Coronavirus: SARS, MERS, and COVID-19
In the past few months, the world has been fighting against COVID-19, which has caused the worst pandemic in the 21st century. As mentioned in our previous blog post, the COVID-19 disease is caused by the novel SARS-CoV-2 virus, which belongs to a class of viruses known as coronaviruses. Though this class of viruses commonly infect birds, it also affects mammals. So far, seven coronaviruses have been identified as the cause of respiratory tract infections of varying degrees in humans . Mild to moderate illnesses caused by human CoVs (HCoVs) resemble the common cold or manifest as bronchitis and pneumonia, whereas the more severe illnesses are SARS, MERS, and COVID-19 [1, 2]. In this blog post, we will be looking at these three diseases and showing how they compare to the common cold and influenza. Before deepening our discussion into these illnesses, let’s begin by talking about the structure of the infectious agents that cause them.
Structure: Coronaviruses vs Influenza Virus and Rhinovirus
Coronaviruses are enveloped RNA viruses distinguished by their spherical shape and spike proteins adorning their surface, giving the crown-like appearance. Its envelope consists of a lipid bilayer with three proteins — membrane proteins (M), envelope proteins (E), and spike (S) proteins — associated with it. The M and E proteins are involved in the assembly of the virus, whereas the S protein mediates the virus’ entry into the host cells .
Similar to coronaviruses, influenza viruses are characterized by having a spherical shape and proteins embedded into their envelope made of lipid bilayer. They contain three types of proteins anchored into its envelope: two spike glycoproteins — hemagglutinin (HA) and neuraminidase (NA), that enable binding to the host cell — and a proton channel — M2. Influenza viruses also contain RNA as their genetic material [4,5].
Rhinovirus are RNA viruses distinguished by their icosahedral structure. This type of virus does not contain an envelope, instead, they have a capsid made out of four proteins: VP1, VP2, VP3, and VP4. The first three form the major part of the capsid, whereas VP4 is embedded, anchoring the genomic material to the capsid .
Coronavirus Diseases vs Influenza and The Common Cold
Influenza and common cold are respiratory illnesses, like SARS, MERS, and COVID-19, but caused by different viruses. Influenza is caused by influenza virus, whereas the common cold is caused by rhinovirus. They all have similar symptoms, thus it can be difficult to differentiate between them based on symptoms only. The flu is worse than the common cold, but less severe than SARS, MERS, and COVID-19. The flu symptoms can include fever or chills, cough, sore throat, runny or stuffy nose, muscle/body aches, headaches, and fatigue; whereas the most common symptom of a cold is runny or stuffy nose .
Severe acute respiratory syndrome (SARS) is caused by SARS-associated coronavirus, SARS-CoV. The first reported case of SARS was in Guangdong, China, in November 2002 — there haven’t been any known reported cases of SARS since 2004. Even though the SARS outbreak was contained before becoming a global pandemic like COVID-19, the disease spread to 26 countries affecting 8,098 people worldwide from which 774 died [8,9].
Clinical Features, Symptoms, Spread, and Treatment
According to the World Health Organization (WHO), the incubation period — time period from infection to symptom onset — of SARS-CoV was 2–10 days and the global case fatality ratio or case fatality risk (CFR) was approx. 9.6%. As reference, CFR for influenza pandemics in 1918, 1968, and H5N1 pandemic of 2008 were 2.5%, 0.1%, and between 14–33%, respectively . CFR is a measure of the severity of the disease and can be calculated by dividing the proportion of deaths due to a condition by the total number of cases presenting the condition.
SARS affected mostly adults whereas infected children showed a milder illness. Those infected by SARS-CoV usually presented high fever with temperature greater than 100.4 F accompanied by chills, headache, body aches, feeling of discomfort, dry cough (after 2–7 days), and/or diarrhea (10–20 % of cases). Most patients developed pneumonia and 10–20% of cases required mechanical ventilation. The method of transmission is similar to SARS-CoV-2. The virus can be spread through direct (person-to-person via droplet spread) and indirect (infected surfaces and airborne) contact. To date, there is no proven treatment for SARS disease, though supportive measures are recommended [8, 9].
Since the beginning, it was suspected that animals were the immediate source of the disease. The primary suspects were, at first, infected civet cats being sold in live-animal markets. However, recent studies suggest that bats are the natural reservoir — habitat where an infectious pathogen lives, grows, and multiplies — of SARS-CoV, and infected civets were only intermediaries. This means that SARS is considered a zoonosis which is any disease that is spread from vertebrate animals to humans [9, 11, 12].
