SARS-CoV-2 vs. SARS vs. MERS: A comparison of three coronaviruses

By Yidan Gao

COVID-19 has become a big name in the news. No one had ever imagined that in 2020, our lifestyles would be significantly changed due to a viral outbreak. This is, however, not the first time when mankind was challenged by a virus, more specifically, this viral family. The coronavirus family has posed a health threat to the world three times from 2002 to 2020. These lethal varieties are known to cause SARS, MERS and COVID-19. This article addresses some comparisons among the three viruses of the coronavirus family and the lessons we learned from the previous outbreaks.

Before getting started, it is important to distinguish between the illness itself and the disease-causing agent. Following a chronological order, the first illness, SARS (referring to severe acute respiratory syndrome) was first reported in southern China in 2002. Patients manifested flu-like symptoms, which includes dry cough, fever and shortness of breath after a week of infection. It was caused by a SARS-associated coronavirus (SARS-CoV). It affected a worldwide population of 8,098 as of December, 2003. Fortunately, among the infected population, the US constituted a very small amount, in which only 8 people tested positive for SARS-CoV infection. The death rate for SARS was estimated to be 9.6%.

After 10 years, another coronavirus came along as MERS-CoV. The first case was identified from a sputum sample of a Saudi Arabian in September 2012. The disease is named MERS, which is short for Middle East Respiratory Syndrome, with very similar symptoms to SARS. MERS cases concentrated in Saudi Arabia and Korea due to a travel-related outbreak, with only 2 cases found in the US. The total infected population was about 4-fold less compared to that of SARS. Because of strict contact tracing and patient isolation, the Korean government declared an end to the MERS epidemic in 2015. The death rate for MERS was found to be 34.3%.

It was 4 years later when the first case of COVID-19 was reported in Wuhan, China. COVID-19 is short for coronavirus disease 2019. With the research effort from the previous outbreaks, scientists quickly identified the viral agent as SARS-CoV-2 since it is genetically more related to SARS-CoV than MERS-CoV. The death rate for COVID-19 is currently estimated to be between 1.38% and 3.4%.

Although these viruses arose asynchronously, they do have some similarities. Sharing the same family, these three viruses all contain spike proteins on their surfaces, which give them the name of coronavirus or a "crown-like" virus. The family as a whole enjoy hopping, meaning they are transferable from an animal host to human. SARS-CoV was identified to hop from a horseshoe bat onto civets and other animals in wet markets and eventually onto humans. MERS-CoV was thought to come from a bat, who then infected camels as an intermediate carrier. Patients in Saudi Arabia were in contact with camels and started the spread among humans. SARS-CoV-2 is also suspected to spread from an animal reservoir, most likely to be bats.

Despite the similarities defined previously, there are many differences among these viruses, potentially driven by natural selection within the animal host. One of the most notable differences is the epidemic potential, setting apart SARS-CoV-2 from the other two viruses. Epidemic potential refers to the ability of the virus to spread itself within the population. Although it is less virulent than SARS-CoV and has a much lower mortality rate compared to MERS, SARS-CoV-2 has the greatest epidemic potential due to asymptomatic infections. The transmissibility of the virus starts before the onset of the symptoms. Some patients never even develop a noticeable symptom after infection, which can be ascribed to differences in their immune system. A survey performed with passengers from the Diamond Princess cruise ship has shown that among the infected individuals, half of them showed no symptoms. Compared to SARS-CoV-2, infected individuals of MERS-CoV and SARS-CoV have fairly high transmissibility a number of days after the onset of the symptoms. This characteristic of SARS-CoV-2 made it very difficult for health care workers to track or to isolate infected individuals. With no viable vaccine at this point, the only preventative measure to implement is behavioral change, including strict contact tracing and social distancing.

Some other defining characteristics of SARS-CoV-2 include the location of infection and viral persistence. SARS-CoV-2 has a different tropism when compared to SARS-CoV and MERS-CoV. The word tropism refers to the ability of the virus to infect different cell types and to produce more viral copies within them. For SARS-CoV and MERS-CoV, their tropism concentrated more in the lower airway (trachea and lungs) and less in the upper airways (nose and part of the pharynx). In SARS-CoV-2 patients, a relatively high viral load (6.8× 10^5 copies per swab) was discovered in the upper respiratory tract in the first 5 days of infection. In severe COVID-19 patients, the virus is found to comparatively persist longer in the lower respiratory tract.

The two previous viral outbreaks from recent years brought us not only experiences, but also insights for medical inventions and vaccination. With the work laid down by previous scientists, researchers took less than one month to identify this current virus as a relative to SARS-CoV and to develop a diagnostic test. It took us four years to develop a diagnostic test for HIV because it was a novel virus. In light of vaccine development, one of the studies demonstrated that a patient's serum with a SARS-CoV component could potentially prevent the entry of SARS-CoV-2. In other words, patients that were once infected by SARS-CoV might provide protection against SARS-CoV-2. In addition, a genetically engineered ACE2 protein was found to reduce lung injury arising from SARS-CoV. Another study showed that antibodies that target the MERS-CoV component could inhibit the entry of the virus. Overall, this previous research provided us with research targets in development of vaccines or medication against the current outbreak.

In conclusion, the three viral outbreaks have provided us wisdom, which fuels vaccine developments. Understanding SARS-CoV-2 is just the first step in providing a competent treatment. But social distancing and contact tracing have proved to be effective as the cases of MERS-CoV and SARS-CoV infection decreased years before. Applications of these two measures have made a significant difference up until now.

Results from ongoing research and the current understanding of COVID-19 are constantly evolving. This post contains information that was last updated on September 17, 2020.

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