History shows that the viruses, bacteria and other microbes behind pandemics stop spreading when they no longer find hosts in which to proliferate. And there are only three ways to reach this epidemiological wall, lists Patrice Bourdelais, demographer, historian specializing in epidemics and director of studies emeritus at the École des Hautes Études en sciences sociales in Paris.
“There are the deceased who did not resist, those who had it and who survived it, and those who have built up immunity [avec un vaccin]. »
The smallpox virus still remains today the only pathogen that humanity has managed to wipe off the face of the Earth. The last known case of smallpox was recorded in 1977. “Smallpox disappears when the three categories of the population are 100%. But we got rid of it after a massive vaccination, which lasted for more than a century”, puts Patrice Bourdelais into perspective.
“A shift” is currently taking place with COVID-19, according to him, since immunization rates – by vaccination or contamination – are approaching 90% in several Western countries.
“Hence the idea that we are coming out of the pandemic. Except that the pandemic doesn’t just mean an epidemic in the West. There are all the countries of the South which are not very vaccinated. »
Sanitary standards
Several exit routes make it possible to slow down pandemics, without however stopping them. Diseases of bacterial origin have declined with the acceleration of public hygiene and urban sanitation policies. Cholera, for example, was brought to a halt when it was discovered that the bacterium responsible for it was transmitted through dirty water. As soon as the cities were equipped with purification plants with sand filtration, “before and after, it was no longer the same curve of infectious disease”, maintains Patrice Bourdelais.
Cholera has thus practically disappeared from Western countries, where it now remains endemic. The disease still strikes millions of humans each year in places less well equipped with public hygiene.
Plague, a disease activated by a zoonosis with infected rodents and fleas, has also become endemic due to advances in hygiene. However, it never completely disappeared. From 2010 to 2015, the WHO recorded 3248 cases of plague worldwide.
“There are sorts of plague outbreaks, then the deaths go down. Then, two or three years later, hop!, in a region, there is the plague again. It relativizes the notion of the disappearance of an epidemic,” said Mr. Bourdelais.
“Every time, it’s the same story. The whole population has either been in contact, or has become resistant, or the most fragile have died. »
Major epidemics have the particularity of being followed by adjustments to health standards, which limit their spread. For example, the so-called “Hong Kong Flu” coronavirus outbreak of 1968 sparked early interest in antiviral vaccines. It was after this epidemic, which claimed 25,000 lives in one month, that the annual vaccination of the most vulnerable against the flu began.
At the sources of the contagion
The Spanish flu pandemic reveals how a coronavirus pandemic evolves. This disease appeared and then disappeared in the space of three years. “There was the first wave in the spring of 1918, which was not very serious, no doubt because the first virus was less serious than the one that arrived in the fall, relates Patrice Bourdelais. It is in the fall that the devastation is very heavy. In 1919, there were practically none. And there is a little rise in 1920, and then it disappears. »
The author of numerous works on the subject argues that the “advantage” of these pandemics of yesteryear is found in the absence of precaution (“or almost”) on the part of the authorities. “The contamination spreads very quickly and immunity is acquired very quickly too,” he points out.
This means that the current health measures prolong the duration of the COVID-19 pandemic. “It’s mathematical and almost modelable. The more you protect people, the more you leave important parts of them that did not come into contact with the virus quickly. You leave unscathed pockets, and these pockets will gradually be affected by the virus. »
If SARS-CoV-2 evolves into a fragile virus and mortality is limited by vaccination, this virus could end up being confused with seasonal influenza, like other coronaviruses of the past.
Remember that until the arrival of COVID-19, about one billion people around the world suffered from the flu each year. Of these, the WHO estimates that between 290,000 and 650,000 patients lost their lives.
In the short term, the director of human and social sciences at the CNRS places the future of the COVID-19 pandemic in the hands of chance. “Or we are lucky, and there are no new viruses emerging in unvaccinated areas. Or if they emerge, they are more fragile. Or, we are less lucky and within six months, a year, we have a new variant that avoids vaccines, is very contagious, and we are off again. »
This text is taken from our newsletter “Coronavirus mail” of January 24, 2022. To subscribe, clickhere.