At the end of 2022, many people were infected with the new crown virus, some people know how they were infected, and they had contact with people who were already positive before infection. But there are still many people who have not been confused, obviously did not go to a place with many people, and changed clothes and disinfected before taking a courier into the door, how can it be yang? Where did you get the virus?
This issue of Food & Heart takes you to understand the relevant research on the transmission mode of the new crown virus, and how does the new crown virus infect people? Which path is more effective to truncate to avoid infection? This is by no means an afterthought, I don't know when, what virus will emerge or the coronavirus will make a comeback. Knowledge is the lesson of life and is worth collecting.
One. The most important mode of virus transmission - respiratory droplets
We already know that the coronavirus is highly contagious. Studies have found that the new coronavirus spreads infection mainly in the following three ways:
Respiratory droplets. A large amount of virus is present in the respiratory droplets of an infected person, and an susceptible person becomes infected after inhaling a certain amount of virus.
Airborne transmission, i.e. transmission through aerosols. The tiny virus, like other particles such as PM2.5, spreads through the air to susceptible people, causing infection.
Contact transmission. A susceptible person becomes infected after contact with an infected person that contains a large amount of the virus, or after contact with an object contaminated with the virus.
In fact, the first two methods can be grouped together, the vast majority of respiratory droplets are converted into aerosols, which is also the most important way for the virus to spread. [Aerosol refers to a gaseous dispersion system composed of solid or liquid particles suspended in a gas medium, and the particle size is usually between 0.01~10μm]
The vast majority of aerosols end up depositing on solid surfaces, which can be transmitted through contact. However, the classic poker player experiment (1987) suggests that aerosols are far more effective than exposure to respiratory viruses.
The virus used in this study was rhinovirus, and the infection rate was 56% when uninfected people and infected people could only be infected with the virus through the air and could not contact the pollutants; When uninfected people and infected people were infected through both air and contact with pollutants, the infection rate was 67%, but there was no statistically significant difference between the two groups. When the uninfected and the infected can only be infected by contact with pollutants (the air space is separated, the pollutants are the infected person's saliva soaked chips and bridges), no one is infected, and the infection rate is 0.
Some people may be concerned about toilet wastewater spreading the virus. Indeed, when the concentration of viruses in feces is very high, viruses may also be present in aerosols produced by aerosols from aerosols from toilet wastewater atomization. However, the good news is that the new coronavirus is very sensitive to oxidants such as chlorine and has a rather limited ability to survive in water, and our daily water is often disinfected with chlorine.
2. Source and content of respiratory droplets
Aerosols formed by respiratory droplets are the main mode of transmission of respiratory viruses, including the new coronavirus. So, how are respiratory droplets produced, converted into aerosols, and how do they infect people?
You may never have thought that the human respiratory system is actually like an open container, exchanging gases all the time, providing us with the oxygen necessary for survival and expelling metabolic exhaust gases (mainly carbon dioxide and a small amount of water). Like a triangular bottle without a lid, the human upper respiratory tract (mainly including the nose, pharynx and larynx, but also the mouth) is smaller, and the lower respiratory tract (including the trachea, bronchi and lung organs) is larger. The difference is that the inner diameter of the triangular flask is expanding from top to bottom, while the diameter through which the airway gas can pass from top to bottom is constantly decreasing. From the trachea downwards, the respiratory tract branches 23 times, the first 7 branches correspond to the bronchi, subsequent branches correspond to the small bronchi, and finally reach the alveoli.
When a person exhales, makes a sound, coughs, or sneezes, droplets erupt from the respiratory tract, and droplets range in size depending on the source. These droplets originate in the lungs, trachea, larynx, and oral/nasal passages and are made up of ≥95% water when they are first produced, and are precisely described as small droplets that leave the respiratory tract to form respiratory droplets.
1. Breathe
Adults at rest breathe an average of about 7 liters of air per minute (about 10,000 liters of air per day). On inhalation, air enters the respiratory tract, flows down through the upper part of the respiratory tract, and finally reaches the alveolar area. During exhalation, when the passage contracts, the air flows in the opposite direction and is eventually exhaled. The exhaled air flow passes at high speed through the surface of the water-based liquid lining the respiratory tract and atomizes the liquid to form respiratory droplets. The number of respiratory droplets produced varies from person to person and increases dramatically after a viral infection.
The total volume of liquid exhaled particles at rest in healthy people is usually very small (≤ 15 pL/L of air), but the volume of breathing air during aerobic exercise can be an order of magnitude larger, so it is not surprising that the rate of virus transmission is higher in aerobic exercise facilities.
