This section discusses the various epidemiological and clinical factors that have been considered to understand the spread of pandemic coronavirus. The contributing factors have been classified into risk and protective factors. Risk factors represent environmental and some other factors which are beyond human control. Examples of risk factors are age, humidity, and temperature etc. However, protective factors are under human control and thus can be adjusted in response to requirement. Few examples of protective factors are personal hygiene, social distancing etc.
I. VIRULENCY
Coronavirus (CoVs) is an unparalleled damaging virus for human, livestock, gastrointestinal, hepatic, infecting respiratory, and central nervous system [13]. During the 2002 SARS outbreak and 2012 MERS outbreak, the likelihood of transmission of CoVs from animals to humans was already confirmed [14]. Prior to 2019, there were only few CoVs that could infect respiratory system in human viz. HCoV-229E, HCoV-OC43, HCoV- NL63 and HKU1. These viruses could trigger only mild upper respiratory disease in normal conditions although it could also lead to serious infection under extreme circumstances. SARS-CoV and MERS-CoV are other similar viruses which can infect lower respiratory tract and cause severe respiratory syndrome [13][15].
This recent virus COVID-19 has been unanimously accepted to be the most severe virus till date as its spread is quite fast and uncertain. According to the recent guidelines by Chinese health authorities' [16] three are major transmission modes for COVID-19: 1) transmission through droplets, 2) transmission of aerosols and finally 3) transmission by contacts. Apart from these, digestive system is also considered to be a possible transmission mode for COVID-19. The distance covered by these droplets is normally limited to 2 meters. Nevertheless, this airborne transmission also takes place with much smaller droplets that can float and travel longer distance[17]. Moreover, these airborne droplets remains in environment for hours and even days under particular environmental conditions [18].
II. Host Defense Potential
Clinically, SARS-CoV-2 or any similar virus triggers the immunity system to work in two stages. Initially, it requires an immunological response to eradicate the virus during the incubation stage and thus prevent its progression to severe stages. These two stages are clinically known as anti-sera or pegylated IFNα. For the same, health of the host should be in good general health. Additionally, it also necessitates a favorable genetic history as it elicits adaptive antiviral immunity for the growth of an endogenous defensive immune response at the incubation and non-severe stages.
Genetic differences in the immune response to pathogens are believed to respond to human variations. However, when a defensive immune response is compromised, virus spreads and massive tissue destruction starts particularly in intestines and kidneys leading to severe viral infections [19] [20]. Thus, immune insufficiency or misdirection leads to viral replication and hence damages tissue. By study, it is evident that Immune insufficiency or misdirection can increase viral replication and cause damage to tissues [21].
III. Underlying Health Conditions
Health of an individual plays an important factor considering in susceptibility of COVID-19. It appeared prematurely in the COVID-19 epidemic that infants or healthy people are safe from this virus, but later it was understood that it could be just because they were unlikely to have visited the Wuhan wet market. Another reason for the same could also be that they were more probable to have asymptomatic or benign infection and therefore implausible to have been screened for it. Nevertheless, COVID-19 affected infants (1 month old). Also no cases of negative child outcomes have been reported for COVID-19 positive moms pregnancy [15].
IV. Atmospheric Temperature and Humidity
According to a research, it is claimed that high temperature and humidity can reduce the spread of coronavirus. This claim is in agreement the similar claim for spread of Influenza [22]. At the same time, it is also too early to make any strong claim based on the small historical data of not more than 4 months. Primarily, this claim is based on the strong belief that the coronavirus primarily transmits through respiratory droplets [23]. For the same, the behavior of coronavirus at different temperatures and humidity is being studied intensively. According to the study in [24], authors claim using linear regression framework that high humidity and high temperature lead to reduction in transmission of COVID-19. Authors also claim that each 10 Celsius increase in temperature and 1% increase in humidity results in reduction of daily reproductive value by 0.0225 and 0.0158, respectively.
Similar study for SARS coronavirus in [23] revealed that the SARS coronavirus can retain for 5 days at temperatures of 22–25°C and relative humidity of 40–50% . However, the virus is lost at higher temperature (>38°C) and relatively higher humidity (>95%). Hence, it is evident that low temperature and low humidity environmental conditions favors expediting the virus spread.
V. Airflow and Ventilation
During the intensive study of contributing factors responsible for the spread of coronavirus, a shocking conclusion was drawn while understanding the outbreak of the virus in a southern chinese city of Guangzhou [11]. In this incident, it was noticed that the direction of droplet transmission was consistent with airflow direction. This incidence sidelined the prevalent opinion about its spread by giving an eye opening result that coronavirus can also ride on currents created by air-conditioning. Resultantly, HVAC (Heating, Ventilation and Air Conditioning Systems) engineer‟s society itself made an official statement that airborne transmission of COVID-19 is most likely that leads to implementation of necessary changes in the building ventilation. Authors in [25] also attempted to establish the transmission mechanism of this virus. In the study, authors in [25] conclude that HVAC, if not implemented correctly, may enhance and expedite the spread of the virus. Hence, a major shift is mandated to prevent virus spread by implementing structural changes to buildings and HVAC systems.
VI. Personal Hygiene factors
It is quite evident since the explosion of COVID-19 that this virus transmits through physical contact. Hence, the spread of this virus may be controlled by maintaining sophisticated hygiene standards. Relation between personal hygiene and virus spread has already been established by authors in [26]. Moreover, the evidence of hand hygiene in preventing nosocomial infections was demonstrated as early as 1847.However, standard personal hygiene is not limited to hand hygiene only. It also considers safe waste management, respiratory hygiene, environmental cleaning and sterilization of patient-care equipment etc. [27]. For COVID-19, maintaining personal hygiene is most crucial as it spreads through air droplets of infected people. Hence, it is of paramount importance that the patient must cover their nose and mouth while coughing or sneezing [28].
VII. Social Distance
As the novel COVID-19 outbreak continues to proliferate around the globe, healthcare experts and government officials strongly advise individuals and families to position space between themselves and others. The idea of social distancing seems to have gone from emergency healthcare preparedness and mitigation jargon to a household expression till a proven vaccine is available. As COVID-19 is known to spread through human-to- human transmission, the transmission is known to affect contacts within 6 feet radius. The virus can be transmitted airborne and stay on objects for as many as three hours; hence social distancing is considered to be the viable and easiest solution to contain this virus.
VIII. Population Density
Another most important factor in analysing the susceptibility of the virus is population density. COVID-19 has taken hold and struck hard in different places of the world. For instance, largely dense cities, like New York, London, with the huge inflows of tourists, and hugely dense residential areas, industrial centers like Wuhan, Italy, Detroit have shown the spread of the epidemic exponentially. At the same time, its many other factors along with population density such as population size, age, education, level, work culture, also play prominent roles in the spread of epidemics.