Middle east respiratory syndrome (MERS) is caused by MERS coronavirus, MERS-CoV. The first reported case of MERS occurred in Jordan in 2012. Though MERS has spread to 27 countries, 80% of the cases have occurred in Saudi Arabia. Since 2012 and as of January 2020, there have been 2,519 laboratory confirmed global cases with 866 associated deaths. From this total, 2,121 cases and 788 deaths were reported by Saudi Arabia. Thus far, those who contracted MERS got infected through contact with infected camels or infected people in Saudi Arabia. Cases reported outside the Middle East usually got infected while travelling there [13, 14, 15].
Clinical Features, Symptoms, Spread, and Treatment
According to the CDC and the WHO, the average incubation period is about 5 to 6 days after exposure, but it can vary between 2 to 14 days. The CFR for MERS is approx. 34.3 % global and 37.1% in Saudi Arabia, both much higher than SARS. Even though MERS can affect anyone, those at higher risk of acquiring the disease are in the age group of 50–59 years. It appears that the virus affects more severely those with underlying medical conditions like diabetes, cancer, or chronic diseases (lung, heart, or kidney). The most common symptoms of MERS are fever, cough, and shortness of breath. These could be accompanied by pneumonia, diarrhea, nausea, and vomiting. In some cases, infected people had cold-like or no symptoms at all. The method of transmission can be non-human to human, through dromedary camels, and human-to-human. The later one is rare, thus close contact with an infected person is required. Usually person-to-person transmission occurs among family members or by providing care to an infected patient without the required protection. Similar to SARS, there is no treatment for MERS and patients receive care to relieve symptoms and support for vital organ function [13,14, 15].
Just like SARS, MERS is considered a zoonosis given that the disease has been transmitted from infected dromedary camels to humans. Just like SARS-CoV, studies suggest that MERS-CoV originated from bats [16, 12].
SARS-CoV-2 virus causes coronavirus disease 2019 (COVID-19). The first reported case of COVID-19 was in December 2019 in Wuhan, the capital city of Hubei province in China. According to the WHO, as of April 23, COVID-19 has spread to 213 countries, with 2,549,632 (yes, almost 3 million!) positive cases and 175,825 deaths have been confirmed. To get the most updated number of cases by region and by country visit the maps and visualizations posted by the WHO here [17, 18].
Clinical Features, Symptoms, Spread, and Treatment
The CDC estimates, based on MERS-CoV, that the incubation period for SARS-CoV-2 is between 2–14 days. An approximate CFR of 6.89% was calculated based on reported confirmed cases and deaths as of April 15. Similar to SARS and MERS, infected individuals usually present fever, cough, and shortness of breath. The method of transmission is similar to SARS, the virus spreads through direct and indirect contact. People with COVID-19 can be asymptomatic or present mild to severe illness. Usually, adults 65 years and older are affected with the most severe form of the disease [ 17 ]. Unfortunately, there is no specific treatment for COVID-19 yet, but potential vaccines are under development.
When the COVID-19 started to spread in Wuhan, many of the patients were linked to a live animal market suggesting animal-to-person spread. Therefore, it is suspected that similarly to SARS-CoV and MERS-CoV, SARS-CoV-2 emerged from animal reservoirs .
To find a more thorough explanation of what it is like to be infected with SARS-CoV-2, its transmission methods, and suggested guidelines for prevention visit our previous blog post, The COVID-19 Pandemic of 2020. If you’re a visual person and prefer videos and visuals to understand things better, we recommend the video, How Coronavirus Attacks the Body, published by The New York Times on April 6th 2020, showing how SARS-CoV-2 invades the lungs.
Citations and links:
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Li, Fang. “Structure, Function, and Evolution of Coronavirus Spike Proteins.” Annual review of virology vol. 3,1 (2016): 237–261. doi:10.1146/annurev-virology-110615–042301
 Vincent, Racaniello. “Structure of Influenza Virus.” Virology Blog. 30 April 2009. Retrieved from: https://www.virology.ws/2009/04/30/structure-of-influenza-virus/
 World Health Organization. “Influenza (Seasonal)”. 6 November 2008. Retrieved from: https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal)
 Jacobs, Samantha E et al. “Human rhinoviruses.” Clinical microbiology reviews vol. 26,1 (2013): 135–62. doi:10.1128/CMR.00077–12
 United States. Dept. of Health and Human Services. Centers for Disease Control and Prevention. “Cold Versus Flu”. 30 December 2019. Retrieved from: https://www.cdc.gov/flu/symptoms/coldflu.htm
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 World Health Organization. Middle East respiratory syndrome coronavirus (MERS-CoV). 2020. Web. 14 April 2020. Retrieved from https://www.who.int/emergencies/mers-cov/en/
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