key: cord-0723973-dgb7es15
authors: Rayan, Rehab A.
title: Seasonal Variation and COVID-19 Infection Pattern: A Gap from Evidence to Reality
date: 2021-02-20
journal: Curr Opin Environ Sci Health
DOI: 10.1016/j.coesh.2021.100238
sha: daa70f054b0564fa8cb0580d8162a3cb37271abf
doc_id: 723973
cord_uid: dgb7es15

In December 2019, the coronavirus disease (COVID-19) was discovered in China, causing many cases and deaths. Several studies have explored the role of environmental factors in the spread of COVID-19, emphasizing the effect of two weather parameters, humidity and temperature. Those parameters are evidently vital affecting outbreaks of infectious respiratory diseases, like influenza, yet such an effect on COVID-19 remains controversial. This review explores the relation between the change in weather-related factors and the transmission of the COVID-19. With seasonal variation from winter to summer and in the absence of adopting thorough public health measures, elevated temperature and humidity might not limit the COVID-19 cases. Hence, we need multidisciplinary strategies and interventions to limit the burden of this pandemic over the healthcare systems.

Since December 2019, a growing number of cases for the novel coronavirus disease , a worldwide health disaster of this time, that has been discovered in China [1, 2] . COVID-19 evidently has high transmission rates among humans with about 17,396,943 and 675,060 confirmed cases and deaths respectively as reported until August 1, 2020 [3] , J o u r n a l P r e -p r o o f worldwide, hence declared the pandemic state and enforced lockdowns everywhere [4, 5] .

In addition to the contacting humans and public mobility, environmental variables could affect droplet transmission and viral survival such as influenza virus, yet it has not been well investigated for COVID-19. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) varied genetically from both the severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1) and the Middle East respiratory syndrome coronavirus (MERS-CoV), therefore, it might spread differently with the changing climatic variables [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] .

Former studies have endorsed the epidemiological hypothesis of dry and cold environments, with lower absolute humidity and temperature, support the survival and transmission of viral infections spreading via droplets while humid and warmer environments, with higher absolute humidity and temperature, suppress such transmission.

Absolute humidity, the atmospheric water content, and temperature have been significantly associated with other viral transmission. For instance, the survival of influenza viruses, either in droplets or on surfaces, is longer in dry and cold air, hence growing the potential transmission [16] [17] [18] [19] [20] [21] [22] [23] [24] ]. Yet there is limited research on the effect of both temperature and absolute humidity on COVID-19 spread.

Globally, COVID-19 has overwhelmed the entire economy and the public health systems in their attempts to prevent and/or control such a virus via slowing the transmission down. Scientists in one study developed a model to explain the sensitivity of the respiratory droplets to relative humidity and temperature, and their association to COVID-19 as shown in Figure 1 . The model assumed that COVID-19 spread only by inhaling infected respiratory droplets. The vaporizing and drying time of droplets, an indicator for the infection rate constant, relies on relative humidity and temperature both indoors and outdoors. And the number of infected cases is directly associated with this infection rate constant. The study suggested that relative humidity and temperature have affected the COVID-19 spread. They found that with an alike relative humidity across two sites, COVID-19 transmits further fast at cooler temperature compared to warmer ones along with negligible effect upon including the measures of social enforcement [41] . and temperature, and their association to COVID-19 [41] .

