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On July 4, three local confirmed cases were found in the Ruili District of Yunnan Province, belonging to the border trade zone, and the circulation work is still in progress
.
In addition, the mutant strain of the new coronavirus that has recently spread in Guangzhou is also the B1.
617.
2 (Delta) mutant strain that is mainly raging in India.
It spreads very quickly and has a strong transmission power
.
An analysis of nearly 1,000 clustered cases in China found that 83% of clustered cases are based on families, and the risk of transmission of COVID-19 in a family environment is 10 times the risk of transmission in other environments
.
By constructing a mathematical model, this paper quantitatively studies the role of droplet transmission, contact transmission and aerosol transmission in the spread of COVID-19 in a three-person home environment, and gives targeted opinions
.
The pathogen of the new type of coronavirus pneumonia is the new type of coronavirus (SARS-CoV-2), and the World Health Organization has officially named it Corona Virus Disease 2019, or COVID-19 for short
.
As a respiratory infectious disease, it is currently believed that droplets and contact transmission through the respiratory tract are the main transmission routes, and there may be risks of fecal-oral, aerosol, and mother-to-child transmission
.
Different transmission routes have different prevention and control measures.
Therefore, exploring the relative importance of each transmission route in different environments and the effects of different prevention and control measures is crucial to formulating targeted COVID-19 prevention and control measures
.
The latest research shows that the relative importance of different transmission routes of COVID-19 in different environments is different
.
Considering that the home environment is a high risk area for COVID-19 transmission, based on the characteristics of contact between people in the home environment, a mathematical model was constructed to explore the relative importance of different transmission routes of COVID-19 in the home environment and different measures to prevent COVID-19.
The effect of control
.
Definition of SARS-CoV-2 transmission route
Definition of SARS-CoV-2 transmission routeIt is assumed that SARS-CoV-2 mainly has the following three transmission routes: droplet transmission, contact transmission (direct and indirect contact) and aerosol transmission
.
SARS-CoV-2 through droplet transmission, aerosol transmission and contact transmission
SARS-CoV-2 is transmitted through droplets, aerosols, and contact.SARS-CoV-2 is transmitted through droplets, aerosols, and contact.
Droplet transmission refers to the infection caused by the droplets produced by the patient through sneezing, coughing, speaking and breathing, inhaled through the respiratory tract of the susceptible person, or adhered to the mucous membranes (eyes, nostrils, lips) of the susceptible person
.
The radius of the inhaled droplets is generally between 5-50 μm
.
Droplets with a radius greater than 50 μm mainly adhere to the mucous membranes of susceptible persons, and the maximum horizontal distance of these large droplets is generally 1.
Contact transmission is divided into direct contact transmission and indirect contact transmission.
Direct contact refers to the transmission caused by direct skin contact with the patient (such as shaking hands); indirect contact transmission refers to the transmission caused by susceptible persons through contact with the environmental surface contaminated by the patient
.
Aerosol transmission is also called air transmission.
There are many small particle sizes in the SARS-Cov-2 droplets exhaled by the patient.
If the radius of these small particles is less than 5 μm after evaporation, they can be suspended in the air for a long time.
, And move to a long distance with the airflow, causing aerosols to spread
.
The relative importance of different transmission routes and the effects of different prevention and control measures
The relative importance of different transmission routes and the effects of different prevention and control measuresThe relative importance of different transmission routes of SARS-CoV-2 in the home environment
First, simulate the infection risk and total risk of SARS-CoV-2 spread through various channels within a period of 1 day
.
When the infection coefficient of SARS-CoV-2 on the mucosa and in the lower respiratory tract is the same, the risk of aerosol transmission is almost negligible compared to droplet transmission and contact transmission
.
SARS-CoV-2 is mainly transmitted through droplets and contact transmission, and the risk of infection between the two is close
.
When the infection coefficient of SARS-CoV-2 in the lower respiratory tract is 300 times that on the mucosa, the risk of aerosol transmission is greatly increased, but it is still less than droplet transmission and contact transmission
.
The effect of ventilation on aerosol transmission
The effect of ventilation on aerosol transmissionAnalyze the risk of susceptible people contracting COVID-19 through aerosols under different ventilation conditions
.
In the case of different air changes, susceptible persons are at risk of contracting COVID-19 through aerosols
In the case of different air changes, the risk of susceptible people getting COVID-19 through aerosols.The risk of susceptible people getting COVID-19 through aerosols under different air changes.
As the air exchange rate increased from 0.
5 times/h to 5 times/h, the probability of susceptible persons being infected with SARS-Cov-2 through aerosol route decreased from 3.
7% to 1.
3% during the 24 h exposure time
.
When the concentration of SARS-Cov-2 in the droplets is as high as 107 mRNA copy/mL, as the air exchange rate increases from 0.
5 times/h to 5 times/h, the probability of susceptible persons being infected by aerosol route increases from 32 % Is significantly reduced to 12%
.
The effect of distance from the patient on the spread of droplets
The effect of distance from the patient on the spread of dropletsAnalyzing the risk of susceptible people being infected with COVID-19 through droplets at different levels of distance from the patient, the results show that the level of distance from the patient can significantly reduce the risk of infection
.
