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Reducing Infection by reducing viral dose
One way to prevent infection is to reduce the amount of virus to which people are exposed. This is sometimes called the "viral dose." It is different from the average number of viral particles needed to establish an infection, which is is known as the "infectious dose." Research suggests that the infectious dose for COVID-19 is relatively low, which is why it is easy to catch and spreads so rapidly. The infectious dose, also known as minimal infective dose for COVID-19 is estimated to be just a few hundred or thousand particles.

"Viral load" is the number of virus particles in an infected person after the virus begun to replicate in their cells. People with higher viral loads tend to shed more virus, meaning they are more contagious, increasing their risk of spreading disease more widely. In the case of influenza and other viruses, a higher viral load is also associated with worse symptoms and outcomes. Research has shown that animals infected with higher doses of the SARS and MERS coronaviruses experienced worse outcomes. According to David Hardy, professor of infectious disease at Johns Hopkins University School of Medicine, higher viral load is likely associated with more damage to the lungs. To complicate matters, COVID-19 has a high viral load at presentation, victims are infectious two to three days before experiencing symptoms, and its viral RNA has been detected in some COVID-19 patients' deep throat saliva for 20 days or longer, suggesting a long infectious period.

While COVID-19 is related to SARS, there are also important differences among them that aren't yet fully understood. Edward Parker at the London School of Hygiene of Tropical Medicine says until there is more research on the relationship between the initial level of virus at infection and disease severity, people should minimize exposure to the virus to reduce the chances of catching it and passing the virus on to others.

Health-care authorities around the world are looking for effective ways to prevent community transmission of COVID-19. Some of the methods currently in use include social distancing, wearing a mask, thorough cleaning, being outdoors, ventilation and filtration of air in indoor spaces, and containing and disinfecting aerosols from toilet flushing. Risk is relative, a matter of degree, and cumulative.

Lowering the amount of virus in indoor air
There have been numerous reports of COVID-19 clusters in poorly ventilated and heavily populated spaces such as households ("family clusters"), workplaces, restaurants, stores, religious gatherings, public transportation, nursing homes, and other indoor settings. In such spaces, frequent viral transmission occurs in spite of social distancing efforts because the virus builds up in the shared air space. Consistent with this, people who work close together, sharing the air in open offices with no barriers between them are absent due to illness more often than those working in separate offices. This has led to the closure of non-essential businesses. These businesses will continue to close until this risk is sufficiently reduced.

Staying at home theoretically limits the viral dose by limiting number of infected people with which one comes into contact. If there are multiple household members and visitors, however, precautions are needed because there is a greater probability of contact with infected, contagious, asymptomatic individuals because you are sharing the same limited air.

Everyday sneezing, singing, talking, breathing and flushing the toilet can result in the expulsion of thousands of large and small respiratory droplets carrying the virus. These droplets, which range in size from submicrons to approximately 10 μm diameter, and have been shown to contain viral particles. Larger, heavier droplets drift to the ground quickly unless there is air movement but droplets less than 5 microns in diameter, called "aerosols" can remain viable and infectious in aerosols for 3 hours or more. These aerosols build up in small, poorly ventilated areas, increasing the likelihood of someone receiving an infectious dose. These small droplets also travel further, remaining aloft longer. They have more potential to spread the virus throughout the room and building than the large droplets. Proximity to the infected person, air flow, and timing are particularly important determinants for aerosol transmission. Aerosols containing even a small concentration of virus in poorly ventilated spaces, combined with low humidity and high temperature, might result in an infectious dose over time.

The implications of the information above are critical to understanding the importance of reducing the concentration of the virus and consequently the risk of becoming infected in all home and public settings where one might accidentally come into contact with an asymptomatic, COVID-19-infected person.

Enhanced ventilation. The CDC recommends ventilation of all public spaces to help dilute and clear out potentially infectious aerosols. This can be achieved by increasing the percentage of outside air, increasing the total airflow to occupied spaces, disabling demand-control ventilation (DCV) controls, and using natural ventilation (e.g., opening windows and doors if possible and safe to do so).

Enhanced filtration. The CDC recommends enhancing filtration in building heating and air conditioning (HVAC) systems as part of an overall risk mitigation plan in conjunction with consideration of "compatibility with HVAC system capabilities for both temperature and humidity control as well as compatibility with outdoor/indoor air quality considerations." It should be designed in consultation with a professional or it may be counterproductive. The CDC recommends increasing air filtration as high as is possible (MERV 13 or 14) without reducing design airflow. This may require replacing the current HVAC system with one that can better handle such filters which create more resistance to air flow. Other recommendations include using appropriate, correctly installed filters, running ventilation systems continuously, and creating separate ventilation zones for higher and lower risk areas of the building (e.g., individual offices on a different zone than office gym).

