Characteristics of COVID-19

■ "Characteristics of COVID-19" / Chu Chul-hyun (Doctor of Medicine)

 At the end of last year, a new family of coronaviruses first appeared in the world as a member of the Hwangju (Wuhan) land protected by Guan Yu in the Three Kingdoms, hitting the world in just three months with unprecedented high propagation.  Of course, the clinical picture of a viral infection is determined by the nature of the virus itself.  Many alumni are familiar with it, but we have compiled a basic knowledge of the virus to make it as easy as possible.  For more detailed mechanical and molecular biological knowledge, please refer to the references at the end.  Interpreting the references in particular, it is important to keep in mind that epidemiological data from Wuhan, China, include redundant exposures due to the rapid spread of viruses and exceptional cases (outliers) due to high levels of infection.

 The new Corona mutant, which was called various names such as Wuhan Corona and New Corona, was officially named Corona 19 (COVID-19) for the accuracy of keyword search.  This is a subtype strain of the corona virus, one of the common causes of seasonal colds that we learned from undergraduates.  Subtypes that primarily cause pandemic flu in humans include corona 229E and OC43.  In the past, in addition to these common coronas, there may have been intermittent emergence of mutant coronas, but the lack of inspection techniques could not confirm.  However, the development of molecular biological virus testing techniques, particularly reverse transcriptase polymerase chain reaction (RT-PCR) techniques, facilitated the immediate identification of new RNA viruses.  As a result, several corona variants have been reported since the 2000s.  Corona viruses that have been identified to date include SARS-CoV (2003), HCoV NL63 (2004), HKU1 (2005), MERS-CoV (2012), and COVID-19 (SARS-CoV-2).  There is.  Of these, SARS and MERS were the most notorious.  The term COVID-19 was used to unify the name of this periodic corona virus.  Being named like COVID-19 means that the outbreak will happen periodically in the future.  If a new corona comes out in 2022, it's called COVID-22.

 COVID-19, a member of the corona virus, belongs to Orthocoronavirinae, which is characterized by muco-affinity, and is the largest RNA virus that infects the body.  Positive single-stranded RNA genes are 27-34 kbase in size and 120-140 nm in physical size.  It has a phospholipid shell that originates in the host's cell membrane.  This phospholipid bilayer is the biggest feature of respiratory infections, which are spread through mutations.  This is because the phospholipid membrane increases the tolerance for mutations in the foreign proteins of the virus and increases the probability of avoiding immunity.  In the case of the corona virus, the phospholipid shell is densely packed with sticky spike proteins that have an affinity for respiratory mucus.  When observed under an electron microscope, the shape resembles that seen from the top of the crown, and it is named Corona Virus, and these spikes give rise to respiratory mucosal evolution.

 Mutations of the virus occur randomly and have a certain probability per number of multiplications.  Therefore, in order for a favorable variant to be selected and emerge as a new virus through mutation, a host must be fully developed.  In the case of corona, the bat is the host.  As you know, because bats are mammals, the barrier of viral cross-border between humans and heterogeneity is very low.  Bats are high in metabolism because they are both mammals and flying beasts.  This characteristic allows the corona virus to multiply in the long term without activating immunity, resulting in various mutant viruses.  When a new mutavirus from a bat crosses a species and infects humans, humans encounter a whole new structure of antigen that they have never encountered before.  In other words, the collective immunity is zero, and the risk of global pandemic increases.  However, because humans rarely come into direct contact with bats, they need an intermediate medium (amplification host) to link the infection from bat to human.  Cats in SARS and camels in MERS played this role. COVID-19 is still uncertain about this intermediate.  There is also a hypothesis that it is a snake or a fish, but it is less likely as an intermediate mediator because it is far from mammals.  Worst of all, like the epidemic of Ebola's subtype, the German Malberge virus, it is possible that he came to a human right away while experimenting with bats in a biotechnology laboratory in Wuhan.

