Covid-19: is the fifth wave really different from the previous ones?

The Covid-19 epidemic is on the rise again in France, as everywhere in Europe. During a joint press conference on Monday, December 6, the Prime Minister and the Minister of Health warned of this perilous situation and a possible congestion of hospitals, already affected by nearly two years of health crisis. However, hammered MM. Castex and Véran, the vaccination of a large part of the population (more than 88% of French people over 12 years of age have received at least two doses of vaccine) is a marked change compared to the previous waves that hit the country.

“This situation calls us to lucidity and vigilance, without giving in to some sort of panic, because if the number of cases is again very high, the situation is not the same as there is year. (…) Without vaccination, our hospitals would already be at saturation levels equivalent to what we had during the first wave ”, a résumé M. Castex.

What is it really? Can we really compare this wave to those that swept over France in March 2020, October 2020, March and August 2021? Moreover, does this comparison make sense both in terms of the parameters – seasons, respect for barrier gestures, variants, restrictions in force, etc. – evolve over time?

Response elements in graphics.

  • A daily number of cases at a very high level

With 45,043 new contaminations on December 6 – on average per day over the previous seven days – this fifth wave has already exceeded the peaks of the two previous epidemic waves (around 24,000 cases in mid-August and 35,000 at the beginning of April); it is approaching the peak of November 2020 – 47,000 daily cases at the start of the second confinement.

This high level of circulation of the virus translates into an incidence rate of 445 cases per 100,000 inhabitants at the national level on December 4 (against 502 cases per 100,000 inhabitants at the beginning of November 2020).

Comparing the waves is a difficult exercise, especially because they were not caused by the same variants, do not occur in the same seasons, etc. The epidemiologist Mircea Sofonea, researcher in the infectious diseases and vectors laboratory at the University of Montpellier, cites as the causes of this fifth wave “The relaxation of barrier gestures, the winter season and the shorter days that push people to stay indoors, or even the vaccine efficacy decreases over time”. Above all, the Delta variant, clearly more contagious than the strains that preceded it, is now responsible for almost all infections in France.

For Daniel Lévy-Bruhl, head of the respiratory infections and vaccination unit at Public Health France, “The Delta variant has been a game-changer in general, but more specifically with regard to the proportion of children. Vaccination has, in fact, made it possible to control the effects of this variant in adults, but not in children, who are too young to be vaccinated. We can no longer speak of the low contribution of children to viral circulation ”. In fact, the incidence is twice as high in children aged 6 to 10 (978 cases per 100,000 on December 4 – on average per day over the previous seven days) than in the general population (444 infections per 100,000 inhabitants), although the screening rate for this age group is three times higher than for the entire population (18,810, against 6,936, per 100,000 inhabitants).

The number of cases detected depends directly on the testing policy in force. “The more we test, the more we will generate positive and measure asymptomatic”, summarizes M. Lévy-Bruhl.

At the start of the pandemic, for example, in the spring of 2020, screening was very low. While more than 7,000 people were in intensive care, the number of cases detected was particularly low, the tests being mainly carried out on severe cases.

Today, more than 700,000 daily tests are performed. As we screen a lot, including in people who are not very symptomatic or asymptomatic, it is therefore logical that the number of cases is very high, as is the incidence rate.

The positivity rate (number of cases compared to the number of tests) is logically undergoing a downward trend: it does not reach the levels of previous waves. There are 6.4% positive tests today, compared to nearly 16% in November 2020 and more than 9% in April. It was 4% this summer, when the health pass went into effect and the number of tests had peaked.

  • Hospitalizations contained … for the moment

For the moment, the number of hospitalizations, especially in critical care, remains at a contained level compared to previous waves. However, there is a constant increase in the number of people in critical care. A little more than 2,400 people are now treated in critical care units; according to models from the Institut Pasteur, they could be 3,000 in two weeks.

For Daniel Lévy-Bruhl, “eIn terms of incidence, this is a significant wave. In terms of hospitalization, it is, at this stage, a wave much smaller than that which one would have expected based on the incidence data ”. He specifies :

“There is a dissociation between the epidemic dynamics linked to new infections and the epidemic dynamics linked to new hospitalizations, and it is of course the vaccination which is at the origin. When we look at the curves, we see that we are beyond the peak of what we called the fourth wave, beyond the peak of what we called the third wave and very close to the peak of the second wave (in the fall of 2020), with already significant testing capacities at the time. On the other hand, the picture is not at all the same when we look at the hospitalization curve: we find ourselves at about the same level as the fourth wave, which was a small wave this summer, and well below what we observed during the second and third waves. “

If we superimpose the incidence and hospitalization curves of the current wave on those of the wave that hit the country in the fall of 2020, we see that at equal incidence the number of hospitalizations is lower.

  • What vaccination has changed

« If we didn’t have the vaccination today, with the level of incidence we are at, we wouldn’t be facing », Notes Daniel Lévy-Bruhl. Indeed, among hospitalized people, those who are not vaccinated are 11 times more numerous in critical care than those who have received at least two doses, according to data compiled by the direction of research, studies, evaluation. and statistics. In other words, if there were the same proportion of vaccinated people as unvaccinated people admitted to critical care, there would be around 500 daily admissions, against 100 recorded on average over the period considered (from November 15 to 21).

  • The very high effective R does not encourage optimism

How will critical care admissions evolve?

“The vaccine protects 85% against the risk of hospitalization, which still leaves a low risk of going to the hospital in case of infection. Applied to the whole country and with the unvaccinated, it still represents a lot of people. There is therefore a risk of saturation of hospital services during an epidemic wave. Hence the recommendation for the third dose and – above all – to continue to mobilize unvaccinated people. Because the unvaccinated will be infected! The question is not and, most when and in what form », summarizes Judith Mueller, epidemiologist, professor at the School of Advanced Studies in Public Health.

To anticipate the dynamics of the epidemic, a tool used by epidemiologists is the effective R (abbreviated “Re”, effective reproduction rate of the virus). It is estimated that a person carrying the virus in turn infected, on average, at the end of November, 1.35 people. “This indicator is valid for two weeks, it allows provide with ten days in advance what will happen in the hospital, in short “, explains Mircea Sofonea, who adds:

“The length of the period in which the reproduction rate is greater than 1 is a good predictor of future hospital pressure. When it comes to a day or two, the impact on the hospital is not significant. If, on the other hand, this number is consolidated over a week, we can begin to warn hospitals that admissions will increase sharply. “

This is precisely what is playing at the moment, according to him. If the third dose is essential for lowering Re and will have medium term effects, so will further measures “Non-pharmaceutical”, its generalization probably comes too late to avoid saturation of hospital services at Christmas time, in barely two weeks.

According to projections by the Institut Pasteur, the peak of hospitalization will be reached in January. “This peak can vary and of course depends on many factors (whether people come for their third dose as early as five months or wait seven months, how teleworking is really followed, the vacation effect, etc.). But the real question, in my opinion, is: how are hospitals doing? It is a fourth wave of magnitude which is announced for them – that of the summer having been rather minor -, while their numbers are reduced, and the fatigue very great ”, redoute Mme Mueller.

To cope, the white plans have been triggered in hospitals in many regions and this while the Omicron variant arrives with its share of uncertainties, both on its possible escape from the vaccines and on the virulence which remains uncertain. “The cases observed for the moment do not turn out to be more severe”, remark Mme Mueller.

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Covid-19: is the fifth wave really different from the previous ones?