Covid-19: what percentage of French people should be vaccinated to gain collective immunity?

The end of the tunnel is near. During his televised speech Tuesday evening, Emmanuel Macron announced that vaccination could begin “late December, early January” subject to the results of clinical trials and validation by the competent authorities. A perspective that the President of the Republic described as “tremendous ray of hope”, but that doesn’t inspire all French people. According to a BVA survey exclusive to Europe 1, 60% say they are ready to be vaccinated against Covid-19, but only 20% plan to do so as soon as possible. To achieve collective immunity and the hope, finally, of returning to normal soon, what percentage of French people should be vaccinated?

Last July, a study published inAmerican Journal of Preventive Medicine showed that to achieve collective immunity, vaccine efficacy should be at least 60%, in case 100% of the population were vaccinated. In the event that vaccination coverage drops to 75%, when for example vaccination is not mandatory, the effectiveness of the vaccine should reach 80%. But even an efficiency rate of between 40 and 60% could already be a game changer in keeping the pandemic under control. Therefore, the researchers calculated that a 40% effective vaccine would have prevented 89.5,000 days of hospitalization and 2.8 million people would have put on artificial ventilation. By announcing efficacy rates of more than 90%, vaccine candidates should therefore end the epidemic. But another criterion raises uncertainties.

In a report on vaccination strategy presented in early July, the Scientific Council estimated, based on the work of Swedish and British researchers, that a “Group immunity could be achieved by protecting 43-49% of the population.” This is on the sole condition that the vaccine is sterilizing, that is, that it prevents the transmission of the virus by vaccinated people. But on Tuesday, Moderna’s chief medical officer Tal Zaks told HBO that the results still don’t show that the vaccine prevents transmission of the virus. “When we start rolling out this vaccine, we won’t have enough hard data to show that it reduces transmission.”, He said. Pfizer Laboratories have not yet communicated on this topic.

The Institut Pasteur, on its website, explains that collective immunity depends above all on R0, i.e. “the average number of immunologically naive individuals a subject will infect after contact”. The higher this baseline reproductive rate, the higher the percentage of immune subjects must be, he says. So, while the R0 of Covid-19 is now less than 1 in all regions, it would be sufficient, according to the calculation indicated, that just under one in 10 people are immune to obtain collective immunity. In Auvergne-Rhône-Alpes, where the R0 was 0.64 last week, vaccination of 0.3% of the inhabitants would be sufficient. For influenza, the R0 is 2, which means 50% of people need to be vaccinated.