Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has infected tens of millions of people worldwide since its outbreak in late 2019. Millions of people have died from Corona virus disease 2019 (COVID-19), which is caused by infection (Li, 2020; Li and Zhong, 2020; Mason, 2020). The common signs of human infection with SARS-CoV-2 include respiratory symptoms, fever, cough, shortness of breath dyspnea, etc. These symptoms are similar to severe "influenza" in North American and European (Agyeman et al., 2020; Grant et al., 2020; Huang et al., 2020). In more severe cases of COVID-19, SARS-CoV-2 infection can lead to pneumonia, severe acute respiratory syndrome, kidney failure, and even death, which is closely resemble the severe acute respiratory syndrome (SARS) which occurred in 2003.

For these reasons, in the early days of the outbreak and spread of SARS-CoV-2, most countries in Europe and the United States thought it was just a "great influenza", and the coming warm weather would slow or even stop the spread of the disease. The outbreak of SARS in 2002 was still fresh in Chinese people’s memory, which also emerged in winter but ended in summer 2003, which also seemed to mean that summer and high temperatures could prevent the transmission of SARS-CoV-2.

With the continuous spread and outbreak of SARS-CoV-2 in the whole year of 2020, the influence of seasonal changes, temperature and humidity changes on SARS-CoV-2 itself and its transmission has become a problem of great concern. Is the transmission of COVID-19 affected by seasonal temperature changes? What are the effects of temperature and humidity on SARS-CoV-2 itself and its spread?

This study attempted to explore the above problems and provide a theoretical basis for controlling the transmission and spread of SARS-CoV-2 by analyzing the relationship between SARS-CoV-2 activity and temperature, SARS-CoV-2 transmission and temperature, and SARS-CoV-2 transmission and season.

1 Relationship Between the Activity of SARS-CoV-2 and Temperature

Generally, coronaviruses are sensitive to heat, and can be stored at -60℃ for several years. With the increase of temperature, the activity of the virus decreases, and the activity is moderately stable in the suitable maintenance fluid at 4 ℃, but the virus can be inactivated only when it reaches 92℃ for more than 15 h (Pastorino et al., 2020). Sars-CoV-2 can survive for 2~10 days on different body fluids or surfaces at room temperature (Liu et al., 2020; Kampf et al., 2020). Temperature mainly affects the survival time of the virus, not its infectivity.

When the temperature reaches 56 ℃ for 30 min, SARS-CoV-2 could be effectively inactivated. Obviously, it is impossible to inactivate SARS-CoV-2 by increasing the temperature of environment with air conditioning or using heating installation. The radiation temperature of the sun cannot reach 56℃, and the intensity of ultraviolet rays cannot reach the intensity of ultraviolet lamps. Therefore, both sunlight and ultraviolet rays cannot achieve the effect of inactivated new coronavirus (https://archive.today/20200316233945 ; http://www.nhc.gov.cn/yzygj/s7653p/202003/46c9294a7dfe4ce f80dc7f5912eb1989.shtml).

2 Relationship Between the Transmission of SARS-CoV-2 and Temperature and Humidity

Previous studies have suggested that the efficiency of virus transmission decreases with the increase of absolute humidity. In the calculation formula of statistical data adopted in a piece of paper in Science in 2018, absolute humidity appeared as a key parameter representing the air environment, confirming the significant influence of absolute humidity (Dalziel et al., 2018).

For the epidemic of SARS-CoV-2 in 2019, although there is no authoritative research report on the influence of temperature and humidity on the transmission of SARS-CoV-2, "SARS" in late 2002 and early 2003 can still be used as a reference. The severe acute respiratory syndrome coronavirus (SARS-CoV) epidemic began in the winter of 2002 and ended abruptly in the early summer of the following year. This is in sharp contrast with the SARS-CoV-2 which broke out at the end of 2019, prevailed in the summer and rebounded severely in the winter of 2020.

A paper in 2011 conducted experiments on SARS-CoV and found that SARS virus remained active on smooth surfaces for more than 5 days in a typical air-conditioned room environment (Temperature: 22~25 ℃; Relative humidity 40%~50%). However, at 38℃ and 95% relative humidity, the virus quickly lost its activity (Chan et al., 2011). The study made it clear that the effects of high temperatures and humidity could explain why SARS did not transmit to large scale in hot and humid countries such as Indonesia and Thailand, and in similarly hot and humid Singapore, it only transmitted in hospitals and hotels with high levels usage of air conditioner. Some scholars also studied the outbreak of four cities including Beijing and Guangzhou, and pointed out that SARS-CoV was significantly correlated with temperature (Tan et al., 2005). The paper pointed out that temperature, humidity and wind speed were the three key factors of SARS transmission. The study highly affirmed that the practice of opening doors and windows for adequate ventilation was an effective measure in Guangzhou hospitals at that time (Yuan et al., 2006).

A study in 2013 showed that the respiratory syndrome coronavirus in the Middle East, MERS, regardless of whether it is in liquid droplet or aerosol state on solid surface, its activity can be maintained for a long time under low temperature and low humidity, and the activity of MERS decreases significantly with the increase of absolute humidity (van Doremalen et al., 2013). Direct testing of coronaviruses other than SARS and MERS has led to similar findings that low temperatures and humidity significantly facilitate coronavirus transmission and survival (Kim et al., 2007; Casanova et al., 2010).

On February 25, 2020, researchers from Sun Yat-sen University published a study on the preprint website medRxiv, which explores the role of air temperature in the transmission of SARS-CoV-2 and proposes a possible nonlinear dose-response relationship between them. When a certain temperature is reached, the rate of virus transmission is the highest, and the subsequent rise in temperature may inhibit the virus transmission (Wang et al., 2020).

