A Quebec team investigated the effect of temperature on the interaction between the COVID-19 virus and human cells. The researchers’ findings and reflections help understand the concept of seasonality.

Since the beginning of the pandemic, scientists have wondered whether the SARS-CoV-2 virus is affected by the seasons. After all, coronaviruses usually follow annual cycles, peaking in the winter, as shown systematic review November 2020.

So far, the virus has wreaked havoc in every potential climate and season, from Florida and the Indian summer to the Canadian winter to the Brazilian fall. This does not exclude the possibility that they are sensitive to different times of the year.

Seasonality only appears when the virus has become endemic, which means that a large portion of the population is less susceptible to it because they have already been infected or have been vaccinated. Before that, the situation was too volatile to draw any conclusions. Health measures put in place by governments, for example, confuse epidemiological data: it is difficult to isolate the “season” factor when we compare winter to strict confinement to summer when measures are relaxed. The arrival of new variants also blurred the lines.

University of Toronto professor David Fisman summarized the issue at a round table convened by the World Meteorological Organization on September 22. Globally, “Currently, we are seeing different waves. Their frequency is about four months. They are not seasonal. These multiple waves overlap. Above all, we lack perspective.” We don’t have enough perspective to be able to talk about seasonality, he added. .but I think we end up seeing her show up.”

Better affinity

However, the scientific community is working to clarify the topic. This is the case of a doctoral student at the Research Center of the Center Hospitalier de l’Université de l’Université de Montréal (CRCHUM) Jérémie Prévost, who conducted a series of experiments to test the ability of the virus (or rather the spiky protein) to enter human cells (using the ACE2 receptor). at different temperatures (4, 22 and 37 ° C).

This work was performed on cells in culture. It is clear that it does not exceed 4 degrees in the human body. Although… “These four degrees of nose can happen,” explains Jeremy Prevost. This is where you are exposed to the virus for the first time. In your nose, there is like a temperature gradient. Depending on the temperature around you, you will get a higher or lower temperature in the nose which will be more soothing in the respiratory tract. “

In the lab, the team found that the affinity between the virus protein and the receptor in human cells is best when it’s cold. Therefore, it is possible for the virus to infect cells more easily when the air entering the nose is cold. “It is not the reproduction that occurs at lower temperatures, it is the initial exposure to the virus. The effect at this initial attachment is thought to act as a boost for replication. [qui, elle, se fait plus loin dans les voies respiratoires] », identifies Jérémie Prévost. These results It was just published by the team and other colleagues in Quebec and America in Journal of Biological Chemistry.

The CRCHUM group also continued its work by deepening the state of the variables, in second article Newly appeared in Virology. Scientists note that the disturbing variables have a different relationship to room temperature. wild type virus [la souche initiale] It has to be at a very low temperature to have good affinity with the ACE2 receptor, while some variants, such as alpha, beta and gamma, that have the N501Y mutation, have better affinity with the receptor and are therefore less dependent on temperature,” explains Jonathan Richard, a researcher on the same team It is possible that this is an adaptation of the virus to warmer regions, but that remains only a hypothesis.

For the delta variant, “in terms of temperature sensitivity, it serves as an intermediate between alpha and wild [la souche initiale]. It has better affinity at a higher temperature than in the wild, explains Jeremy Prevost, but not as much as alpha.

The next step will be to study all of this in an animal model, where the effect of temperature may vary in “real life”, outside of culture panels. A project is being started in this aspect.

In addition to works to be published soon in Nature Connections By a British team based on epidemiological and meteorological data from 500 cities around the world also indicates the influence of temperature. The study covers the period from February 2020 to May 2020, when health measures were less, and the results were adjusted according to the interventions that were underway in different countries.

“We found a relative increase in risk [de contamination] At temperatures around 10 degrees, that risk drops again as the temperature rises to 20 degrees Celsius, explained Rachel Lowe, a professor at the London School of Hygiene and Tropical Medicine, during the international roundtable. However, they indicate that health measures had a sixfold effect on the variability of transmission from weather conditions.

Beyond the temperatures

Jérémie Prévost, of CRCHUM, insists seasonality is complicated. “We focused on temperature in our study, but there are a lot of factors that fluctuate by season, such as atmospheric pressure or human behavior.”

He named two, but he could easily have named twenty! In addition to the virus’ “preferences” for weather and human behavior, there are all seasonal variations in human immune function (vitamin D deficiency, for example).

David Fisman believes that scientists would benefit from studying unusual cases, such as those in the southern United States, which have been the scene of many midsummer outbreaks. How is this possible? “A friend who studied in Arizona told me that July happens indoors in Arizona,” he told the science committee. We don’t spend much time outside because it’s very hot! So it’s a manifestation of an aerosol-borne disease where people take refuge indoors. Relationships with weather are complex. “

Because the effects of seasonality are unclear, so-called “operational” epidemiological models, that is, those used for decision-making, generally do not take this factor into account. Better knowledge about the seasonality of COVID-19 will one day help governments better plan hospital resources and put in place the right policies, according to Alan Diner, a health economist who is asking for modeling in hospitals. Hospital Beds at Health Canada. “If we want to manage our healthcare system effectively, we have to really decide instead of taking drastic measures as we cancel all elective surgeries, we have to really understand who is entering the healthcare system,” which includes anticipating an increase in cases, he said during the roundtable. .

A comprehensive understanding of seasonal issues will also help in choosing the appropriate timing for future promotion campaigns, Elizabeth Lee, a researcher at the Johns Hopkins Bloomberg School of Public Health, said at the same event.

Some loose studies:

• Since the beginning of the epidemic, it has been found that The virus lives longer at low temperatures.
• in may 2020, study It did not find any correlation between the temperature of different Chinese cities and the strength of the epidemics.
• In June 2020, research Posted in open gamma network Comparing the cities hard hit by the epidemic with other cities less affected, to find that the former share a similar climate, with temperatures ranging between 5 and 11 degrees Celsius and low humidity.
• In September 2020, study from college environmental sciences It reported that 60% of confirmed cases of COVID-19 worldwide occurred in an area where the outside temperature ranged between 5 and 15 degrees Celsius.
• More modern Chinese works (April 2021) conducted by modeling indicates that the hot season means a 46% reduction in injuries in the Northern Hemisphere. It was published in the scientific journal Environmental Research.
• Another study Dated last spring looked at the number of infections and proximity to the equator. They concluded that a country located 1,000 km from the equator than another country had 33% fewer cases.