[Mechanism of respiratory droplet production. The separation of two wetted surfaces creates filaments (a) or fluid films (b), which thin as the surfaces separate; These filaments and films are broken by the exhaled air, creating droplets that are added to the exhaled air stream (black arrow). These processes work in the small-diameter airways (during inhalation), vocal cords, and mouth. (c) During rapid exhalation, coughing or sneezing, turbulent airflow causes instability at the gas-liquid interface, causing the mucosal layer to form waves, resulting in droplet release through various mechanisms, including film rupture, wave undercutting, and shearing away from the top of the rolling wave]
2. Coughing and sneezing
Coughing and sneezing are widely recognized as the source of eruptions of droplets containing pathogens and are often considered symptoms of respiratory illness. Both are caused by the simultaneous closing of the glottis and the contraction of the abdominal muscles, the sudden opening of the glottis releases the air pressure that subsequently accumulates in the lungs, causing the central and upper respiratory tract to expel compressed air at a rate of more than 15 meters per second, in the process leading to the formation of droplets through various mechanisms.
A dry cough is one of the common symptoms of coronavirus infection, a cough that does not have thick phlegm but may be closer to an autonomous cough. Cough droplets range in size from less than 1 μm to over 100 μm and more than six orders of magnitude in volume.
Sneezing is a common reflex to remove nasal irritants, and the extreme air velocity associated with sneezing also produces large droplets. There is no doubt that droplets from sneezing by infected people can also spread the virus, but sneezing is not a common symptom of new coronavirus infection.
3. Rap
During vocalization, the liquid in the larynx is atomized due to the vibration of the vocal cords; During the articulation process, saliva in the mouth is atomized due to the interaction of the tongue, teeth, palate and lips.
The droplets produced in the mouth consist mainly of saliva and range in size to the number and size of droplets produced by coughing and sneezing, although some may exceed 100 μm.
4. What's in droplets
Respiratory droplets may include saliva produced in the mouth, mucus produced in the nasal passages, mucin produced by bronchial and throat goblet cells, or cleansing-rich respiratory lining fluid produced by cells lining the distal airways and respiratory bronchioles.
In people infected with the virus, respiratory droplets also contain large amounts of viral particles. The viral load in respiratory aerosols reflects the concentration of virus in the particle-derived liquid. Since many cells in the human oral epithelium express ACE2 and TMPRSS2 at the same time, the new coronavirus can replicate in large quantities in the mouth, so the virus content found in oral and nasopharyngeal swabs is often highly overlapping, and throat swabs can also effectively detect whether the virus is infected.
Droplet virus concentrations vary greatly from infected people to infected, spanning at least 7 orders of magnitude (tens of millions of times), and some people even have viral loads as high as 10 9/ml of liquid.
As soon as respiratory droplets enter the air (generally in a state of unsaturated moisture), they begin to evaporate (dehydrate), and their radius shrinks over time and as the water content decreases. Particles larger than 100 μm in diameter can carry tens or even hundreds of virions, but will fall to the ground quickly and are less likely to be inhaled. Aerosols produced by droplets smaller than 0.1 μm can stay in the air for many days, but are negligible because they are smaller than the diameter of coronavirus particles.
Viruses in aerosols formed by dehydration of respiratory droplets between 0.1-100 μm may pose a risk of infection. These virus particles can be suspended in the air for a few minutes (tiny particles, non-stop irregular movement), accumulate in confined spaces, or travel long distances through convective airflow, like the smell/smoke of a neighbor's meal, spreading several floors or even several buildings.
For example, dehydration of 45 μm and 15 μm droplets produces aerosol particles of about 15 μm and 5 μm, respectively, both of which enter the upper respiratory tract smoothly, which is also the most common case with SARS-CoV-2, mainly smaller coughing or talking droplets.
Smaller particles are able to descend and infect the lower respiratory tract, which can exacerbate the severity of the disease. The typical viral infection path is gradually spreading from the upper respiratory tract to the lower respiratory tract, but high concentrations of tiny particles, such as the new coronavirus and PM2.5, can be deposited directly into the bronchi and even the alveoli.
Although respiratory droplets are small in size and do not necessarily carry the virus, they still pose a risk. Because breathing is a continuous activity, and breathing aerosols last up to several hours in the air. The sum of respiratory particles produced by hospitalized patients per hour may contain thousands or even millions of virions, especially in seriously ill patients. Coupled with their small size, which can penetrate deep into the lower respiratory tract, and the difficulty of filtering these particles without masks, the high incidence of severe illness and death in hospital wards may be related to the large accumulation of viral particles. The high number of infections among healthcare workers in the early stages of the outbreak may also be involved, and increased air humidity and air exchange can reduce the risk of infection.
iii. What kind of droplets are involved in super-spreading events
A key factor in the spread of infectious diseases, the super-spreader, directs a single spreading individual with a large number of secondary cases. In most infectious diseases, including covid, there is such a rule - the 20/80 rule, that is, 20% of the infected population causes at least 80% of transmission.
Studies on many international super-spreading events show that these super-spreads often contain 3 common factors:
at least one highly contagious host;
Closed environments with poor ventilation;
Vocalize loudly, such as speaking or singing loudly.
