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A well-fitting face mask fully covering nose and mouth reduces airborne particles with COVID-19 from spreading. Infectious particles spread in air by droplet nuclei can be carried 5 meters (almost 17 feet) when coughing or sneezing. For normal breathing to avoid the droplet nuclei and to prevent direct contact a distance of 2 meters (6 feet) between persons is sufficient in uncrowded and well-ventilated areas. A fitted mask (respirator) with high efficiency filtration is needed to fully protect against tiny viruses carried by aerosols. This is most important for trained professional health care workers who need to be close to people under their care. (https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/types-of-masks.html)
Respiratory infections caused by many different microbes can be transmitted by direct close person-to-person contact (e.g., during kissing or hugging or hand shaking), and through the air, or from fomites (things the infected person has touched, coughed on, or sneezed on). Transmission on fomites is less common for COVID-19. Wash your hands before touching your face and items like your cell phone or your computer. Disinfect surfaces around you that you touch, like tabletops, door handles, and light switches.
This COVID-19 virus has an envelope to enhance infectivity through attachment to cells in the respiratory tract. However, the envelope makes the virus vulnerable to disinfectants, even soap and water. The most effective disinfectants have sodium hypochlorite (bleach), hydrogen peroxide, or alcohol. These viruses can survive in a variety of environments and on surfaces for at least 4 to 5 days. Disinfection of surfaces and frequent hand washing will reduce spread of infection by many different infectious agents.
It is not possible to predict how bad a coronavirus infection will be for each person. Some coronaviruses like COVID-19 spread easily from person to person and can cause more severe disease in more people. Healthy people without other illnesses are less likely to have severe illness, but there is no guarantee. You can keep getting exposed and ill. Persons who feel well or just mildly ill can spread the virus to others who are at greater risk. Risk goes up with age and with existing health problems like diabetes, obesity, respiratory disease, kidney disease, and heart disease. Taking precautions to prevent the spread of infection protects you and others.
Vaccines work. They can prevent COVID-19 infection, prevent more serious illness or death, and reduce spread of the infection. (Nat Rev Immunol. 2021;21(10):626-636. doi:10.1038/s41577-021-00592-1).
Biology of COVID-19
COVID-19, also called severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is an enveloped RNA virus. The envelope acts like a sticky bubble around its genetic material composed of the RNA directing viral replication. This envelope with spike-like projections (the "corona") sticks readily to infect cells lining the respiratory tract. However, the bubble is easily burst out in the environment, and it is highly vulnerable to disinfectants. Therefore, viral spread is highly dependant upon close human contact.
This virus produces the characteristic pathologic findings of many viral respiratory illnesses. Infected cells show cytopathic effects of damage to cell cytoplasm and membranes, leading to cell necrosis, called apoptosis. Remaining cells begin to proliferate and try to repair the damage.
The most common symptoms are cough, fever, and shortness of breath. Persons short of breath have more severe disease. Like many viruses, COVID-19 can produce a variety of systemic symptoms including headache, fatigue, muscle pain, chills, nausea, abdominal pain, vomiting, and diarrhea.
Only a small amount of this virus establishes infection, starting in the nose and throat. This virus can proliferate rapidly. An infected person can begin passing this virus to other people before feeling ill, maybe having only coryza (runny nose), scratchy throat, or something resembling allergies. It may take 2 days to 2 weeks for symptoms or signs of infection to appear following exposure. Many viral illnesses act in this manner, so early diagnosis and tracking of contacts as a coordinated public health effort is crucial to limit spread.
The infection may stay in the upper respiratory tract, but it often goes to the lungs, where it causes inflammation around the air spaces, called interstitial pneumonia, so there’s not much to cough up, accounting for a dry cough early on. In healthy people, the infection may be cleared in a week with few symptoms. There is random variation to all biologic events, there’s no guarantee it won’t progress to a severe pneumonia, so anyone is potentially vulnerable and could die. Older people are more likely to have chronic illnesses, and even young people can have poor health, putting them at risk.
