The rapid outbreak of coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that is spread via airborne, droplet, contact, and fecal–oral transmission modes constitutes a worldwide and disruptive challenge to societies [1
]. Both symptomatic and asymptomatic carriers have been recognized to transmit the SARS-CoV-2 virus via various modes of transmission [3
], causing severe respiratory infections among patients receiving medical care in hospitals [6
]. Hence, the COVID-19 outbreak has been declared a global public health emergency of international concern by the World Health Organization (WHO) [12
]. The original source and the way of transmission to humans are the subject of current studies [13
]. It was stated in January 2020 that the virus of SARS-CoV-2 causes COVID-19, creating infections typical for a second-class infectious disease, but that management measures for a first-class infectious (dangerous) disease are required [12
]. At least a 70% genetic similarity of SARS-CoV-2 to the original Severe Acute Respiratory Syndrome (SARS), as well as it being a subspecies of Sarbecovirus, has been reported [18
Coronaviruses such as SARS, endemic human coronaviruses (HCoV), and Middle East Respiratory Syndrome (MERS) can remain biologically active for ≤9 days on the surface of plastic, metal, and glass [19
]. Because the incubation time of SARS-CoV-2 vary from 2 to 14 days, infected persons not displaying symptoms and unaware of their infection are active carriers for transmission to others [12
]. Generic symptoms of COVID-19 disease are common to SARS and MERS infections including fatigue, nausea, diarrhea, vomiting, breathing difficulty, dry cough, fever, and bilateral lung infiltration in severe cases [13
On 7 February 2020, COVID-19 cases existed in twenty-six countries around the world: Canada, the United States, Australia, India, Sri-Lanka, Cambodia, Thailand, Vietnam, Malaysia, Singapore, Taiwan, South Korea, Japan, Philippines, Nepal, United Arab Emirates, Russia, Italy, Germany, Sweden, Finland, Belgium, Spain, France, and the United Kingdom [9
]. Most of the confirmed cases were travelers connected to Wuhan or other Chinese cities; however, locally transmitted cases outside of China had also been reported [10
]. Moreover, respiratory transmission of the novel coronavirus by airborne mechanisms from aerosols and droplets gained consideration [21
]. SARS-CoV-2 can be spread via near contact with an infected person coughing or sneezing and emitting small droplets [21
]. Transmission via droplets occurs when a SARS-CoV-2 carrier talking, coughing, and sneezing emits droplets with the virus that can expose other people’s nose, mouth, and eyes causing infection [24
]. Those generated droplets are relatively large and cannot stay in the air for long time and deposit relatively rapidly [24
]. Hence, near contact with an infected person is the main way for virus intake causing infection [9
In addition, indirect long-distance transmission (aerosol transfer) is possible and can happen [9
]. The virus emitted in small particles may be transferred a long-distance [9
]. In other words, airborne transmission by small droplet nuclei or particulates including the virus is favored due to their extended time suspended in air, and their farther distance traveled from the emitter [24
]. For example, concentrated airborne droplets (fragmented) are released through respiration or from sneezing by patients with SARS-CoV-2 virus, reaching several meters away to also remain in suspension for about 30 min, and once settled onto objects can still be biologically active for a few days [22
Because there is no specific therapeutic drug or vaccine for COVID 19 yet [20
], it is important to reduce exposure to SARS-CoV-2 [9
]. Thus, specific control measures for SARS-CoV-2 transmission are needed [43
], such as those listed by the US Centers for Disease Control and Prevention (CDC). The CDC measures include suitable sanitary habits such as utilizing regular hand washing with soap and water, or an alcohol-based hand rub (ABHR) with at least 60% alcohol, using protective masks in public, restricting social contacts (restricting human to human transmission pathway), cleaning and disinfecting surfaces continuously, staying at home as much as possible, and applying proper ventilation such as displacement ventilation (DV) to decrease infection risk [6
]. Wearing protective masks in public is perhaps the most cost-effective way for preventing to SARS-CoV-2 spreading. However, temporary shortages in supply of facemask have occurred during the outbreak of COVID-19 due to limitations in production and import/export of KN95/N95 respirators and medical protective masks [10
To the best of our knowledge, this is the first study reviewing the recent literature for SRAS-CoV-2 transmission by grouping them into four structured sections that including an introduction, a methodology section, the results and discussion, and the concluding remarks. The work explains common symptoms of COVID-19, modes of transmission of SARS-CoV-2, concentration, and infectious dose for airborne transmission of SARS-CoV-2, the influencing factors on the airborne transmission of SARS-CoV-2, and protection approaches to prevent the transmission of SARS-CoV-2. Finally, several proposed protocols for SARS-CoV-2 prevention and a future perspective of the current situation are presented.