This may be because an infected person can release respiratory droplets of various sizes containing large amounts of the virus when they speak loudly. When coughing, sneezing, talking, and breathing, infected people release droplets that contain virus particles that can be converted into longer-lived aerosols as the water evaporates. However, larger droplets (such as coughing, sneezing, or droplets flying from the mouth) tend to be small in number and will land quickly (affected by gravity, follow a ballistic trajectory), and play a smaller role in transmission; The most worrying thing is speech aerosols, although these aerosols are small in size and carry a low amount of virus, but because speech tends to last for a period of time, these virus particles can be suspended in the air for a few minutes (tiny particles, non-stop irregular movement), accumulate in confined spaces, and may even directly infect the lower respiratory tract.
Read what happened when you passed the sun 2 - friends who how to prevent and treat infection from an immune perspective may remember: after infection with the new crown virus, the upper respiratory tract quickly produces an infectious hormone reaction to clear the virus and avoid serious infection; However, it is too late for the virus to produce an interferon response when the virus infects the lower respiratory tract, and infected people are more likely to become severely ill. Therefore, when the new coronavirus passes through the upper respiratory tract and directly infects the lower respiratory tract, it escapes part of the host's immune surveillance. Without interference from the immune system, the symptoms of infection tend to be worse.
During the coronavirus pandemic, many restaurants in Japan had "silent food" signs on the tables and "silent" warning signs in elevators, prompting not to speak when eating or riding the elevator during special infections, which are indeed positive practices to prevent the spread of droplets.
Studies have found that most of the transmission of the new coronavirus occurs before the onset of symptoms, that is, before coughing. At this time, although there are no obvious symptoms, a large number of viruses are already replicating in the upper respiratory tract and mouth of infected people, so the speech aerosols formed by speech droplets released by asymptomatic or mildly infected people when speaking are likely to infect unprotected uninfected people. Our usual dinners usually take place indoors and almost everyone is talking loudly, so it's only natural that bars, restaurants and elevators are at the center of multiple super-viral events.
Studies have shown that verbal aerosols are one of the main ways of airborne transmission of the new crown. Speaking loudly without a mask in an enclosed space may be the activity that poses the greatest risk to others. Infected people speak loudly in closed spaces, which will release a large number of droplets containing the virus and infect others; For uninfected people, speaking loudly also increases the chance of viral aerosols in the air entering their upper or even lower respiratory tract.
4. Who is protected by a mask – infected or uninfected
Basic hygiene tells us that wearing masks can help uninfected people reduce the risk of infection. When they are not infected, most people actively wear masks to prevent infection. However, some people think that it makes no difference whether they wear a mask after infection/after impotence, right?
In fact, most people feel that they should also wear masks when they are yang to avoid infecting others; But there are also a few people who feel that it is useless to wear a mask anyway, whether it will worsen the symptoms, and even some people not only do not wear masks after infection, but also wander around maliciously spreading the virus.
From the perspective of national epidemic prevention, the act of not wearing a mask to a public place after infection is undoubtedly a violation of the epidemic prevention law and an immoral behavior. Then we also need to understand scientifically what the impact of wearing masks has on infected and uninfected people.
Masks serve two main purposes: to limit the escape of potentially infectious respiratory droplets; At the same time, the entry of aerosols carrying the virus is reduced. The former role can not only protect the wearer, but also reduce the risk of infection of those around him, while the latter role is undoubtedly the mask wearer.
Studies have found that wearing masks is also beneficial to infected people themselves, mainly with the following three benefits:
Virus-containing body droplets leaving the mouth/nose also contain salts, mucus, lipids, and other non-volatile components and remain highly hygroscopic after dehydration. As a result, foreign respiratory aerosols will reabsorb moisture and swell as they pass through the damp mask, increasing the likelihood that they will be trapped.
Masks minimize the possibility of breathing the virus that self-captures its speech aerosol into the lower respiratory tract, where the virus otherwise migrates from the upper respiratory tract to the lungs and worsen symptoms.
Masks/face shields humidify inhaled air, promote mucociliary clearance and interferon-mediated innate immune responses.
Food and heart warm summary: the cold air from late autumn to winter is more likely to make the moisture in droplets and aerosols volatilize rapidly, making the virus drift more easily, although the temperature has begun to rise in spring, but the spring air is still very dry, and it is also the season when virus particles are easy to spread. Whether it is the new coronavirus or influenza virus, whether it is human or livestock infection, most infections are more epidemic and difficult to control during this special period.
Most of the new coronavirus transmission is spread by infected people who are asymptomatic or have not yet shown obvious symptoms, and people often do not know whether the person sharing the air space is infected. Respiratory droplets released by asymptomatic infected people contain a large number of virus particles, which can form aerosols after dehydration. This virus-containing aerosol is very small in size and weight, and does not fall to the ground like a thrown stone, but floats in the air like PM2.5 particles and constantly moves, is not blocked by walls and glass, and can float like smoke for a long distance.
Contrary to many people's intuition: masks protect not only those who are not worn, but also the wearer; It can not only protect the uninfected, but also protect the infected. The virus particles released by the human body after impotence may not only infect others, but also cause severe disease through self-inoculation of aerosols containing the virus, and even infect the lower respiratory tract.
Although it may be uncomfortable to wear, wearing a mask during an infectious disease is beneficial.
Reference materials
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8242678/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9430019/