In China, a study examined the relations between daily average relative humidity and temperature and the daily counted cases of COVID-19 in 30 districts using Generalized J o u r n a l P r e -p r o o f Additive Model (GAM). They found that COVID-19 daily cases' counts were adversely associated with both the average relative humidity and temperature. The study suggested that both daily relative humidity and temperature affected the incidence of COVID-19 in some districts of China. Yet, such an effect was not consistent throughout the entire districts, where there was a spatio-temporal heterogeneity that could be caused by the meteorological attributes and intervention measures among districts [42] . Globally, a cohort study of 50 cities worldwide, having and lacking COVID-19, investigated whether the SARS-CoV-2 is associated with seasonality and the possibility of estimating transmission. The researchers claimed that COVID-19 outbreak-predominant areas had less absolute humidity and average temperature than those reported lower virus spread. Hence, the study found that the pattern of significant COVID-19 community outbreaks across controlled measures of temperature, humidity, and latitude was coherent with a seasonal respiratory virus behavior; suggesting modeling for likely predicting susceptible regions for significant COVID-19 community spread [44] . Likewise, to that report from another study applied ecological modeling to compare the spread and the weather showing less transmission in higher temperature and humidity in confirmed cases from cities in both China and the United States [34] . Similarly, a study in Spain analyzed COVID19spatiotemporally via modeling the incidence of the reported cases regionally while counting for humidity, temperature, and sunshine. The study findings supported the hypothesis of lower disease at higher humidity and temperature [45] . Another study investigated relative humidity and temperature effects on daily-reported new COVID-19 cases and deaths for 166 countries, except for China, applying the log-linear generalized J o u r n a l P r e -p r o o f additive model. The study found that both relative humidity and temperature adversely associated with daily-reported new COVID-19 cases and deaths (fewer cases and deaths with higher relative humidity and temperatures), suggesting that the COVID-19 spread might be slightly diminished at higher humidity and temperature [46].

Most studies, added to those showing that the previous SARS-CoV-1 survived unfavorably in elevated humidity and temperatures, were considered as an adequate proof to expect that summer might enable controlling COVID-19 [47], however, those observations might be confounded by the global pattern of traveling that led to slower transmission in temperate areas [48] . The worldwide transmission of the COVID-19 implies that seasonal variation cannot be regarded solely as a major modifier of spread. However, the warmer climate might moderately limit the spread of COVID-19, no proof has advised that warmer weather would limit the potency of COVID-19 spread leading to less extra required actions to restrict transmission [49] . Hence, it is vital to interpret these observations by considering the ongoing transmission of COVID-19 worldwide.

The studies, supporting the conventional hypothesis stating that the more the absolute humidity and the temperature, the lower the transmissibility and survival of COVID-19, showed an initial investigation and held some limitations [6, 34, 41, 42, [44] [45] [46] . For instance, the data were analyzed variably among studies where some investigators gathered either suspected, confirmed, or the total number of cases, while others quantified the incidence rates. The probability of infection is variable for the variable regions and the population size is known to critically affect the rate of COVID-19 transmission. Different studies investigated various locations such as countries and even cities in the same country with variable sizes and densities of population while the incidence rate per unit population might better represent the epidemic. Hence, investigating the effect of environmental parameters on COVID-19 spread requires studying epidemic-centered areas separately from epidemic-sporadic ones. Further, the reported cases and moralities usually vary across countries for the variable testing availability, the tests' sensitivity and specificity, and the reporting patterns. Moreover, the ecological time-series design in some studies might hold some ecological fallacy [50] . The chosen periods for the study are critical as well where a longer J o u r n a l P r e -p r o o f model periods might render higher resolving data for identifying correlations between the spread of COVID-19 and the environmental parameters, beyond just gathering pre-and post-epidemic data and missing chances to emphasize how epidemics start and end.

Besides, most of the studies used correlation analysis between the pandemic data and the meteorological factors at the early stages of the pandemic, yet, studying COVID-19 under various environmental conditions is missing. Hence, the entire ecological profile determining COVID-19 survival and transmission are yet less appreciated [51] . Further, many other key human factors (political, economic, demographic, or societal) were not included in some studies for the complicated analysis and the shortage in updated data [50] . 

