COVID-19 is a highly infectious form of coronavirus that can cause a severe pneumonia as well as damage to multiple other organs of the body. .
Respiratory tract infections can be spread person-to-person via droplet nuclei and aerosols that are breathed and coughed out from the mouth into the atmosphere, traveling as far as 5 meters (almost 17 feet). Spread via aerosols with airborne viral particles is enhanced by crowding and poor ventilation. People with infection who appear well and feel well can spread the infection.
A cloth 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 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.
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 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 from others. 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. Following a nationwide vaccination program in Israel, the estimated 2-dose vaccine effectiveness during the follow-up period a week after the second dose was 94% for symptomatic and 92% for severe COVID-19. In over half a million people vaccinated there were just 15 people hospitalized, 4 with severe infection, and no deaths. (N Engl J Med 2021;384:1412-1423 DOI: 10.1056/NEJMoa2101765) This level of success exceeds expectations for control of the pandemic.
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 has the ability to 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, concentration 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 and other organ damaage. Persistent mental health issues include depression, worsened by chronic fatigue.
This virus infects the surface of airways where it can dodge 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 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.
Biology of Viral Illness
Viruses must live and reproduce within human cells. Those cells can be on surfaces like our respiratory tract, gastrointestinal tract, or skin. Infected cells can be 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 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.
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 growth and 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. Further doses of a vaccine may be needed to maintain immunity, as with yearly flu vaccines.
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 more challenging to limit the potential damage. Passive immunization does not provide long-lasting immunity. Producing the antibodies for passive immunization is complex and expensive.
Drugs to treat viral diseases are difficult to develop, so few are available, even after decades of research. Drugs to treat microbes target the things microbes do but our own cells don't. A virus may use a protein to reproduce that our own cells do not have. A drug that blocks a viral protein can slow the virus from reproducing. Very few drugs kill microbes outright.
In order for a drug to get to the place in the body where it needs to attack a virus, enough has to be given to be effective. At such dosing levels, the drug can have side effects that are annoying, like nausea and vomiting, or are more serious, like damaging our own cells. Developing an effective drug that does not have serious side effects is hard to do.
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.
Coronaviruses come in many forms, but they all have RNA comprising their genes. They are found in many animals, not just humans, but they can spread between animals and us. 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.