- On Monday evening, Boris Johnson announced national confinement for at least three weeks in the UK.
- Long criticized for his strategy of "group immunity", the Prime Minister changed his mind. This is not the case for Sweden.
- But what is this principle? Why is it a problem in the case of coronavirus? 20 Minutes goes around the question in four points, to see more clearly.
The United Kingdom had for a time chosen to bet on "collective immunity" to avoid confinement and blockage of the country. The Netherlands also. But for the past few days, these two countries seem to have changed their minds. And opt for more severe containment measures to limit the spread of the coronavirus pandemic. But what are we talking about exactly? 20 Minutes helps you see more clearly.
What is "collective immunity"?
It is a principle that the more people are infected with a disease, the more antibodies they develop against this virus, and the less the epidemic spreads among the population. “For Covid-19, it is estimated that the reproduction rate [contagiousness] is around 2.5, explains Jean-Stéphane Dhersin, mathematician specializing in epidemic modeling. This means that the first 1,000 people infected will transmit to 2,500 people. If we do nothing, this curve increases exponentially. Once half the population is immunized, an average person will infect 1.24 people. And if 60% of the population has been in contact with the virus, you will only transmit it to one person. When you reach a reproduction rate of 1, the epidemic no longer spreads. "
This is why some countries have chosen not to confine populations, hoping that a rapid spread of the virus, once it has affected 60% of citizens, would provoke protective collective immunity in the long term.
Find our file
To illustrate this concept, Mircea T. Sofonea, lecturer in epidemiology and evolution of infectious diseases at the University of Montpellier, takes an analogy: "An epidemic can be represented as a forest fire that spreads quickly. The difference with trees is that humans move: we have contact not only with neighbors, but at work, during races. It is enough that a fraction of the population is resistant to the disease for it to prevent the reproduction of the pathogen. It's like having a less dense forest. "
Historically, where does this concept come from?
The first models were published in the 1930s. Working on Spanish flu at the start of the 20th century, mathematicians discovered that an epidemic does not die "for lack of combatants" - a situation in which the infectious agent would eventually disappear with the patients he kills - but by acquiring "gregarious immunity", explains Antoine Flahault, specialist in public health and epidemiology at AFP.
But this concept is mainly used to fight epidemics when you have a vaccine, to determine the rate of vaccine coverage necessary, to be sure that smallpox or measles will disappear, for example.
Which countries have chosen this strategy?
Little by little, some countries that wanted to build on this approach have revised their copies. At first, the British Boris Johnson popularized the term "collective immunity". While France imposed confinement on the entire population (with some exceptions), our British neighbors were invited to wash their hands. But faced with a nightmare scenario, which estimated that if nothing were done, 250,000 Britons could die from the coronavirus, the Prime Minister finally decreed Monday evening a national confinement for at least three weeks.
In the Netherlands, too, this reversal is taking shape, but more slowly. If the Prime Minister, Mark Rutte, no longer excludes a total containment of the population, he hopes "that it will not be necessary". Mark Rutte had declared last week wanting to favor the development of a collective immunity in the Netherlands of which, he had warned, most of the inhabitants would be contaminated.
The only European country to go it alone, Sweden has not followed the path of containment. If high schools and universities are closed, primary schools, restaurants and bars remain open. Critics rain down on the decisions of certain politicians, accused of putting the disastrous economic consequences of confinement before the protection of the most fragile.
Why is this a problem for coronavirus?
First of all for human reasons: waiting for more than half of the population to fall ill, with a mortality rate between 1 and 5% depending on the country, is to let a large number of vulnerable people die. In addition, remember that there is no treatment or vaccine today. It is also a risky bet because it ignores the catastrophic consequences of a massive influx of patients in hospitals with insufficient resources.
Another concern: there are still a lot of questions about this Covid-19. And one of the questions is about long-term immunity to this coronavirus: basically, can you get this disease twice? "For Sras, a coronavirus close to Covid-19, studies have shown that antibodies can drop after two years," says Mircea T. Sofonea. This means that for this close virus, immunity is not guaranteed for life. Please note that this cannot be transposed to Covid-19, which has only been discovered since December. No one can tell you if people who have been infected will have a persistent immune memory within a year. We advance in the fog. "
Also, can this virus mutate? "Nothing says that group immunity is sufficient if the pandemic continues in other countries, that it circulates there quietly," continues the expert. And come back in a few months with mutations such that our immunity would not recognize it. So the flu reappears every winter with different strains, that's why you have to get vaccinated every year.
For many, the question of post confinement is beginning to arise. What will happen if, as soon as the strict measures are lifted, the French return to the parks, schools and nursing homes? "In the absence of a vaccine, knowing that it takes between 6 and 18 months to develop on average, the epidemic is likely to restart, including through imported cases," warns Marcea T. Sofonea. This is what seems to be observed right now in Hong Kong. "As long as this 60% threshold is not reached, States should maintain vigilance on the chains of transmission so that the epidemic does not restart," continues the epidemiologist. In particular, we keep new cases in quarantine and trace all contacts. Or by maintaining confinement for the most fragile people. Politicians have not finished facing difficult dilemmas.
World
Coronavirus in the United Kingdom: Faced with nightmarish forecasts, the British government changes its strategy
Lille
Video. Coronavirus: Is the Franco-Belgian border really closed?
- Coronavirus
- Health
- Covid 19
- World
- epidemic
- UK
- Sickness
- Netherlands