of the European Center for Disease Prevention and Control).

Knowing that the alpha variant spreads more among young people and that in addition, the oldest have been vaccinated as a priority, we can naturally expect that the youngest will become the reservoir of circulation of the virus in the months to come. .

The real question is to know what ambition we set ourselves for the start of the school year: do we want, for example, to get rid of wearing a mask completely, not only outside, but also at home? inside? Re-allow all cultural events, regardless of audience size? In this case, the continuation of the vaccination campaign this summer will be crucial.

TC: What are the estimates in terms of potential deaths?

MS: Currently, if 90% of the adult population is vaccinated with two doses at the start of the school year, we would still be under threat of 15 hospital deaths nationwide - the equivalent of a major epidemic of seasonal influenza. This is an order of magnitude, under current conditions (excluding immune escape of a variant, which for the moment does not seem relevant).

Are we ready to accept 15 additional deaths? Knowing that we have already collectively accepted more than 000 deaths due to this pathology, there is little reason to imagine the opposite ...

From a purely pragmatic point of view, the risk is that, if these 15 deaths occurred in close proximity, they could again put some local hospitals under tension. For this reason, it is crucial to prepare for the start of the school year now. For hospitals, it will notably be a question of determining how to organize themselves to best manage the residual Covid-000 activity: will it be necessary to maintain a dedicated platform or to distribute hospitalizations between departments?

TC: To prevent this type of problem, does the vaccination necessarily have to be uniform throughout the territory?

MS: Not necessarily. In a modeling initiated by Olivier Thomine based on OpenStreetMap data, not yet peer-reviewed, the spatial heterogeneity of the epidemic suggests that it is important to achieve high vaccine coverage levels, above all in large metropolitan areas.

This calls for differentiated territorialized measures, including for vaccination coverage: having lower vaccination coverage in the Gers than in Paris, in Lyon or in Seine-Saint-Denis is not necessarily a problem. However, if the virus were to reach areas with far too low vaccination coverage, there would be risks of local epidemics. This is what has been observed for example in the Netherlands with measles.

In addition to the problem of the delta variant (of Indian origin), whose dynamics in France are closely monitored, the end of the epidemic in France will be determined by the vaccination coverage. Adherence may weaken as the hospital situation improves and the perceived risk of infection decreases.

Epidemiological models do not yet integrate human behavior, although this is an active avenue of research. In the meantime, the models must produce scenarios based on a range of realistic and documented hypotheses, so that they cover a range of possibilities capable of anticipating the epidemic risk.

Mircea T. Sofonea, Lecturer in epidemiology and evolution of infectious diseases, MIVEGEC laboratory, University of Montpellier

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

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