A number of covid cases that doubles in seven days, but no “wave” in hospitals. How to explain the phenomenon which is currently observed in France? The question arises, as government spokesman Gabriel Attal announced on July 19 that “we entered a fourth wave “.

Read also: Covid: who could be affected by the fourth wave?

A normal time shift

First point to understand: there is always a delay between the increase in the number of cases and that of the number of people hospitalized and in critical care. There is an additional delay before an increase in the number of deaths is observed.

And for good reason: people who declare a covid are not immediately hospitalized and a week generally elapses between contamination and the need for hospitalization for a serious form. Same reasoning for death, which usually occurs one to three weeks after contamination.

These two graphics, published by engineer Guillaume Rozier, founder of the site CovidTracker, show it well. On the first graph, the blue curve represents the number of cases and the red that of admissions to critical care. A slight shift is observed between the two curves.

On this second graph, the purple curve represents the number of cases and the red the number of deaths. The time lag is here even more marked.

Still young cases

Another factor to take into account: the age of positive people. As Guillaume Rozier explains, today it is young people who contribute the most to the circulation of the virus. These populations are less likely to report a severe form of covid and also less likely to die from it.

But if the virus begins to infect older people, the number of hospitalizations and the number of deaths may increase, as was the case in September / October 2020.

Enough vaccines to prevent an outbreak?

But can we expect the wave of last fall to repeat itself? Everything will depend on the vaccination coverage. Because the more people there are vaccinated, the more the population is protected.

Thus, countries where the percentage of vaccinated population is high seem for the moment to contain a surge in severe cases requiring hospitalization and deaths. This is what shows, curves in support, a British analyst of the Financial Times, who compares the evolution of the epidemic in various unequally vaccinated countries.

Here, the death curve has been moved forward in time to be “aligned” with that of the number of cases. In the United Kingdom (first curve “UK”), where 52% of the population has a complete vaccination schedule, mortality is currently contained despite a marked increase in the number of cases.

This is not the case in Tunisia or Namibia (third and fourth curve), where vaccination coverage is still very low (5.8% and 1.2% respectively).

And in France ? To date, this figure is 43%. It remains to be seen whether this figure will be sufficient to stem a fourth wave in hospitals and intensive care units in the coming weeks.