SARS-CoV-2 emerged in December 2019 in the city of Wuhan, Hubei Province, China, and by January 30, 2020, it had spread to 18 countries outside of China. On February 12, 2020, the World Health Organization (WHO) announced that the disease would be called COVID-19, and a month later, considering the rapid increase in cases (over 118,000 cases in 114 countries and 4,292 deaths), the disease declared a pandemic.
SARS-CoV-2, which belongs to the same family (Coronaviridae-b) as the SARS-CoV and MERS-CoV coronaviruses that caused epidemics in 2002 and 2012 [1, 2], has a positive polarity RNA genome and a cellular structure that consists of structural proteins (S, E, M, N) and a nonstructural polyprotein replicase. The S protein is involved in receptor recognition, virus attachment, and its entry into human cells [3, 4]. The virus is mainly transmitted by respiratory droplets produced by individuals infected with SARS-CoV-2 [5] through human contact or by touching contaminated surfaces [6].
The virus is spread by coughing or sneezing of an infected person and by human contact or contact with contaminated surfaces (copper, cardboard, stainless steel, and plastic). After an incubation period of 1–14 days [7] the virus causes a respiratory disease (COVID-19) characterized by mild to very severe symptoms (cough, myalgia, fever, fatigue, and shortness of breath). The clinical presentation of the disease ranges from asymptomatic or mild respiratory infection to uncontrolled pneumonia with acute respiratory distress syndrome, multiple organ failure, and death [8–10]. Complications after the acute phase of infection, such as rare multisystem inflammatory disease in children and adults, 2 to 5 weeks after initial infection [11], have also been identified in the cardiovascular and gastrointestinal systems, with dermatologic and mucosal manifestations, such as Kawasaki disease, in children.
COVID-19 has spread globally through air travel, travel, and trade, with countries adopting restrictive measures to stem the spread of the disease. Twenty months after the start of the pandemic, with vaccination campaigns continuing (a total of 5.019.907.027 doses of vaccine have been administered), the fourth wave of the pandemic is sweeping humanity with enormous health impacts (217.558.771 confirmed cases and 4.517.2405 deaths, 4 pm CEST, 1 September 2021) [12] and socioeconomic activities.
In Greece, the first confirmed case of COVID-19 was reported on February 26, 2020. The country's response was immediate, imposing a national lockdown on March 23, 2020. At that time, a total of 695 confirmed cases and 17 deaths had been recorded [13]. On 5 May 2020, as the number of daily cases began to decrease, the restrictive measures gradually relaxed and on 7 July 2020, flights from abroad were allowed to arrive.
Since the beginning of August 2020, cases have shown an increasing trend, with the country gradually entering the second wave of the pandemic. In the months of October-November, a rapid increase in cases was observed and on November 7, 2020, a new national lockdown was imposed. On November 12, 2020, the epidemiological curve peaked, reaching 3.316 cases per day. After a gradual decrease in cases until January 31, 2021 (484 daily cases), the country entered the third wave of the pandemic in early February. In the next two months, the number of cases increased rapidly, and on March 30, 2021, the epidemiological curve reached its highest point, with 4,340 cases daily. From the beginning of the pandemic to 1/9/2021 counts a total of 590.832 cases and 13.743 deaths occurred [13].
The effects of climate change and weather conditions on the spread of infectious diseases have been the subject of many studies [14, 15]. It has been found to affect ecosystems, the life cycles of pathogens and vectors (insects, rodents, mammals), and their geographic distribution. For example, rising temperatures contribute to the spread of infectious diseases (e.g., malaria, dengue fever, yellow fever, and Lyme disease) from lower-latitude areas to higher-latitude areas.
On the other hand, heavy rains and floods favor malaria and leptospirosis as environmental conditions (humidity, stagnant water) influence the mosquito reproduction, which is the primary cause of these diseases [16]. Malaria outbreaks in Asia and South America have been associated with widespread flooding, [17] as was an outbreak of leptospirosis in Mumbai, India, in 2000 [18].
Additionally, most viral respiratory diseases are characterized by seasonal behaviour, such as the influenza virus, the spread of which is favoured in cold and dry weather conditions [19, 20] or even in areas with temperate and tropical climates [21]. The SARS-CoV and MERS-CoV coronaviruses that appeared in 2002 and 2012 respectively exhibited seasonal changes. According to Tan [22] SARS-CoV weakened with increasing temperature and gradually disappeared by July 2003, while according to Chan [23], weather conditions characterized by high humidity (RH) (> 95%) and high temperature (e.g., 38°C) contributed to the inactivation of SARS-CoV activity. Accordingly, the transmissibility of MERS-CoV increased in the winter months, with a significant increase under low temperature and humidity conditions [24]
Weather conditions are estimated to have a similar effect on the infectious behavior of SARS- CoV-2. Many studies have been conducted since the start of the COVID-19 pandemic to determine whether and what this correlation is (positive or negative). However, further research is required because the research data to date are conflicting.
Most studies concluded that an increase in temperature and humidity led to a decrease in the rate of disease spread [25–31]. For example, research conducted in 166 countries worldwide showed that an increase in temperature (+ 1°C) and relative humidity (+ 1%) was associated with a decrease in daily cases and deaths [30]. Additionally, according to Qi [32] the daily incidence in China decreased significantly when the mean daily temperature and relative humidity increased. However, there are also studies that have reported different results. Some studies have concluded that a temperature increase positively affects disease spread [33, 34], while others have reported no correlation between temperature and disease incidence [35, 36].
Likewise, the conclusions of studies on the effect of humidity are conflicting, sometimes showing a positive and sometimes no correlation with disease transmission [37, 38].
Another parameter studied was the critical temperature, which led to a decrease in the rate of exponential transmission of the disease, and it was found that at an ambient temperature of 30°C, the basic rate of reproduction (R0) was approximately 1, while it increased significantly (R 0 ≅ 2,5) when the temperature reached 0°C [39].
However, the effect of weather conditions on the transmission of SARS-CoV-2 is not entirely clear, as according to some researchers, these conditions alone cannot reduce the spread of COVID-19 and prevent the re-emergence of new outbreaks without taking measures to protect public health [40–43]. According to Oliveiros, [42] weather variables explain 18% of the variation in disease doubling time while the remaining 82% may be related to containment measures, general health policies, population density, transport, or cultural aspects.
In this paper, we studied the effects of air temperature, humidity, and wind on the spread of COVID-19 in Greece. More specifically, we examined the effect of weather conditions on COVID-19 admissions to ICUs in two major cities, Athens in central Greece and Thessaloniki in northern Greece.