What are coronaviruses?
Coronaviruses are a group of related viruses that cause diseases in mammals and birds.
They are roughly spherical particles with surface projections (spikes that give the virus its name). The average coronavirus has 74 spikes. The average diameter of a coronavirus is about 125nµ - the sphere is about 85nµ and the spikes on each side are about 20nµ. A micron (µ) is one millionth of a meter; a nanometer (nµ) is one billionth of a meter.
Where do coronaviruses come from?
Viruses are thought to be as old as life. There are trillions of species of viruses but fewer than 7,000 have names. Only 250 or so have the mechanics to infect us. Four of them cause most common colds. It is not uncommon for infectious diseases to spread from animals to people (known as zoonotic diseases), and many human coronaviruses have their origins in bats.
Viruses are typically not dangerous to the animals that host them prior to their transmission to humans. Viruses typically mutate as they spread, and an analysis of changes in the genetic material (RNA) can help reveal the path of the spread of the virus. More than 1300 variants of the current coronavirus have been identified.
Analyses of these variants have led scientists to conclude that the virus almost certainly came from bats in China, and was not engineered in a laboratory. Analyses also indicate that most of the cases in NY likely came via Europe.
How do coronaviruses affect humans?
The virus enters the body through the nose, mouth, or eyes and then attaches to cells in the airways. A spike of the virus attaches to a host cell receptor. The virus then fuses its membrane with the membrane of the cell. Once inside, it releases a snippet of its RNA. The infected cell begins to produce new copies of the virus while trying to keep the immune system at bay.
Each infected cell can release millions of copies of the virus before the cell dies. The viruses may then infect nearby cells or end up in droplets that escape the lungs, and have the capacity to infect other people.
The human immune system typically develops antibodies to fight the virus. When the immune system works effectively, the virus is vanquished quickly. The symptoms of coronaviruses range from mild (eg, the common cold) to lethal (SARS, MERS, and COVID-19).
The most common symptoms of COVID-19 include fever or chills, cough, shortness of breath, fatigue, muscle aches, headache, diminution of sense of smell and taste, and nausea.
How do coronaviruses spread?
The coronavirus primarily spreads from respiratory droplets released by infected individuals when they exhale, talk, cough, or sneeze. Shouting or singing releases more droplets than normal speaking. The droplets are typically much larger than the virus itself. The virus is smaller than 2nµ, while droplets are typically 5nµ to 10nµ (but can be bigger). The droplets can travel short or long range from the source depending on the size and shape of particles, the initial velocity, and environmental conditions (humidity, airflow, temperature).
Large droplets typically remain airborne only briefly before settling because of gravity. By the time someone walks out of a room, droplets have most likely settled on the floor or nearby surfaces. Droplets can also be emitted during aerosol generating procedures (AGPs) such as intubations.
Aerosol transmissions are often smaller than 5nµ but can be up to 30nµ. Because of their small size they can remain suspended in the air for longer periods. Aerosol transmission is more of a concern in a hospital setting, but it can also happen in other settings. The virus can also be spread by touching a contaminated surface (known as fomite transmission), but this entails someone releasing droplets onto a surface, the virus remaining alive on the surface, and then someone subsequently touching the surface and picking up a large dose and infecting themselves by touching their eyes, mouth or nose.
This is possible, but the CDC has said that contaminated surfaces are not thought to be a main cause of virus spread. Studies have shown that the virus can survive for up to 3 days on some surfaces (plastic and steel) and on cardboard for up to 24 hours - but those studies tested for genetic material, not the live virus which degrades more quickly.
How does this pandemic compare to other pandemics?
Epidemics are not new. The most deadly viral pandemics in history include: The Bubonic Plague (the “Black Death”) in the 1300s (200 million deaths), Smallpox starting in the 1500s (more than 50 million deaths), Yellow Fever in the late 1800s (more than 100,000 deaths), the Spanish Flu in 1918 (more than 40 million deaths), HIV starting in 1981 (more than 25 million deaths), SARS in 2002 (770 deaths), Ebola in 2014 (11,000 deaths), MERS in 2015 (850 deaths), and COVID-19 in 2019 (more than 1 million deaths and counting).
The viruses which caused SARS (Severe Acute Respiratory Syndrome), MERS (Middle East Respiratory System), and COVID-19 (Coronavirus Disease 2019) are known as SARS-CoV, MERSCoV, and SARS-CoV-2, respectively. SARS-CoV-2 is closely related to SARS-CoV. The SARS epidemic originated in Guangdong in 2003, most likely having come from horseshoe bats.
The total number of reported cases of SARS was just over 8,000 and the fatality rate was about 11%. The virus is most infectious in severely ill patients which usually occurs during the second week of the illness, meaning that quarantine is a highly effective preventative measure.
People quickly became hyper-conscious of the virus and super-vigilant about cleaning surfaces, and the virus was essentially eradicated by July 2004. The first MERS case was identified in Saudi Arabia in 2012 and it is believed to have come from camels. Most transmissions of the virus came from close contact with severely ill persons and there is no evidence of transmission from asymptomatic cases. The number of reported cases is only about 2,500, but the virus is particularly deadly (killing an estimated one-third of those infected).
There have been a small number of reported cases of MERS in recent years. As our understanding of viruses has improved, society has been able to more effectively mitigate their impact. Thus the expectations of many were that we would quickly develop an effective response to COVID-19 which may help explain our complacency during its early spread.
