The Novel Coronavirus of 2019 (COVID-19) has caused morbidity and mortality in various parts of the world over a short period of time. This post will look at what is known of this mutated virus as of right now.
COVID-19 consists of a single-stranded RNA genome that ranges from 26 to 32 kilobases in length (Liu et al., 2020). It belongs to the subgenus of betaviruses and has been compared to both the Severe Acute Respiratory Distress (SARS) coronavirus (also a betacoronavirus) and the Middle-Eastern Respiratory Distress (MERS) coronavirus. Although compared to these viruses, COVID-19 has been shown to be sufficiently divergent in genome sequence to be considered a new human-infecting betacoronavirus (Liu et al., 2020). It is newly discovered that COVID-19 is actually more similar to two bat-derived coronavirus strains (bat-SL-CoVZC45 and bat-SL-CoVZXC21) than to viruses that caused SARS outbreak of 2003 (Liu et al., 2020).
COVID-19 causes diseases by attaching to the ACE2 receptors in the lungs (European Centre for Disease Prevention and Control, 2020). This is the same mechanism as SARS but not MERS, which attached to the CD26 cells in the human body (European Centre for Disease Prevention and Control, 2020). The COVID-19 has a circular shell that is peppered with spike-shaped proteins, which helps it attach to the host’s ACE2 receptors (Woodward, 2020). It is worth noting that COVID-29 has been shown to have a 10x higher affinity for ACE2 receptors than SARS, which contributes to it being more contagious and allows it to more easily jump from person to person (Woodward, 2020). It has taken only 8 weeks to infect more than 75 000 people where SARS only affected 8 000 in 18 months (Woodward, 2020).
COVID-19 is a zoonotic virus and although it is thought to use bats as a reservoir, there is profound evidence to show that bats could not have been the host that transferred the virus to humans. There are four main reasons why this is so:
1. The COVID-19 outbreak was reported in late December 2019. During this period most bats would be in hibernation.
2. No bats were sold at the Huanan seafood market where the outbreak is said to have begun.
3. The sequence identity between COVID-19 and bat-SL-CoVZC45 and bat-SL-CoVZXC21 was less than 90%, therefore there is a relatively long branch difference between them. It is thus shown that they are not direct ancestors of the COVID-19 virus and that there must be mutations occurring inside another host that allows It to become human-infecting.
4. In both SARS-CoV and MERS-CoV, bats acted as the natural reservoir but had another animal acting as the intermediate hosts (masked palm civet and dromedary camel respectively) and humans as the terminal hosts. (Liu et al., 2020)
The most likely intermediary host for the 2019 coronavirus is thought to be pigs, civets, or (most likely) pangolins (Liu et al., 2020).
The mechanisms of transmission of COVID-19 between people have been widely debated. It has been settled that the main routes of transmission are through sneezing/coughing, contact with secretions, contact with infected surfaces, and through the faecal-oral route (Heyman and Shindo, 2020). A scary knowledge about the period of infectiousness has recently been discovered. It was found that some patients can still carry the virus and be infectious after recovery (Heyman and Shindo, 2020). This was discovered as patients need to test negative twice in order to be found negative but they are allowed to go home between the testings, leading to them infecting other people (Heyman and Shindo, 2020). There was a debate around this as some people claim that the tests were false negatives and they were never cured while some believe that the viral load decreased so that only traces of the virus remained and therefore could begin proliferating once again after the patient was released. There is no resolve to this debate as of yet.
COVID-19 replication occurs in the upper respiratory tract, where ACE2 receptors are abundant (Heymann and Shindo, 2020). In this stage the infected person has less abrupt onset of symptoms (similar to conventional human coronaviruses that cause common colds) (Heymann and Shindo, 2020). The difference in the COVID-19 outbreak is that the infected individuals produce large quantities of the virus in their upper respiratory tract during their prodrome period where they are asymptomatic and thus are highly infectious without an idea that they are ill (Worldometers.info, 2020). The prodrome period of COVID-19 is between 2 to 14 days (Worldometers.info, 2020) but a recent case only presented 27 days after being infected and was still infectious in this period, which is high cause for concern (Malaysiakini, 2020). This infectious prodrome period is very different to SARS, which was not only much shorter (7 days) but SARS was not contagious until the infected person became seriously ill (Woodward, 2020).
