Review of the risk factors for SARS-CoV-2 transmission

Table 1 Public health emergency of international concern announced by the World Health Organization

Public health PHEIC announced by WHO
H1N1 influenza pandemic in 2009
Polio eradication in 2014
Ebola virus outbreak in West Africa in 2014
Zika virus outbreaks in 2016
Ebola outbreak in the Democratic Republic of Congo in 2018
SARS-CoV-2 outbreak in 2020

H1N1: Influenza A; PHEIC: Public health emergency of international concern; SARS-CoV-2: Severe acute respiratory syndrome coronavirus 2; WHO: World Health Organization.

Table 2 Survival of severe acute respiratory syndrome coronavirus, Middle East respiratory syndrome coronavirus, and severe acute respiratory syndrome coronavirus 2 on various materials

Ref.	van Doremalen et al[78]	van Doremalen et al[80]	Chan et al[79]	Rabenau et al[82]	Lai et al[83]	Duan et al[81]
Year	2020	2013	2011	2005	2005	2003
Location	United States	United States	Hong Kong, China	Germany	Hong Kong, China	China
Virus	SARS-CoV-2, SARS-CoV	MERS-CoV	SARS-CoV	SARS-CoV	SARS-CoV	SARS-CoV
Load applied (TCID50)	SARS-CoV-2: 105.25 in aerosols, SARS-CoV: 106.75-7.00 in aerosols	106 in aerosols, 105 on steel and plastic	105 on plastic	NA	Dilution solution (102-104)	106
Substrate (s)	Aerosols, plastic, stainless steel, copper, cardboard	Aerosols, steel, plastic	Plastic	Polystyrene petri dish	Paper, disposable gowns, cotton gowns	Wood board, glass, mosaic, metal, cloth, paper, filter paper, plastic
Temperature/RH	21-23 °C/40%	Variable	Variable	Room temperature (21-25 °C)	Room temperature	Room temperature
Viability	Viable SARS-CoV-2 detected after 3 h in aerosols, no viable SARS-CoV-2 detected after 4 h on copper and 24 h on cardboard, stable after 72 h on plastic and stainless steel; no viable SARS-CoV detected after 8 h on copper and 8 h on cardboard	The viability of MERS-CoV decreased 7% at 40% RH and 89% at 70% RH in aerosols; Viable MERS-CoV tested after 48 h at 20 °C/40% RH, 8 h at 30 °C/80% RH and 24 h at 30 °C/30% RH	SARS-CoV survived for 5 d at 22-25 °C relative humidity of 40%-50% with only 1 log10 loss of titer and was viable for more than 20 d; SARS-CoV was more stable at relatively low temperatures (28 °C vs 38 °C) and humidity (80%-89% vs > 95%)	SARS-CoV survived for more than 6 d and retained its infectivity for up to 9 d	SARS-CoV survived no more than 5 min to 24 h on paper, 1 h to 2 d on disposable clothing, and 5 min to 24 h on cotton clothing	SARS-CoV survived for > 72 h on the surfaces of eight materials, and > 120 h on metal, cloth and filter paper
Reduction in infectious titer (TCID50)	SARS-CoV-2: from 103.5 to 102.7 in aerosols, from 103.7 to 100.6 after 72 h on plastic, from 103.7 to 100.6 after 48 h on stainless steel; SARS-CoV: from 104.3 to 103.5 in aerosols, from 103.4 to 100.7 after 72 h on plastic, from 103.6 to 100.6 after 48 h on stainless steel	NA	The reduction in infectious titer was similar in solution compared with virus dried on surfaces	NA	NA	NA
Half-life	SARS-CoV-2 :1.1 h in aerosols, 6.8 h on plastic, 5.6 h on stainless steel, 0.8 h on copper, 3.5 h on cardboard; SARS-CoV :1.2 h in aerosols, 7.6 h on plastic, 4.2 h on stainless steel, 1.5 h on copper, 0.6 h on cardboard	The half-life of MERS-CoV ranged from 0.6 to 1 h on steel and from 0.4 to 1 h on plastic	NA	NA	NA	NA

MERS-CoV: Middle East respiratory syndrome coronavirus; RH: Relative humidity; SARS-CoV-2: Severe acute respiratory syndrome coronavirus 2.

Table 3 Measures to prevent the transmission of severe acute respiratory syndrome coronavirus 2

Measures to prevent SARS-CoV-2 transmission
Strengthen the environmental hygiene of the medical sector and the personal hygiene of medical staff
Standardize the management procedures for confirmed and suspected cases to reduce nosocomial transmission
Equip health-care workers with PPE to protect their safety
Strictly assess hospitalization criteria and limit nonessential visits
Increase public awareness and education on infectious diseases and measures to prevent the spread of diseases on an individual basis
Enhance supervision and management of the flow of people in public places to reduce large-scale gatherings

PPE: Personal protective equipment; SARS-CoV-2: Severe acute respiratory syndrome coronavirus 2.

Table 4 Unanswered questions about severe acute respiratory syndrome coronavirus 2

Unanswered questions about SARS-CoV-2
Where does SARS-CoV-2 really originated from and how does it affect humans?
Will the spread of SARS-CoV-2 be a seasonal outbreak?
Why is the prevalence of SARS-CoV-2 infection lower in children than in adults?
Is the infectivity of a patient positively related to the severity of the disease?
What is the proportion of asymptomatic carriers worldwide and what role do they play in transmission?
What is the probability that a cured patient is re-infected with SARS-CoV-2?
How does SARS-CoV-2 invade other organs than the lung?
Can animal experiments find out the specific pathogenesis of SARS-CoV-2 infection?
How long will it take to develop effective vaccine or medicine against SARS-CoV-2?

SARS-CoV-2: Severe acute respiratory syndrome coronavirus 2.