Based on the above statements, high temperature and high humidity have a clear inhibitory effect on the transmission and activity of multiple types of influenza and coronavirus. This is obviously contrary to the global epidemic of SARS-CoV-2 epidemic in the whole year.

3 Relationship Between the Transmission of SARS-CoV-2 and Seasons

3.1 The SARS-CoV-2 is not a "seasonal" virus

According to an article published in PLOS Pathogens, all infectious diseases can be seasonal, and despite their differences, they all have one thing in common: they rise and fall with the seasons (Lipsitch and Viboud, 2009). Thus, most of us will have the view that seasonal factors will slow the transmission of SARS-CoV-2 around the world as temperatures rise.

As everyone knows, influenza, cough, common cold and other respiratory infections have seasonal ebb and flow, which makes it easy for CDC and epidemiologists to predict and prevent influenza-like viruses (Lipsitch and Viboud, 2009).

At the beginning of the outbreak of SARS-CoV-2 in 2019, some infectious disease experts, especially many ordinary people, are hoping that SARS-CoV-2 will end abruptly and disappear without a trace by early summer of the following year, like the SARS in 2003. Instead, SARS-CoV-2 wreaked havoc throughout 2020.

"We can only compare it to other diseases that spread in a similar way," says Paul Hunt, an infectious disease expert at the University of East Anglia in the UK. Certain environmental factors may also help SARS-CoV-2 transmit. Cold weather, humidity, and the way people behave in winter may all influence the trajectory of SARS-CoV-2 disease (Steel et al., 2011).

An epidemic expert at Brown University said relying on seasonal factors to contain epidemics can be a dangerous idea. Seasonality may reduce infection rates, but we are not close to solving epidemic on its own, and it is more important to rely on people's preventive measures to slow transmission.

3.2 Winter can accelerate the transmission of respiratory diseases

Many respiratory infections, including COVID-19, are transmitted by droplets. These droplets are released when an infected person coughs or sneezes. Experts say, “When the air is cold and dry, these droplets are more likely to float in the air for a longer period of time, thus traveling farther and infecting more people. In winter, it's easier for people to stay indoors and congregate, which increases the risk of infection.”

"Cold weather is thought to cause the transmission of coughs, colds and influenza because cold air irritates the nasal and respiratory passages, which makes us more susceptible to viral infections," says Simon Clark, a cellular microbiologist at the University of Reading in the UK.

3.3 Spring does not help control COVID-19

The SARS virus in 2003 came to an abrupt end in the early summer of the following year. It gave politicians in many countries around the world the wrong signal that the epidemic of SARS-CoV-2 in 2019 would "cure itself" by the summer of 2020, which led to the epidemic out of control in many countries. The United States is a prime example.

In general, influenza-like virus diseases have an intermittent period in early summer, especially in the northern hemisphere, influenza-like virus diseases are often likely to decrease significantly. The epidemic of SARS-CoV-2 is a complete exception. It did not disappear in the summer, and rebounded again in the winter, especially in the United Kingdom, and even in China, which has done the best in the world in the prevention and control of SARS-CoV-2.

4 Conclusion

COVID-19 does transmit like influenza, which both lead to typical mild respiratory illness, and both developed severe, life-threatening pneumonia. But the rate of transmission and severity of COVID-19 is much higher than that of influenza, and it has been circulating globally throughout 2020. Quite obviously, people cannot determine whether the spread of COVID-19 will be affected by seasonal changes of temperature.

For influenza, as spring and summer come and temperatures rise, there will naturally be a significant drop in the number of cases until the epidemic ends in late summer and early fall. The seasonality of this influenza is thought to be caused by the sensitivity of the virus to different climates, seasonal changes in the human immune system and behavioural patterns.

There are three possible reasons for the seasonal ebb and flow of influenza. First, the influenza virus survives better in cold, dry weather with less ultraviolet light. Second, for humans, many people will experience lower levels of Vitamin D and melatonin during the shorter winter months, which can affect the function of the human immune system. Third, in winter, humans spend more time indoors and in closer contact with others, which further increasing the chances of the virus transmitting.

Up to now, we can't say what effect temperature and humidity have on SARS-CoV-2 itself and its transmission, but it is certain that some other coronaviruses are seasonal. For example, SARS in 2002~2003 began in the winter of the northern hemisphere and ended in the summer of 2003. Most scholars believed that the number of cases of SARS peaked in May. The end of the epidemic in July may simply reflect that the containing of the virus needs time, rather than the impact of summer weather on its spread.

Similarly, the influenza pandemic from 2009 to 2010 began in the spring. The transmission and prevalence of influenza viruses increased in spring and summer, and peaked in the subsequent fall and winter. Obviously, in the pandemic, the virus is able to circulate and spread continuously throughout the summer. As a result, warmer weather may reduce the spread of the virus in the northern hemisphere, but it is unlikely to end the growing epidemic.

To summarise, there is no clear conclusion about the relationship between virus and climate. As COVID-19 continues to spread around the world, long-term observation and analysis of the relationship between COVID-19 and climate, as well as the relationship between COVID-19 and season are needed, which is a very important research focus and a hot spot of public concern for the control and prevention of viral pandemic.

Authors’ Contributions

GH was the executor of this study, responsible for data analysis and writing the first draft; ZJ is the person in charge of this study, responsible for the revision and proofreading of the article; ZJY participated in data collation. All authors read and approved the final manuscript.

Acknowledgments

This study was funded by the Special Fund of "Clinical Research and Control Strategy of COVID-19" of Hainan Institutes of Biotechnology (Project No. 20200124).

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