This study examined the association between daily temperature variation and daily fluctuations in the number of new COVID-19 cases and on transmission rate on a global scale. What emerges from the results is a negative correlation between day-to-day temperature variability and COVID-19 incidence. Moreover, we found that warmer temperatures slowed the transmission rate, as higher temperatures were positively correlated with a longer time to reach a standard number of confirmed COVID-19 cases. Especially, countries with daily average temperatures of 0 °C − 20 °C, had a faster transmission rate compared to countries with higher daily average temperatures.
These data accord with earlier studies in this area demonstrated that ambient temperature dips increase the incidence of viral acute respiratory tract infections [3, 4 9, 10, 17]. Our findings also broadly support current observations, which showed that temperatures had a negative linear relationship with the number of confirmed cases [8, 9].
Evidence from several experimental studies has established a relationship between temperature and viral epidemics supporting that cold temperature usually provides a conducive environmental condition for virus survival. Existing research recognized a rapid decrease in MERS-CoV and SARS-CoV viability at higher temperatures [18, 19], further supported that direct contact transmission via aerosols, and not fomite transmission, would be the most likely route of zoonotic and human-to-human transmission of the viruses in outdoor settings [18, 19].
In accordance with the present results, a recent study has documented that temperature decreases spread parameters of the new COVID-19 case dynamics . Chan et al. previously supported that virus viability was rapidly lost at higher temperatures (e.g., 38 °C) . Furthermore, another study documented a relatively short survival of another type of coronavirus, Turkey coronavirus (TCoV), at room temperature as compared to a lower temperature (4oC), claiming the limited virus survival during summer months . More recently, it has been reported that first MERS human cases in Saudi Arabia were more likely to occur when conditions were relatively cold and dry, which is similar to seasonal patterns described for other respiratory diseases such as influenza in temperate climates . A different study on this topic has shown that during the epidemic, the risk of increased daily incidence of SARS was 18.18-fold higher in days with a lower air temperature than in days with a higher temperature, in Hong Kong  and other regions .
Shaw Stewart proposed four mechanisms that could be the main drivers of the seasonality of viral epidemics . He suggested that winter temperatures are associated with increased crowding that might enhance viral transmission. Furthermore, he suggested that low temperatures can increase the stability of virions outside the body, the host susceptibility, and may activate dormant virions .
Winter conditions suppress humans’ innate immune response leading to reduced blood supply and decreased availability of immune cells to the nasal mucosa. Additionally, low humidity can impair mucociliary clearance as nasal mucus becomes dry, the innate immune defense, and airway cells’ ability to repair damage; thus, the nasal cavity is vulnerable to virus invasion . Therefore, relative to temperature, low humidity seems to be a critical environmental factor influencing the outbreak of human coronavirus disease.
The origin(s) of coronaviruses are still unknown; however, some general epidemic patterns are noticeable . If bats were the natural hosts of SARS-CoV and SARS-CoV-2, cold temperature, and low humidity in these times might provide environmental conditions favorable for prolonged viral survival in these regions concentrated with bats . The widespread existence of these bat-carried or -released viruses might have an easier time breaking through human defenses when harsh winter makes human bodies more vulnerable .
Nevertheless, over the years, contradictory claims have debated the impact of temperature in coronavirus infectivity. Some studies argued that high temperature and low relative humidity contributed to increased MERS-CoV spread . The seasonal variation of MERS-CoV was inversely associated with temperature and relative humidity among children during MERS-endemic to the Riyadh Region . In experimental conditions, the survival ability of SARS coronavirus in human specimens and environments was found to be relatively strong in various temperatures (4oC, 20oC, and 37oC) for at least 2 hours without a remarkable change in the infectious ability in cells. Only heating and ultraviolet irradiation could efficiently eliminate viral infectivity .
Our study provides further insights into the hypothesis that there is an association between temperature and COVID-19 spreading. A limitation of the study was that the number of confirmed coronavirus cases reported by provinces could be profoundly affected by underreporting, low number of people who got tested, and different measures of detection and prevention of SARS-CoV-2 as sampling strategies differ vastly between countries. Our design is fit for hypothesis generation to be later tested in more detail in each country’s setting, taking into account other environmental parameters.