 In the case of COVID-19, the propagation rate is still in progress, so accurate epidemiological statistics are hard to come out, but the data of Korea, which is being tested quickly and extensively, shows that the mortality rate will be about 1% or less, and 0.5% in good control cases.  Is expected to be  It's a bit higher than the mortality rate of the flu.  The mortality rate was 7% for SARS and 34% for MERS.  The inverse relationship between infection rate and lethality is due to the nature of viral infections, which only allow direct transmission between hosts.  In case of MERS, the mortality rate is very high and it is difficult to spread because it limits the daily life of the infected person.  However, in this case of COVID-19, many people pass like a common cold, so it has good propagation characteristics.  This is because, even in the eyes of the virus, the host must maintain the daily life as possible to benefit his genes.

 However, these mechanical properties alone do not fully account for the high infectivity of COVID-19.  All respiratory viruses must have mucus affinity.  If you don't adsorb to mucus, you're less likely to get into respiratory epithelial cells.  This mucus affinity is gained by the spike portion of the corona mentioned earlier.  The new corona should be essentially a variation of this spike.  This area is recognized by the body's immune cells, so if there is no mutation, it is quickly removed by immunization.  In the case of COVID-19, the spikes were not only mutated, but mucus affinity was about 50 times higher than in previous coronas.  In other words, the mucoadhesion is improved and the worst mutations can be avoided.  Due to this molecular biological property, even when a small amount of virus comes into contact with mucus, the probability of infection of the respiratory epithelium increases, and the proliferation is active from the beginning.  In other words, unlike previous viruses, which have reached a level that can be infected by others after a certain clinical course, it is more likely to spread to others early in the infection.

 Since COVID-19 also begins with respiratory epithelial infections, there are no distinguishing clinical features that distinguish it from other colds, especially influenza infections.  However, there is a high likelihood that there will be a difference in the progress of the clinical picture (the order in which symptoms develop over time) due to molecular biological characteristics.  Influenza flu often begins with nasal infections through aerosols due to the size and receptor characteristics of the virus.  COVID-19, on the other hand, is often the beginning of oral infections through splashes due to its large size and spike characteristics.  The virus that enters the oral cavity infects the respiratory epithelial cells of the larynx and starts its first proliferation.  Viruses with low mucus affinity usually enter the oral cavity with food or water.  But COVID-19 has a strong mucus affinity, which infects respiratory epithelial cells without being washed off.  One of the clinical features of COVID-19 is that it has less runny nose and less dry cough and abdominal pain.  Of course, COVID-19 does not mean that a runny nose does not come out unconditionally.  The infection may have begun through an aerosol or the virus may pass from the throat to the nasal cavity as the infection progresses.  A runny nose can not rule out COVID-19, but if the first symptom of a cold is a sore throat or dry cough, there is ample doubt.  Accompanied fever has individual differences, depending on the state of immunity.  However, due to the nature of the fast-growing virus, early fever is often accompanied.

 Confirmation is based on RT-PCR, like most viruses these days.  This technique is fast and accurate, but it is important to take into account that negative judgments take time because the gene fragments can remain for a period of time even if they are too sensitive and have no virus growth.  In other words, not all RT-PCR positives are possible.  Clinically, confirming the imaging modality is an important determinant of prognosis and has high diagnostic value.

 Finally, if you look at the nature of the spread of infection, the virus particles in the aerosol do not easily rise above the required level because of the large particle size of the virus.  In other words, the mask is not important in outdoor or well-ventilated places.  However, if you have someone who continues to cough indoors, such as in an inpatient room, you should assume that the concentration of aerosol quickly rises to dangerous levels.  In many cases, only the mask is considered when the infection spreads through the droplets, but in the case of corona, contact with the secretions through the hands is more important.  In hospitals, the handle of toilets is the main source of infection.  It's no use if you have to wash your hands and touch a dirty handle.

 Therefore, the hospital's bathroom is better to push without a handle and if there is structurally possible, it is better to have no door at all.  In this case of COVID-19, which is characterized by high infectious droplets, it is dangerous to talk to each other, eat or drink.  In addition to the habit of not touching your face with your hands, it is also important to wash your hands often. Since it is a virus with a phospholipid shell, it is effective even with a soap as a surfactant even without using a special hand sanitizer.  The virus is said to prevent, not cure.  In particular, COVID-19 concludes by highlighting the importance of each individual's lifestyle in preventing infections because of the high probability of transmission by close contact.  Thank you.

 references

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 (Provided by Prof. Chul-Hyun Ju, Department of Microbiology, Ulsan Medical College)