Effect of weather on Covid, what scientists say

As bar and restaurant terraces reopen and sanitary restrictions ease, more good news could come from the onset of summer and its warmer temperatures.

Numerous scientific studies have indeed shown that most epidemics and infectious diseases are characterized by seasonal movements - we speak of "seasonality": the hot seasons are more conducive to the disappearance or reduction of epidemics born in cold seasons.

This is the case, for example, with the so-called “seasonal” influenza. For researchers Jeffrey Shaman and Melvin Kohn, flu intensity increases during cold periods, when temperatures and humidity are at their lowest. As such, the Covid-19 virus would not be much different, as evidenced by its dynamics across seasons and hemispheres over the past three waves.

2015 WHO influenza video (WHO / Youtube).

But what about scientific studies specifically on Covid-19? And can we really count on a boost from the summer, combined with vaccination, to bring down the masks?

The effect of humidity and solar radiation

It has been shown by several laboratory studies - including that of Günter Kampf and his colleagues published in 2020 - that the resistance of human coronaviruses to which SARS-CoV-2 belongs, both in the air through droplets and on surfaces, is significantly reduced by hot and humid conditions.

On the contrary, under drier conditions, the water contained in the droplets evaporates more quickly and leaves the virus to stagnate in the air, which increases its transmission, especially in a poorly ventilated and confined environment.

The results of studies on other human coronaviruses point to a reduced persistence in hot atmospheres. In 2011, researcher KH Chan and his colleagues confirmed, for example, for SARS-CoV-2, that the viability of the virus deteriorates rapidly. at high temperatures and relative humidity levels.

Cold, dry conditions - particularly in environments using air conditioning and / or polluted conditions - increase the transmission of the virus.

From labs to statistical analyzes

Are the results of laboratory studies statistically verified in real conditions?

For the researcher Florence tushabe, it would be the case:

“The severity of the disease in terms of infection and death is 6 times higher for countries located in the temperate region. Infections in tropical countries represent only 4% of global infections with even fewer deaths (2,5%). "

For Simiao Chen and his team, as their work published in Scientific Reports, the proximity of the equator would work in favor of a reduction in the number of Covid cases per million inhabitants.

According to other studies, there is a range of weather conditions for which the virus would be particularly transmissible, corresponding to moderate levels, as has been mentioned for Italy, at the start of the epidemic in winter 2020.

However, precisely, the industrialized countries of the northern hemisphere (Europe and the United States) have meteorological conditions and therefore temperatures which are on average within this range and for which the meteorological effects on the virus are weak.

This may have contributed to the development of the epidemic within these countries, particularly during the first wave and more particularly, in towns where the air quality was degraded, as in Lombardy and the region of Madrid.

Report in Nembro, the village in Lombardy most affected by the virus (C in the air / Youtube, July 2020).

The relationship between climatic factors and Covid-19 could nevertheless be more complex. “Nonlinear” statistical relationships may exist for temperature levels and ultraviolet in particular: very negative or very positive temperatures reduce the survival of the virus, but moderate temperatures have only a moderate or no impact.

George Luo and his colleagues are more skeptical on the existence of significant relationships between humidity and the survival rate of the virus in China.

Other studies go in the same direction: for Spain for example, Álvaro Briz-Redón and Angel Serrano-Aroca do not find any significant effect of temperatures, by controlling - to understand here "by incorporating the variables" - however for the population density or the number of travelers which could have biased the results of the studies previous ones.

Beyond the role of temperature and humidity, more recent studies have looked at the role played by ultraviolet rays, solar radiation and wind speed.

In January 2021 in the review PNAS, Tamma Carleton and her colleagues have shown, for a panel of 173 countries, that a higher daily level of ultraviolet rays reduces the cumulative growth rate of Covid-19 cases up to a horizon of two and a half weeks; on the other hand, they do not obtain a significant effect of the temperature and humidity levels.

In the end, the link between climatic conditions and Covid-19 would be significant, but of low magnitude, as the specialist Rachel Baker already wrote in the journal Science from summer 2020.