Another option may be to add freestanding air cleaners, which range in size from tabletop units for homes and restaurants to units for large rooms. The CDC recommends considering portable high-efficiency particulate air (HEPA) fan/filtration systems to further clean the air, especially in areas where virus particles are likely to be most concentrated.

UV systems. The CDC recommends ultraviolet germicidal irradiation (UVGI) as part of an overall system to help inactivate the virus. This should be done as part of an overall mitigation plan in consultation with a professional because the specifics of the system are critical to safety and effectiveness.

Bathrooms. The CDC recommends that bathroom fans be working and at full capacity when people are in the building.

Precautions for the home
There are a number of ways one can reduce virus circulation among family members and housemates. Masks. Although inconvenient and not common, masks are still useful within the home to protect housemates from infecting each other within the home. Air filtration.

Reducing risk from toilet spray plume. Research has shown that infected individuals shed COVID-19 in through their stools. When toilets are flushed, they release a plume of toilet water particles, or "aerosols" about 3 feet into the air. Although the larger droplets may quickly fall onto surfaces, smaller ones may stay airborne for long periods of time, e.g., more than half an hour, and be inhaled by subsequent users. The concentration of virus builds up with more flushing, increasing the amount of virus to which toilet users are exposed both through their air and on surfaces. A busy, windowless public bathroom is riskier than a bathroom with an open window, or a bathroom that’s rarely used. It is safer to chat outside with a masked neighbor than be in any of the bathroom situations.

To date, efforts to reduce the virus in bathrooms have been insufficient.

It is important to either dilute this build up (vent it out into an uninhabited space) or aggressively filter it.XXXX Otherwise, it may provide a potent reservoir of inhalable virus that may expose users to an infectious dose. Another option is heavily disinfecting the water in the toilet bowl and tank so that at least some of the virus in feces (so far urine is not known to contain the virus, but there is much we don't know about this virus) is likely to be killed or compromised when it hits the water in the bowl. Common disinfectants are known to be effective in killing the virus.XXXXXX Cleaning. It is important to keep surfaces in the home. Common disinfectants are known to kill COVID-19. XXXXX Isolating sick household members. It is important to isolate any member of the group who becomes ill as well as those exposed to that person. Isolated individuals ideally should not share the same bathroom, living areas, or kitchen as others in the household, but this may not be possible. The whole house should be kept disinfected, ventilated, and the air filtered to reduce the dose members of the household may be exposed to.XXXXX If isolation is not possible, the infected individual should avoid contact with others in the household. Everyone in the home should wear masks until all members of the household are virus-free.

Precautions for outside home
Masks. As Andrew Cuomo, Governor of New York, says, "Masks should be worn in public."XXXX Six feet of social distance is not sufficient protection.XXXXX "In public" is defined as "in view of other people; when others are present."XXXX Masks protect others. ''Mask protect self, though to a lesser degree. Social distancing. Social distancing is difficult indoors, but is recommended. Wearing a mask along with good air filtration and ventilation can help mitigate the risks of exposure to a viral dose. Ventilation and filtration. Stay outdoors. Ventilation and filtration of air indoors. Public bathrooms

Considerations for businesses
Businesses have been allowed to open up when the prevalence of the virus has declined sufficiently in a region. Outdoor businesses that aren't prone to crowding have been allowed to open before non-essential indoor businesses due to the reduced chance of exposure to an infectious dose. If indoor businesses could approximate this lower risk by ventilating their spaces, enhancing air filtration, and reducing crowding, perhaps they could open sooner. Air ventilation and filtration. Require masks. Just as recommended for outdoors, masks should be required indoors when people are "in view of other people; when others are present." Masks designed to allow drinking are available. Social distancing. Bathrooms. Cleaning.

About building immunity Vaccines Accidental low dose exposure

Viral dose is different from viral load, which relates to the number of viral particles being carried by an infected individual and shed into their environment. “The viral load is a measure of how bright the fire is burning in an individual, whereas the infectious dose is the spark that gets that fire going,” says Edward Parker at the London School of Hygiene and Tropical Medicine.

Read more: https://www.newscientist.com/article/2238819-does-a-high-viral-load-or-infectious-dose-make-covid-19-worse/#ixzz6QfbOmiJw

Viral load can either refer to the degree of infection in an individual or the amount of virus in an environment. Cloth face coverings reduce viral load by preventing droplets from entering the surrounding air space. Other ways to reduce viral load are to ventilate an area to dilute the virus in the air, to use filters to remove the virus from the air, and to reduce the amount of infectious spray from flushing toilets by closing the lid or by continuously disinfecting the water in the toilet bowl.