During the first months of the COVID-19 pandemic, one of the hopes of society and administrations was that the transmission of the SARS-CoV-2 virus would be seasonal, as in the case of seasonal flu. This would reduce the incidence of the virus in summer due to the high temperatures.
Many scientific studies have analyzed the relationship between environmental conditions (mainly temperature and relative humidity) and the number of cases or mortality of COVID-19 in different countries or regions, before there was any type of restriction on mobility or lockdown.
The importance of methodology
In general, the studies, in many cases published without peer review, suggest that the number of infections was higher if the environment was cold and dry (low temperature and relative humidity). This relationship seemed to exist on a global level , in the different autonomous communities or in Barcelona, for example.
Unfortunately, many of these studies had some methodological errors:
- At the statistical level, they did not consider other variables that could also have an influence on transmission.
- They did not take into account the time lag that existed in the first months of the pandemic between the day of infection and the day of official notification of the case after performing the PCR test (between 5 and 10 days depending on the country).
- They forgot that temperature and relative humidity are not independent variables and, in general, they confused correlation with causality.
If the possible relationship between temperature / humidity (represented by temperature dew point) is analyzed and incidence considering provinces (or similar administrative entity) in Spain, Italy, Sweden and Germany during the weeks leading to restrictions on mobility, and takes into account the delay between contagion and the official communication of the positive result of the PCR, it is observed that, if there is a relationship, it is not evident at all (see graph).
Furthermore, the influence of environmental factors on transmission is less than the effect of mobility restrictions or decreased social relationships.
Incidence IA14 (cases / 100 000 inhabitants in the last 14 days) and IA14 normalized by the area of the province of each country 10 days after the beginning of the epidemic. In each case, based on the province’s mean temperature and dew point temperature between February 14 and March 15, 2020. The blue asterisk is Lombardy (IT) which is clearly outside the adjustment curve for Italy if its surface is taken into account
Infections have not decreased during the summer in the northern hemisphere . This has occurred especially in those regions with little or no promotion of hygienic measures to contain the virus, such as the southern states of the US In addition, there have been and are very high incidences of the disease in tropical regions. In the case of Spain, the second wave occurred in the middle of summer.
So what is the influence of temperature, humidity, and solar radiation on the transmission of SARS-CoV-2?
To answer this question, it must be taken into account that environmental conditions influence the transmission of the disease, and many others, according to three different aspects.
Sociological factors
Environmental conditions modify our life habits. In summer we tend to spend less time indoors. This should have helped reduce transmission, as it has been shown that the risk of contagion of COVID-19 can be up to 20 times higher indoors.
This may be because transmission occurs, not only through ballistic droplets that fall rapidly to the ground, but also through smaller liquid particles called aerosols that we exhale when we breathe, speak, cough, or sing and which may contain active viruses.
Aerosols can remain suspended in the air for several minutes depending on their size and environmental conditions (any air movements). This implies that COVID-19 can also be transmitted over distances greater than 2 m, especially in poorly ventilated interiors with many people without a mask, if you speak loudly, sing or breathe with intensity.
Microbiological factors
Environmental conditions also influence the time that the virus remains active in the air or on a surface, although this last route of contagion (through fomites) seems to be less important than previously believed.
In a hospital environment, SARS-CoV-2 has been found in the air up to 4 m away from the patient and on various surfaces, where it can remain active between a few hours and more than a day, depending on the surface. However, another study detected the virus in the air and on various surfaces of the plant rooms, but not in the three ICU rooms analyzed.
It is important to note that, for infection to occur, it is not enough for the virus to be present in the air or on a surface; it also has to be active. After a time, which depends on the surface, the virus becomes inactive, that is, it does not cause disease.
Similarly, a person, despite still having a PCR + (virus or virus fragments are detected in a sample), if they do not have symptoms, can come out of isolation 10-14 days after the onset of symptoms or test result because you cannot transmit COVID-19.
Ultraviolet radiation from the sun has been shown to inactivate the virus after a few minutes. Depending on the solar intensity, at 21 ℃ and 40% relative humidity, the virus in the air can go from being active for 1 hour to 2 minutes.
Due to the above factors, parks during the day are environments with a low risk of transmission. Closing them, a measure without scientific support so far, can even be counterproductive: social interactions between children will then be indoors, where there is no solar radiation and ventilation is less.
On the other hand, ultraviolet (UV) C radiation – not present in sunlight reaching the earth’s surface – also inactivates SARS-CoV-2. That is why it has been proposed as a method of reducing the concentration of the virus indoors with many people and without effective ventilation. However, it is necessary to verify that this type of radiation does not cause skin lesions.
Temperature and relative humidity also influence the inactivation time of airborne viruses.
In the case of SARS-CoV-2 we have estimates of the lifetime both on interior surfaces and in the air, although its effect is less than that of solar radiation.
In general, the half-life of the virus is longer in cold environments, especially when the relative humidity is low. This, together with poor ventilation, probably explains some outbreaks registered in food processing industries, mainly meat.
The phenomenon could also explain why the incidence has increased in regions, such as the southern United States, with high temperatures, where air conditioning (which also reduces relative humidity) is common and life is mostly done in the inside. In this sense, it is recommended to keep the relative humidity at high values, but always within the comfort ranges.
In any case, if the distance indoors is not respected, the effect of temperature or humidity on the half-life of the virus and, therefore, on the transmission of COVID-19, is reduced.
Physiological factors
Finally, although there are practically no specific studies on COVID-19, it has been proven that temperature, humidity and solar radiation modify our susceptibility to becoming infected with a respiratory infectious disease.
These variables change our ability to prevent viruses from entering the airways or enhance our immune system. This is a fundamental factor in explaining the seasonality of some respiratory infections due to viruses, such as influenza.
Solar radiation, through UVB, favors the generation of vitamin D in the skin. Vitamin D improves the immune response and helps reduce the risk of catching or dying from an infectious disease.
For its part, breathing air at low temperatures would lower the temperature of the nasal epithelium, reducing the effectiveness of local respiratory defenses. This would make the nostrils partially lose their ability to prevent the virus-carrying particles from entering the airways.
what will happen during the coming months?
In summary, the effect of radiation, temperature and humidity on SARS-CoV-2, and on our body, as well as changing habits when the cold arrives, can facilitate the transmission of the virus in autumn and winter. This could partly explain the increase in cases in Europe today, and what is happening with many other respiratory infectious diseases (including the 1918 flu pandemic). This is why some scientists forecast the second wave of COVID-19 for the fall in the Northern Hemisphere.
However, there are many other factors, such as prevention measures, carrying out PCR tests and subsequent study of contacts and, above all, the lack of immunity of the population, which most probably affect the transmission of the disease more. COVID-19, as unfortunately we have been able to verify in Spain this summer.
The author thanks Dr. PJ Cardona (IGTP, @pjcardona) for his invaluable help in understanding how temperature and humidity influence upper respiratory conditions.
David Pino González, Associate Professor in the Physics Department, Polytechnic University of Catalonia – BarcelonaTech
This article was originally published on The Conversation. Read the original.