Even though the high temperatures and strong UV levels in summer can help reduce the circulation of the virus, the sensitivity of SARS-CoV-2 to weather conditions would not be strong enough to sufficiently reduce its circulation in the population without it being necessary to maintain social distancing policies.

Thus, the climate would play a favorable, but modest, role in the fight against Covid-19, as recently highlighted Tamma Carleton and her colleagues.

An impact that remains marginal

This is also what we show in an article that has just appeared in the journal Plos One, and which concerns a sample of 37 industrialized OECD countries observed during the first wave of the Covid-19 epidemic.

Our results suggest that rising temperatures, levels of solar radiation and humidity reduce the number of cases and deaths from Covid-19 significantly, but marginally.

The effects were measured primarily at 7 days for the number of cases, which corresponds to the minimum incubation period, but also take into account periods of consolidation and hospitalization up to 28 days.

In a related work - not yet peer reviewed - these results are confirmed for hospitalization rates in France, based on data collected in around fifteen large cities for the first wave.

The climate could thus partly explain, although slightly, why the North and East of France were more affected during the first wave of the epidemic.

We also show that air quality would be an important additional determinant of the virus, insofar as pollution would worsen its prevalence in the French cities covered by our study.

The question of mobility

In our study published in Plos One, and conducted, let us recall, on a sample of OECD countries, we went further than the existing literature, by taking into account the way in which the weather conditions were likely to affect the transmission of Covid-19 according to the levels of individual mobility.

We wondered if our movements (car, walking, etc.) were themselves affected by the weather conditions and if so, how this could also have consequences on the transmission of the virus.

Based on data covering the period from January to early September 2020, mobility tends to increase the spread of the virus: the more individuals move, group together, go out, interact, the more the transmission rate is likely to increase, which is hardly surprising.

However, the effects are not visible until three weeks later in our model. This means that it is necessary to take into account the incubation and confirmation periods of the virus to properly identify empirically the effects of weather on Covid. The very short-term relationship is, however, not clear.

However, favorable weather conditions (sunshine, temperatures) tend to increase our propensity to go out (terraces, friends, supermarkets, parks, etc.), which mechanically increases the circulation of the virus.

The double effect of temperatures

Above all, what we show is that the beneficial effect of weather conditions on Covid-19, given its interaction with the mobility of individuals, is partially reduced by this increased mobility: a beautiful hot and sunny day allows certainly to better ventilate one's interior, and therefore to reduce the persistence of the virus, but it also encourages people to go out more, whether on the terrace of a café or in a park for example, and to interact more with others, by increasing the number of contacts, which further reduces social distancing.

All of this can lead to an increase in the circulation of the virus and the number of cases. This mechanism is even more pronounced if some or all of the social interactions take place indoors, in confined and poorly ventilated places - and with air conditioning, such as in summer, which helps increase the transmission of the virus.

The relaxation of the population raises fears of an epidemic rebound (C dans l'air / Youtube, May 2021).

We thus show that individual mobility tends to increase the circulation of the virus when weather conditions are favorable, thus reducing the beneficial effect of the latter on this circulation.

Note that the absence of social distancing and wearing of a mask was probably more marked last year, the indirect effects that we have identified should therefore be less dramatic in 2021.

It is nevertheless advisable to remain cautious in the face of the indirect effects that could provoke a too rapid relaxation of sanitary conditions, coupled with meteorological conditions conducive to an increase in social interactions.

The application under development by Météo France, MyPredict, should be able to alert its users when weather conditions are favorable for the spread of the virus; it could prove to be a useful additional aid for the French population in the coming months and help them to take their precautions.


Clément Mathonnat, lecturer (BETA, University of Lorraine) is co-author of this article.

Olivier Damette, Professor in economics, associate of the chair “Climate economics” (Paris Dauphine University), University of Lorraine et Clement Mathonnat, Lecturer in economics, University of Lorraine

This article is republished from The Conversation under a Creative Commons license.

Read theoriginal article.

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