This review examined how humidity and temperature affect transmission and hence cases and deaths from COVID-19 worldwide. According to previously surveyed evidence, some findings suggested that COVID-19 transmission is inversely proportional to both temperature and absolute humidity. Therefore, the seasonal variation might be a significant element influencing transmission and deaths from COVID-19. However, maintained spread and fast incrementally increasing cases across a varying humidity from dry and cold areas to temperate ones show that weather changes via raised humidity and spread has proven to be inconsistent with the currently evident transmission to tropical areas of the world. Hence, we still need sticking to rigorous social enforcement measures such as social distancing, using non-medical masks to prevent droplets entering the respiratory tract, quarantining, and contact tracing, are yet much needed to control such a pandemic.

[5] Callaway E. Time to use the p-word? Coronavirus enters dangerous new phase. Nature 2020. https://doi.org/10.1038/d41586-020-00551-1. [7] NCIRD. Coronavirus | Human Coronavirus Types | CDC. 2020.

[ 

A Novel Coronavirus from Patients with Pneumonia in China

A new coronavirus associated with human respiratory disease in China

COVID-19) Situation Report -194. 2020

Where Did They Go? Millions Left Wuhan Before Quarantine

A climatologic J o u r n a l P r e -p r o o f investigation of the SARS-CoV outbreak in Beijing, China

Coronavirus envelope protein: current knowledge

Effect of environment and occupational hygiene factors of hospital infection on SARS outbreak

Absolute humidity, temperature, and influenza mortality: 30 years of county-level evidence from the United States

The researchers found that the absolute humidity has been significantly associated with respiratory viral transmissions like the influenza virus

Absolute Humidity and Pandemic Versus Epidemic Influenza

The researchers found that the survival of influenza viruses, either in droplets or on surfaces

Exposure to cold and respiratory tract infections

Urbanization and humidity shape the intensity of influenza epidemics in U

Humidity and respiratory virus transmission in tropical and temperate settings

Effects of temperature, higher temperatures are associated with lower incidence of COVID-19, for cases reported globally up to 29th

Analysis of meteorological conditions and prediction of epidemic trend of 2019-nCoV infection in 2020

Will Coronavirus Pandemic Diminish by Summer?

Roles of meteorological conditions in COVID-19 transmission on a worldwide scale

The Role of Environmental Factors on Transmission Rates of the COVID-19 Outbreak: An Initial Assessment in Two Spatial Scales

High Temperature and High Humidity Reduce the Transmission of COVID-19

Effect of Ambient Temperature on COVID-19 Infection Rate

The impact of temperature and absolute humidity on the coronavirus disease 2019 (COVID-19) outbreak -evidence from China

No association of COVID-19 transmission with temperature or UV radiation in Chinese cities

Impact of meteorological factors on the COVID-19 transmission: A multi-city study in China

Modeling ambient temperature and relative humidity sensitivity of respiratory droplets and their role in Covid-19 outbreaks. ArXiv200410929 Cond-Mat Physicsphysics Q-Bio 2020. The researchers applied a model to explain the sensitivity of the respiratory droplets to relative humidity and temperature, and their association to COVID-19 where they found that with an alike relative humidity across two sites, COVID-19 transmits further fast at cooler temperature compared to warmer ones along with non

COVID-19 transmission in Mainland China is associated with temperature and humidity: A time-series analysis

The researchers examined the relations between daily average relative humidity and temperature and the daily counted cases of COVID-19 in 30 districts of China using Generalized Additive Model (GAM) and found that COVID-19 daily cases

Assessing spread risk of Wuhan novel coronavirus within and beyond China

The researchers concluded that the global pattern of traveling was a confounder and led to slower transmission in temperate areas

Effective transmission across the globe: the role of climate in COVID-19 mitigation strategies

The researchers found that the worldwide transmission of the COVID-19 implies that seasonal variation cannot be regarded as a major modifier of spread

The researchers applied the ecological time-series design that might hold some ecological fallacy where it did not consider longer durations to better reflect the association between the spread of COVID-19 and seasonal factors

Do Humidity and Temperature Impact the Spread of the Novel Coronavirus? Front