This work originated from a systematic review to provide a narrative synthesis performed according to the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) statement [51
2.2. Search Strategy
To perform this work, papers appearing in international databases (Medline/PubMed, Scopus, Google Scholar, Web of Sciences, Science Direct, and Embase) were identified by the following keywords: coronavirus, coronavirus disease 2019, COVID-19, personal protective equipment, transmission, airborne, contact, fecal–oral, droplet, outbreak, beta-CoVs, respiratory, SARS-CoV-2, bioaerosol, aerosol, airborne particle, air, negative pressure, ventilation, air conditioning, social distancing, N95 respirator, mask, alcohol, facemask, ambient air, and indoor air. Due to the high number of articles and to keep the literature up to date, the search was constrained on articles published between 1 March 2020 and 28 December 2020.
2.3. Inclusion Criteria
In addition, preference was given to papers in journals that provide information in the field of airborne and droplet transmission of SARS-CoV-2 virus; the effects of different factors such as environmental conditions, negative pressure ventilation (NPV), air conditioning system, displacement ventilation (DV), noninvasive positive pressure ventilation (NIPPV), and high-flow nasal cannula (HFNC) on the airborne transmission of SARS-CoV-2; and effects of protection approaches for transmission of SARS-CoV-2.
2.4. Exclusion Criteria
The papers with the following contents were considered to be out of the scope of this review and therefore removed: (i) the epidemiology, virology, and clinical features (imaging features) of SARS-CoV-2; (ii) the transfer of SARS-CoV-2 via wastewater (sewage) and solid waste; and (iii) the effects of COVID-19 on the mental health and quality of people’s life, culture, education, politics, and economy of countries.
COVID-19 has become a global health concern creating severe respiratory tract infections and other complications. The pathways of probable transmission of SARS-CoV-2 considered in this work include direct contact, airborne, fecal–oral, and droplet transmission routes. The influence of negative pressure ventilation and air conditioning systems on the airborne transmission was discussed. The major findings of this work are:
The airborne transmission of SARS-CoV-2 is an important contributor to fast spreading of the associated disease.
Droplet transmission occurs from particles >5 µm, which can settle on surfaces under gravitational settling and do not move more than 1 m. Particles <5 µm can stay suspended for an extended period of time (≥2 h) and travel longer distances (up to 8 m) through simple diffusion and convection mechanisms.
The environmental ambient conditions can affect airborne transmission of SARS-CoV-2 over larger distances.
The droplets <10 µm in size can be transferred larger distant when the weather is cold and humid.
The persistence of the virus is remarkable at a low temperature (4 °C), and, by raising the temperature to 70 °C, the virus was no longer detectable after 5 min.
Although warm weather can slow down the growth rates of SARS-CoV-2, rigid social enforcement and other measures such as the widespread use of face masks are still needed to control the COVID-19 pandemic.
Coordinated measures among the public and private sectors are needed to control this disease at the national and international level. Joint solutions from different experts, not only in the biomedical sciences, but also in the environment, chemistry, physics, public health, and areas covering transportation, immigration, economic affairs, and education, are required.
SARS-CoV-2 can also be resuspended from floor surfaces and from protective clothes and shoes of medical workers in indoor environments. Indeed, direct contact with fomites is not the only way for causing viral infection with SARS-CoV-2. As explained above, SARS-CoV-2 is distributed via air. Thus, disinfection of surfaces and protective equipment before removal is needed together with frequent hand washing.
The various coronaviruses survive on surfaces for up to nine days, and they can be eliminated by disinfection with 62–71% ethanol for 1 min or 0.1% sodium hypochlorite.
Healthcare workers must be provided with N95, FFP2, or FFP3 masks combined with gowns and goggles.
Overall, control measures such as using high adequate ventilation, rooms with negative pressure ventilation, practicing social distancing, and wearing N95 and even surgical facemasks are potentially suggested to reduce the SARS-CoV-2 airborne transmission.
5.1. Future Perspective
Future experimental work is certainly needed to expand the current understanding of the findings reported above and on other modes of SARS-CoV-2 transmission by water, wastewater (sewage), and infectious solid waste. In addition, in-depth studies should be performed to explain the effect of air pollution, e.g., PM2.5, PM10, SO2, NO2, Pb, VOCs, and CO, on the airborne and fomite transmission of SARS-CoV-2 to diverse populations. Studying the influence of sunny, rainy, and smog days on the transmission modes of SARS-CoV-2 should be of interest to characterize different scenarios for airborne and fomite transmissions.
This work is novel in that there are no reports to our knowledge of the modes of transmission of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) and factors influencing on the airborne transmission. The results of this work have broad implications for people around the world owing to the pervasiveness of Coronavirus Disease 2019 (COVID-19).