COVID-19 also has an affinity for cells in the lower respiratory tract and can replicate in this area of the body as well (Heyman and Shindo, 2020). During the replication, the lungs are damaged and show exudates, which appear with a ground glass appearance on radiology (European Centre For Disease Prevention and Control, 2020). The damage is seen to the lungs even in patients that did not develop a full-blown pneumonia (Heyman and Shindo, 2020).
There are three patterns seen in the clinical course of infection with COVID-19.
1. Mild illness with upper respiratory tract symptoms (80.9% cases): these patients present with fever, dry cough, fatigue and a sore throat.
2. Non life-threatening pneumonia (13.8% cases): these patients have shortness of breath and are very ill but not in danger of dying.
3. Severe pneumonia with acute respiratory distress syndrome (ARDS) (4.7% cases): these patients begin with mild symptoms for 7 to 8 days then progress to rapid deterioration and ARDS, requiring advanced life support. They go into respiratory failure, septic shock and multi-organ failure (Worldometers.info, 2020).
The individuals with a higher risk of going into the pattern of severe pneumonia and ARDS are those with cardiovascular disease, diabetes, chronic respiratory disease and hypertension (Worldometers.info, 2020).
The mortality rate and R0 have been hard to predict in this outbreak and many studies have thought the estimates to be too low. The case fatility rate estimate was only thought to be 1 to 2% in COVID-19 (compared to 10% in SARS and 35% in MERS) (European Centre For Disease Prevention and Control, 2020). The actual case fatality rate cannot be properly proven until serological tests and serosurverys are available (European Centre For Disease Prevention and Control, 2020). The numbers also appear especially low when put into the context that it took 1 month for 5 people to die from SARS while in that time 213 died from COVID-19 (Woodward, 2020). The mortality rate of COVID-19 is specific for different areas. In Wuhan, it is said to be 4.9% (also deceivingly low for the number of deaths), 3.1% in Hubei province, and 2.1% nationwide (increasing as the virus spreads) (European Centre For Disease Prevention and Control, 2020). The R0 was estimated at 1.4 to 2.5 (WHO.int, 2020) but Chinese researchers in Hong Kong believe the more likely R0 to be 3 to 5 (Woodward, 2020).
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Reference
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Heymann, D. and Shindo, N. (2020). COVID-19: what is next for public health?. The Lancet, 395(10224), pp.542-545.
Jr., B. (2020). Early trial results for potential coronavirus treatments expected in 3 weeks, WHO says. [online] CNBC. Available at: https://www.cnbc.com/2020/02/20/early-trial-results-for-potential-coronavirus-treatments-expected-in-three-weeks-who-says.html [Accessed 22 Feb. 2020].
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Malaysiakini. (2020). Coronavirus incubation could be as long as 27 days, Chinese provincial gov't says. [online] Available at: https://www.malaysiakini.com/news/511844 [Accessed 22 Feb. 2020].
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Who.int. (2020). Statement on the meeting of the International Health Regulations (2005) Emergency Committee regarding the outbreak of novel coronavirus 2019 (n-CoV) on 23 January 2020. [online] Available at: https://www.who.int/news-room/detail/23-01-2020-statement-on-the-meeting-of-the-international-health-regulations-(2005)-emergency-committee-regarding-the-outbreak-of-novel-coronavirus-(2019-ncov) [Accessed 22 Feb. 2020].
Woodward, A. (2020). The new coronavirus has killed nearly 3 times as many people in 8 weeks as SARS did in 8 months. Here's how the 2 outbreaks compare.. [online] Jinolabs.com. Available at: https://www.jinolabs.com/article/158223960425/the-new-coronavirus-has-killed-nearly-3-times-as-many [Accessed 22 Feb. 2020].
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