Like many viruses, COVID-19 can spread from lungs to other organs in the body, causing damage that can be severe and lasting, even after the respiratory infection is over. Damage to the brain can lead to "brain fog" including memory loss, difficulty concentrating, or sleep problems, and increase the risk for neurodegenerative disease. The heart muscle can be damaged to increase the risk for heart failure. Muscle and joint pains can persist. Loss of smell can occur (and with it diminished sense of taste). Viruses can make cells stick together, including blood cells, increasing the risk for blood clots clogging blood vessels, with risk for stroke.
The immune response to the virus can produce damage to many cells. Chemicals called cytokines are part of an immune response and can damage not only infected cells, but also other cells. The immune system may continue to produce these cytokines in abundance. (Malireddi RKS, Sharma BR, Kanneganti TD. Innate Immunity in Protection and Pathogenesis During Coronavirus Infections and COVID-19. Annu Rev Immunol. 2024 Jun;42(1):615-645. doi: 10.1146/annurev-immunol-083122-043545.)
"Long COVID" is defined as an infection-associated chronic condition that occurs after COVID-19 infection and is present for at least 3 months as a continuous, relapsing and remitting, or progressive disease state that affects one or more organ systems. It could last for years. Problems include loss of smell, fatigue, difficulty breathing, cognitive and mental impairments, chest and joint pains, abdominal discomfort, palpitations, myalgia, cough, runny nose, diarrhea, and headache. Affected persons may be unable to do the activities they want, work, or care for their families. (https://nap.nationalacademies.org/read/27768/chapter/1)
This virus infects the surface of airways where it can evade immune responses, spread from cell to cell, and reproduce itself rapidly before an immune response becomes effective. Some people can appear to be asymptomatic while infected, but still passing virus. The incubation period when virus is increasing before illness appears can be 2 to 14 days. Given that COVID-19 functions like many respiratory viruses, the possibility exists that one can be infected but come away with no lasting immunity, and even be at risk for re-infection.
The way COVID-19 spreads, the people at greatest risk, and the bad outcomes are nothing new. We have a very crowded world with extensive interactions among people and lots of travel, making a perfect environment for viruses to undergo high-speed evolution to new variants, and spread more easily.
For infectious diseases, the reservoir of persons infected matters. Where does it arise, where does it spread, and where does it remain? The biggest risk for any illness: poverty. The people with the fewest resources have the biggest risk and become the reservoir. Even prominent and very rich people are vulnerable when there’s a reservoir. The population most at risk for infection, more severe illness, and death becomes those who are old, those who are already in poor health, and those with limited resources, but anyone could be severely affected. More people doing infection control increases safety for all. Denial and lack of precautions reduces safety for all. (Umakanthan S, Sahu P, Ranade AV, et al. Origin, transmission, diagnosis and management of coronavirus disease 2019 (COVID-19). Postgrad Med J. 2020 Dec;96(1142):753-758. doi: 10.1136/postgradmedj-2020-138234.)
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Biology of Viral Illness
Viruses can live and reproduce within human cells. Viruses can infect us by entering cells on surfaces like our respiratory tract, gastrointestinal tract, or skin. Viruses can spread to cells within organs like liver and brain. Viruses have proteins on their surfaces to help them stick onto and enter into human cells, then take over the cells to make more viruses. Viral surface proteins can selectively attach to some cells, explaining the kind of disease they cause, such as pneumonia, diarrhea, skin damage, liver damage, or brain swelling. Some viruses may attach to many different kinds of cells.
Viruses can be spread in many ways: by close contact with another person, through eating and drinking, through the air, from contact with contaminated surfaces, or through animals, and especially insects. Viruses can’t persist in hostile environments. Disinfectants are hostile to viruses.
Coronaviruses come in many forms, but they all have RNA comprising their genes. Many coronaviruses infect the respiratory system, anywhere from the nose down to the lungs. Some of them are responsible for the common cold, which we can experience over and over again because there is no lasting immunity and because there are many different forms of coronaviruses. More severe respiratory infections may leave more lasting immunity.
