With the Delta variant, stemming a 5th wave will be more difficult, warns Institut Pasteur – Le HuffPost

CORONAVIRUS – The gift of showering hopes when things seem to be improving. At the end of June, when the figures for the Covid-19 in France had not been so low for months, the Institut Pasteur warned of a possible fourth wave in the fall. At the time, the Delta variant just imposed itself and was not really taken into account.

After a complicated summer, the resumption of the vaccination campaign and the health pass seem to have succeeded to contain the summer wave caused by Delta. But do not believe that the game is won, warns the Institut Pasteur.

In a new study published this Monday, September 6, scientists have modeled, in view of recent data on the Delta variant and the progress of vaccination, what the coming months could look like. These scenarios are not predictions and make assumptions that could change significantly, but allow us to understand the choices available to us in the weeks to come.

Forty pages which warn of the risk, always present, of a 5th wave if nothing is done, but which also carries its share of hopes, provided that the vaccination campaign does not weaken. And also on the condition that efforts are made to reduce the circulation of the coronavirus, including by vaccinated people.

A risk of a wave if nothing is done

First, the researchers calculated what could happen if no more measures were put in place to reduce the circulation of the coronavirus. In such a scenario, daily hospital admissions would exceed the peaks of previous waves. Despite the vaccination.

To understand, “just dive back into the first wave [de mars 2020], where we had around 3% of the fragile population infected with the coronavirus, which was enough to create a wave ”, explained in May Simon Cauchemez, modeler for the Institut Pasteur and co-author of the study.

In their study, the researchers hypothesized that 90% of those over 60 will be vaccinated in the fall. But the remaining 10% are already much more numerous than the fragile people affected by the first wave. Thus, if the unvaccinated over 60s represent, in the model, only 3% of the population, they are responsible for 43% of hospitalizations.

This observation was already present in the study published in June. But then why does the increase in vaccinations not change anything? Because Delta, more contagious and partly resistant to vaccines, is shaking up models. In June, the scientists of the Pasteur Institute had made the assumption of a rate of reproduction of the virus (R0, the theoretical number of people contaminated by an infected one) around 4. It should rather be counted on 5, estimate the authors of the study.

But since it is difficult to be definitive and the uncertainties are large, the researchers tested alternative scenarios, with an R0 between 3 and 6. The graph below shows what can happen depending on the R0 and the level. vaccination of the population.

With an R0 of 4, as in June, the peak of the fifth wave would have been weaker than that of the first two waves, even with the release of all control measures.

With an R0 to 5, 95% of those over 60 should be fully vaccinated, 90% of 19-59 and 70% of adolescents. Right now we’re at 88%, 81% and 64%. And with an R0 of 6, even those vaccination levels simply won’t be enough. In addition, the study authors took into account vaccine resistance by decreasing their effectiveness in the mathematical model.

Targeting the unvaccinated alone will not be enough

Obviously, this disaster scenario will not happen, because we are not going to abandon all the measures currently in place overnight. And the good news is that given this model, containment does not seem necessary.

The previous two have reduced the transmission of the coronavirus by 70 to 80%. But this fall, given the vaccine coverage and the contagiousness of Delta, a decrease of 20 to 30% would be enough to avoid saturating hospitals. It is not much, but it is more than in the study of June, where a reduction of 10% would have already allowed to have a lower peak than the previous waves.

“This level of reduction can be achieved through protective measures (such as masks, hand washing), a certain level of physical distancing, the sanitary pass and the test-trace-isolate strategy,” note the authors of the study. The great novelty of this model is above all that targeting the unvaccinated only might not be enough.

In June, the math was clear: taking action on the unvaccinated seemed almost as effective as imposing those measures on the entire population. “This model asks the question: if there is a recovery, should we ask everyone to respect new constraints, or should we move towards strategies targeted at the unvaccinated?” Asked Simon Cauchemez. , referring in particular to ethical and social issues.

The end of the mask is not for now

Since then, Emmanuel Macron has chosen. By imposing the health pass on the majority of places open to the public, the President encouraged the French population to be vaccinated and, at the same time, prevented access to the areas most at risk (restaurants, gyms, etc. etc.) to people most at risk of catching and transmitting Covid-19: the unvaccinated.

Targeting the unvaccinated remains more effective. While they only represent 29% of the population in the Institut Pasteur model, it is nearly one in two infected. Minors, little vaccinated, represent a third of infections and are responsible for half of transmissions.

However, the model published on September 6 shows a real difference if we reduce the transmission of the virus (therefore the number of contacts per person) in unvaccinated only or on the entire population. In June, on the other hand, the difference was almost negligible.

This is why even though “control measures targeting unvaccinated individuals can help maximize epidemic control” (with all the ethical issues that follow), “measures aimed at reducing the risk of infection and transmission, such as wearing a mask, should still apply to vaccinated individuals in situations where transmission is possible, for example indoors, ”note the study authors.

Admittedly, these models are far from perfect. Immunization coverage will increase further. In addition, some regions, already severely affected, may achieve collective immunity more quickly. Conversely, the authors of the study did not attempt to calculate a possible decrease in the effectiveness of vaccines over time, which is still debated.

Autumn and winter are still very uncertain, but the mask is unlikely to disappear, even for the vaccinated.

See also on The HuffPost: how to control an epidemic, instructions for use