How transmissible is this coronavirus?
The transmissibility of a disease is estimated byR0 (R nought) which is a measure of the number of people, on average who will be infected by each infected person. A disease that has an R0 well below 1 will fade away, while one that has an R0 well above 1 will continue to spread. The R0 for SARS CoV-2 is estimated to be between 2 and 3 - i.e., each infected person, on average, probably infects around 2.5 others. (The 1918 Spanish Flu pandemic had an estimated R0 of 1.8.)
Measures that reduces R0 below 1 will be effective in ending this pandemic. Some models suggest that a face covering that is worn by 60% of the population and is 60% effective at blocking viral transmission will quickly reduce R0 below 1. R0 by itself does not convey the vast range between how much some infected people transmit the virus (superspreaders) and how little others do, and thus epidemiologists also look at a virus’s dispersion factor known as k.
The fewer the number of cases of infection responsible for all transmissions, the lower k generally is. It is believed that superspreading is a hugely significant factor in the transmission of SARS-CoV-2. One study indicates that just 20% of infected people account for 80% of transmissions, and that 70% did not pass on the virus to anyone, indicating that the k for SARS-CoV-2 is about 0.45. Superspreaders likely shed an unusual amount of virus, release particularly infectious viruses, and have longer-lasting and more close contact with others. Clearly isolating superspreaders would be a huge mitigant, but at his point we have little information on how to identify superspreaders.
But their existence is probably the best argument for universal mask-wearing.
Why is this coronavirus so problematic?
The extent of damage a virus can do to a society is a function of the ease of transmissibility, the point after infection when it is most transmissible, the latency between the time a person first gets the virus and the onset of symptoms, and the severity of the symptoms.
• If transmission is slow or difficult, the overall impact on society will likely be limited. • If the latency is short or transmissibility does not happen until after symptoms first develop, people will know quickly that they are infected, and transmission can be controlled through quarantine.
• If the symptoms are not particularly severe, the overall impact on society will be limited even if many people are infected. If the symptoms are very severe (if you get it and die quickly), it will harm those who get it, but it will likely not spread very far. This coronavirus exhibits a combination of attributes that makes it particularly problematic.
• It is easily transmissible.
• There is a relatively long latency between getting the virus and the initial onset of symptoms (from 2 to 14 days, with an average of 5 days).
• The virus is most contagious in the early days after contracting it, and it is typically spread by people who do not know they are infected. It is believed that somewhere between 40% and 80% of transmissions originate from people who are presymptomatic or asymptomatic.
• Its symptoms are very severe for some (the fatality rate is thought to be between 0.5% and 3%) and almost non-existent for others: it is estimated that 35% of people who contract the virus experience few or no symptoms, and that 80% of people recover without special medical treatment.
What about testing?
There are two types of COVID-19 tests - a diagnostic test tells if you have a current active infection, and an antibody test might tell if you had a past infection.
There are two types of diagnostic tests - molecular tests which detect the virus’s genetic material, and antigen tests which detects specific proteins on the surface of the virus. Both use nasal or throat swabs to get samples. It can be as little as a day or as long as a week to get results from molecular tests, while results from an antigen test can often be available within an hour.
The tests also have varying levels of accuracy. One problem with diagnostic tests is that there is little chance that they will come back with a positive result in the first few days after exposure to the virus, and they might not come back with positive results until as many as eight days after the initial exposure.
This can give a false level of comfort to individuals who are in fact contagious. Antibody tests check your blood to see if your body has made the proteins that help fight COVID-19. Results are typically available in 1 - 3 days.
They are not effective in telling you if you currently have the disease as the antibodies might not show up for 1 - 3 weeks after being infected. While the antibodies help in preventing you from getting infected again, scientists are still not sure if the presence of antibodies means that you are immune to the coronavirus in the future. So testing is helpful in identifying people who should quarantine, but it is not the complete answer.
How and when will we finally get this under control?
Unless we find effective treatments, the coronavirus will remain a major problem until we reach herd immunity - i.e., when enough people have built up immunity so that it become difficult for the virus to spread.
Herd immunity will be reached either when an effective vaccine is widely available, or though the build up of immunity resulting from the spread of the virus throughout the population (although we still do not know how effective that immunity will be or how long it may last). The virus has spread more rapidly in some countries or regions either because governments and citizens underestimated the virus or did not have the resources needed to combat it.
Some countries opted to let the virus spread more or less unchecked in large segments of the population in the hope that herd immunity would be achieved relatively quickly.
However, the near-term consequences of doing this can be severe, and we still do not know the longer-term implications of such a strategy. Rather, most countries have opted to control the spread through social distancing which encompasses varying degrees of isolation and contact tracing (i.e., identifying and isolating those who have been in contact with people who are later found to have the disease).
The ability to test large numbers of people is a key part of this strategy. Social distancing is typically complemented with preventative measures such as mask-wearing, hand-washing and extreme cleaning. Some countries that acted early and imposed more restrictive measures have had better results. Unfortunately, countries that were slower to act or imposed less restrictive measures have had disastrous results.
Efforts to find a vaccine have been intense and considerable progress has been made, but it is unlikely that an effective vaccine will be widely available in the short-term. So we will likely be social distancing and wearing masks for a while.