Human immune systems have many ways to fight viruses. Since viruses live inside cells, the immune response has to destroy the infected cells or neutralize the virus particles outside of cells to limit continued spread of the virus. This takes days to weeks, and sometimes months. Infected cells release lots of new viral particles. These particles infect more cells, either at the original site of infection, or by getting into the bloodstream, called virema. From the bloodstream they can get access to many more cells in many more organs of the body. Some of our immune cells directly attack infected cells where the virus lives. Other immune cells make antibodies that attach to viral particles outside of cells. Since antibodies don’t go inside of cells, they cannot stop the infection within cells, only neutralize viral particles outside of cells.
Available vaccines cause our immune system to make antibodies against specific microbes, like viruses. No single vaccine can protect against all microbes, or even against all forms of one kind of microbe. Giving a vaccine is called active immunization, since the immune system actively makes antibodies in response to the vaccine. Then, when encountering the microbe, the antibodies hold the infection in check while our immune system clears it. The amount of antibodies can diminish with time, reducing protection. The microbe can change, making it a moving target. A high viral mutation rate explains why a new vaccine for influenza, another RNA virus, is developed every year. Further doses of a vaccine may be needed to maintain or boost immunity, as with yearly flu vaccines. In response to ongoing changes in COVID-19, additional COVID-19 vaccines have become available.
Active immunization requires a complex process of developing a vaccine that produces enough antibodies effective in stopping an infection after exposure to the microbe. The best vaccines do this at least 80% of the time, but none do it 100% for all persons. As the microbes change to evade our immune response, and as the antibody protection decreases with time, more doses of a vaccine may be needed. Vaccine development is an arduous, complex, time-consuming process needing lots of safety precautions before use in us.
Passive immunization involves taking antibodies from the blood of another person or animal, or cells modified to produce antibodies. This kind of antibody protection is done routinely for persons who have a deficient immune system who cannot make their own antibodies. Passive immunization can be done for persons recently exposed to a microbe to try and prevent infection, like giving hepatitis B virus antibodies to a person who had exposure to viral hepatitis. This kind of immunization works best when given as soon as possible after exposure has occurred. Giving the antibodies after infection has been diagnosed is less effective. Passive immunization does not provide long-lasting immunity. Producing the antibodies for passive immunization is complex and expensive.
Passive immunization with the long-acting monoclonal antibodies can be provided to persons who are severely immunocompromised. However, for persons who are hospitalized mortality remains high. Drugs are most effective when administered soon after infection. (Huygens S, GeurtsvanKessel C, Gharbharan A, et al. Clinical and Virological Outcome of Monoclonal Antibody Therapies Across SARS-CoV-2 Variants in 245 Immunocompromised Patients: A Multicenter Prospective Cohort Study. Clin Infect Dis. 2024 Jun 14;78(6):1514-1521. doi: 10.1093/cid/ciae026.)
How do viruses defeat immunization and drugs? If a virus stays localized to a surface, like the cells lining the nose or mouth, it may not produce a lasting protective immune response in us. Reinfection is possible. Antibodies or drugs cannot reach all locations in the body in amounts large enough to be effective. Viruses reproduce quickly to release millions of viral particles each day during infection. Viruses change their genetic makeup quickly. Some of these changes make the viruses more threatening to us, and those new viruses can evade our immune responses, be resistant to drugs, reproduce more quickly, spread to more cells, do greater damage to cells, or spread to other people more easily. The more people infected, the more chances a virus has to evolve into new forms that are more threatening to us.
The larger the reservoir of infections, with more people infected, the worse the problems controlling spread of infection. More transmissions are possible with more people contacting each other. The number of COVID-19 infections in 2020-2021 was accompanied by a million viral mutations transmitted per day, and out of that number come new viral variants capable of greater transmission and/or worse illness, and escaping infection-acquired or vaccine acquired immunity.
World Health Organization. Coronavirus disease (COVID-19). https://www.who.int/health-topics/coronavirus#tab=tab_1.
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