Diagnostic tests and antibody blood tests in Antibes, France on April 29, 2020 during the COVID-19 Coronavirus pandemic. (Photo by Lionel Urman / Sipa USA) / 29756055 // 2004291530 – Lionel Urman / Sipa USA / SIPA
Can you be protected against Covid-19 after common colds? Is collective protection higher than previously thought? Behind these questions, a central notion in debate since the beginning of the coronavirus pandemic:
immunity. Some see it as a bulwark against a
second immediate epidemic wave, but for others, these hopes are premature.
“The absence of a resurgence of Covid-19 cases after containment relief in several countries is intriguing,” said Professor François Balloux of University College London on Twitter. Among the explanations, the possibility that “a proportion of the population may have pre-existing immunity to SARS-CoV-2”. It could be “potentially due to past exposure to other common cold coronaviruses.” And “could explain some intriguing observations, such as cases where there is no contamination between spouses,” adds Professor Balloux.
A hypothesis made by Yonathan Freund, professor of emergency medicine at the Parisian hospital of Pitié-Salpêtrière. He builds on what he observed: a significant drop in the number of infections among doctors compared to the start of the epidemic. “This is pure speculation but it could mean that people have natural or acquired immunity,” he told AFP. “The second wave is not happening and probably will not happen” in France, judges Professor Freund, while the indicators are green almost three weeks after the lifting of the containment.
Better protection thanks to other coronaviruses?
The hypothesis that one could be immunized against the new coronavirus after having been exposed to other benign viruses of the same family had already been advanced at the beginning of the epidemic to explain the low contamination of children (since they are often colds). This concept of “cross-immunity” gained momentum with the publication in mid-May of an American study in the specialist journal Cell.
According to her, 40 to 60% of the population could be immunized against Covid-19 without even having been exposed to it, thanks to the action of protective cells, T lymphocytes, previously activated by other coronaviruses responsible for colds. But according to its authors, this should not be taken to mean that the epidemic is ending. “Our study does not cover this hypothesis and does not in any way support it,” say AFP two of the co-authors, Alessandro Sette and Shane Crotty, of the La Jolla Institute for Immunology (LJI) in California. “Some infected individuals form severe forms or even die, while others have very few symptoms: our study suggests that preexisting immunity could be one of the factors, but at this stage it is just a hypothesis that requires other work ”, underline the two researchers.
The idea of a reduced target
Caution shared by WHO. “There is no evidence that a previous infection with another coronavirus prevented contracting Covid-19,” said Michael Ryan, a manager. Cross immunity or not, there is a growing body of scientists who believe that not everyone is likely to get Covid-19. “It is wrong to assume that all individuals are as susceptible [de contracter la maladie] and as much exposed as each other, “said Dr. Gabriela Gomes, a researcher at the Liverpool School of Tropical Medicine, to the AFP. “Little is known about the factors that determine these individual variations: it could be cross immunity but also genetic, age, lifestyle, and more likely a combination of many factors,” she said.
Theoretically, this idea of a reduced target would explain why the first estimates of the proportion of individuals who have been infected with SARS-CoV-2 are so low (approximately 5 to 10% of the total population in several countries ). It would also imply that the threshold for collective immunity is lower than previously thought. Collective immunity is reached when a sufficient part of the population is infected, ending the epidemic. This is
this threshold that Sweden hoped to cross by not opting for containment.
The 70%, a simple belief?
Until then, it is commonly accepted that this bar is around 60 to 70% of the population. But Dr. Gomes objects that this estimate comes from mathematical models that have overlooked “individual variations in susceptibility and exposure to the disease.” According to a study she co-authored (not yet evaluated by other scientists), the collective immunity threshold is in fact much lower, around 10 to 20% of the population. “70% is purely a magic number” that comes under “belief”, abounds for the AFP Professor Jean-François Toussaint, French scientist who does not believe in a second wave.
Dr Gomes does not go that far: she expects “an increase in cases that looks like a second wave in the least affected countries so far”. In contrast, “more affected countries are closer to group immunity”, which would simply lead to the outbreak of “small outbreaks, located in regions least affected by the first wave,” she said. Beyond the immediate consequences of the end of confinement, an unknown factor remains about the seasonality of the virus and the possibility of a second wave in the fall and winter next in the northern hemisphere.
A response to the virus
Whether or not all of these assumptions hold true, we are starting to learn more about the Covid-19 immunity mechanism. A French study by the Institut Pasteur shows that even patients with minor forms develop antibodies that can potentially immunize them.
The objective is now “to study how this response will evolve: will the antibodies be maintained, have this neutralizing activity and for how long”, explains to the AFP Professor Olivier Schwartz, responsible for Virus and immunity unit at Pasteur. And for Michael Ryan of the WHO, all the recent advances in knowledge about immunity are especially encouraging in the perspective of a vaccine.
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