At different distances from the patient, the risk of susceptible people spreading COVID-19 through droplets
Especially within a distance of 1 m, when the horizontal distance between the susceptible and the patient increases from 0.
2 m to 1 m, the risk of infection is reduced from 100% to 31%
.
Continue to increase the horizontal distance from the patient to continue to reduce the risk of infection.
When the horizontal distance from the patient is 2 m, the risk of infection is reduced to 9%
.
The impact of hand washing and environmental surface cleaning on contact transmission
The impact of hand washing and environmental surface cleaning on contact transmissionAnalyzing the risk of susceptible people contracting COVID-19 through contact transmission under different hand washing and surface cleaning frequency conditions, it is found that washing hands and cleaning surfaces can effectively reduce the probability of contact transmission of infection
.
Under different hand washing and surface cleaning frequencies, the risk of susceptible people spreading COVID-19 through contact
Under different hand washing and surface cleaning frequencies, the risk of susceptible people spreading COVID-19 through contact Under different hand washing and surface cleaning frequencies, the risk of susceptible people spreading COVID-19 through contactAs the frequency of hand washing increases from 0 to 16 times a day, the infection probability of susceptible people contracting COVID-19 through contact transmission has dropped uniformly, from 23.
9% to 11.
2%
.
As the frequency of cleaning surfaces increased from 0 to 8 times a day, the infection probability of susceptible persons infected with COVID-19 through contact transmission decreased faster, from 23.
9% to 12.
4%, and when the frequency of cleaning surfaces increased from 8 times a day By 16 times a day, the probability of infection decreased slowly, from 12.
4% to 10.
9%
.
discuss
discussStudies have shown that droplet and contact transmission is the main transmission route of SARS-CoV-2, which is consistent with the views in the COVID-19 diagnosis and treatment plan issued by the National Health Commission of China
.
A study published in Nature investigated the spread of SARS-Cov-2 among golden Syrian hamsters through animal model experiments, and verified that the droplet transmission route has a high efficiency
.
The natural mortality rate of SARS-Cov-2 on the surface is relatively low.
Active viruses can still be detected on plastic and stainless steel surfaces after 72 hours, which increases the risk of contact transmission
.
Although the risk of aerosol transmission is relatively small compared to the other two routes, the risk cannot be ignored when exposed to high concentrations of aerosol for a long time in a relatively closed environment
.
When the susceptible people get along with the patient in a home environment with 1 ACH ventilation for 1 day, and the virus concentration in the patient's droplets is high (107 mRNA copy/mL), the susceptible people are infected with COVID-19 through aerosol transmission The probability of reaching 26%
.
Many documents provide evidence for the aerosol spread of SARS-CoV-2
.
Patients with COVID-19 can produce virus-containing aerosols
.
SARS-CoV-2 is widely present in patients’ respiratory secretions, feces and urine.
SARS-CoV-2 can survive in aerosol for a certain period of time
.
SARS-CoV-2 can be detected in the ambient air of Wuhan hospitals and patient toilets, and under experimental conditions, SARS-CoV-2 can survive in aerosols for 3 hours
Finally, aerosols containing SARS-CoV-2 can reach target tissues and cause infection
.
Studies have found that SARS-CoV-2 receptor ACE-2 exists in the respiratory tract and is expressed in bronchial cells and other tissues
Increasing the ventilation rate can significantly reduce the risk of aerosol transmission
.
Since 1957, the Grantham Hospital in Hong Kong has adopted a natural ventilation system in the tuberculosis ward, which has effectively controlled the long-distance spread of bacteria in the air
In ordinary residences, natural ventilation with intermittent opening windows or enhanced mechanical ventilation can be used to reduce the risk of aerosol transmission
.
For droplet transmission, increasing social distance is an effective control measure
.
A study used a mathematical model to estimate the impact of different measures on the control of the spread of the new coronavirus, and found that simultaneous isolation, closure of schools, and increased social distancing among office workers can reduce more than 78% of patients
Ideally, the new coronavirus pneumonia spreads through droplets and jets
Under ideal circumstances, the new type of coronavirus pneumonia spreads through droplets and the jet indicates that the new type of coronavirus pneumonia is spread through droplets and the jet indicatesSurface cleaning is equally important, and increasing the frequency of hand washing and surface cleaning can help reduce the risk of contact transmission
.
The World Health Organization recommends that people should clean their hands frequently and thoroughly during an epidemic
in conclusion
in conclusionThe research in this article shows that droplet transmission and contact transmission play the most important role in the spread of COVID-19
Maintaining a space distance of 1.
Although the risk of transmission through aerosols is relatively small, when the concentration of pathogens in the patient's exhaled droplets is high (the patient may be a super-infected person), the risk of aerosol transmission can still be as high as 26% under 24-hour exposure in the home environment
.
This study has two limitations: due to the lack of relevant literature, some parameters of SARS-CoV-2 in the model (such as the transmission rate of the virus between hands and surfaces) are estimated based on data from other viruses; due to family The human behavior data in the environment is not perfect, and simplified assumptions are made on human touch behavior
.
In the future, further experimental measurements and touch behavior observation studies are needed to deepen the understanding of the spread of the new coronavirus
.