COVID-19 | From pandemic … to endemic

Over the years, COVID-19 will become an endemic disease. Its impact will no longer be determined by the devastating novelty to the human immune system. Will it then look like the flu? Or the common cold? Will it be seasonal? Two years after the appearance of SARS-CoV-2, science is starting to find answers …

Mathieu Perreault
Press

Flu and cold

Ultimately, when a relatively high vaccination rate is achieved around the world and the unvaccinated are partially protected by the natural immunity conferred by infections, COVID-19 is expected to kill two to four times more. people every year than the flu, a leading virologist, Trevord Bedford of the Fred Hutchison Cancer Research Center in Seattle, concluded in February. His analysis was based on the high mutation rate of SARS-CoV-2 and its high ability to spread from person to person. But David Heymann, an epidemiologist at the London School of Hygiene and Tropical Medicine, believes on the contrary that SARS-CoV-2 will ultimately behave more like the coronaviruses responsible for the common cold. “It is certain that everything will depend on the mutations, but I note that of the fifteen or so variants that the WHO is following, only four have become variants of concern, and none since the spring,” says Mr Heymann. It is therefore too early to predict mortality from endemic COVID. ”

Competetion

In addition, the flu could “protect” against COVID-19. A British study published in theInternational Journal of Epidemiology showed that at the start of the pandemic, flu patients were 58% less likely to have COVID-19. Maryse Guay, flu specialist at the University of Sherbrooke, confirms that “competition” between different respiratory viruses has already been observed. But she notes that the UK results suffer from the lack of data on influenza vaccination and patient behavior. In early 2020, just before the pandemic, a British study had shown in the journal PNAS that rhinoviruses, also responsible for colds, reduce the risk of getting the flu by 70%.

Seasonal

The process that transforms a pandemic disease into an endemic disease should in this case take half a dozen years, estimated in recent studies Rachel Baker, specialist in virus-environment interactions at Princeton University. Ultimately, according to M^me Baker, COVID-19 is expected to be seasonal, like the flu. Andrés Finzi, immunologist from the University of Montreal, agrees with Mr.^me Baker. “But that remains hypothetical for the moment, because there are too many variables,” says Finzi, one of whose studies showed that SARS-CoV-2 binds more easily to its target in the human body at low temperatures. temperature. Various studies published in 2020 had shown that the “naivety” of the human immune system to SARS-CoV-2 played a much more important role than temperature and other environmental factors, which explained a maximum of 17% of transmission.

Other coronaviruses

SARS-CoV-2 is a coronavirus, like four cousins ​​who are responsible for 20% to 30% of colds. Many researchers are therefore trying to find out whether cold coronaviruses protect against COVID-19. In Nature In early November, researchers at University College London studied the immune systems of 58 caregivers repeatedly exposed to SARS-CoV-2 at the start of the pandemic, but who never tested positive for COVID-19. They observe a reaction of a part of the immune system called “T cells” that is similar to that of infected and recovered or vaccinated patients. Their hypothesis: the cold coronaviruses, to which these frontline caregivers were surely exposed just before the pandemic, provide cross-protection. “This study is part of this new field of study which is the understanding of how pre-existing responses, in particular cross-activity, influence the infection, but also the vaccine response”, observes Mr. Finzi.

Surprising reservoirs

Much ink flowed in early November when a study showed that nearly half of American white-tailed deer carried SARS-CoV-2. A new study has identified which of 5,400 animal species could be infected. Cats and other felines, as well as non-human primates, are on the list, as was already known from studies on pets and zoos. But the scimitar-horned oryx, an African gazelle, is also one of them. Less than 10% of the species studied are concerned. Researchers at the Cary Institute for Ecosystem Studies in New York State completed their analysis with artificial intelligence modeling, because they lacked genetic or biological information about the targets of the SARS-CoV-2 only for 5% of the species studied.