What is COVID-19?

Last updated on December 30th 2020

Coronavirus is a large family of viruses that can cause illness in humans or illness in animals. Human coronaviruses are quite common and are usually associated with cold-like symptoms. Some rare cases of animal coronaviruses can infect people and be transmitted from one person to another (1).

  1. The SARS-CoV-2 virus and its origins
  2. How is it different from the SARS-CoV-1 virus responsible for the outbreak in 2003?
  3. How is it different from the flu?
  4. How can I get infected with COVID-19?
  5. Who can transmit COVID-19?
  6. Symptoms of COVID-19 and disease evolution
  7. What are some signs and symptoms of a severe infection?
  8. Who’s more at risk?
  9. Are you immunized from COVID-19 once you are recovered?

The SARS-CoV-2 virus and its origins

COVID-19 is a disease associated with infection by the SARS-CoV-2 virus, a virus that had not been previously identified in humans. It is now the seventh coronavirus known to infect humans. The first case of COVID-19 was identified in Wuhan in the Hubei province of China in December 2019. Genome studies suggest that the SARS-CoV-2 virus comes from an animal source, like a lot of other coronaviruses. Similarities between this virus and bat and pangolin viruses suggests that these two animals could have served as reservoir hosts for SARS-CoV-2 (2).

University of Toronto professor Dr. Nick Woolridge created the following video to help others visualize what the SARS-CoV-2 virus particle actually looks like:

Nick Woolridge (https://www.nickwoolridge.xyz/projects/sars-cov-2)

How is it different from the SARS-CoV-1 virus responsible for the outbreak in 2003?

SARS or Severe Acute Respiratory Syndrome is a respiratory illness that is associated with infection by a coronavirus. There are two major coronavirus that have been identified to be the cause of SARS in the past 20 years, the SARS-CoV-1 and SARS-CoV-2 viruses. The SARS-CoV-1 virus was the cause of the worldwide outbreak in 2003, whereas the SARS-CoV-2 virus is associated with the COVID-19 pandemic of 2019-2020. Both viruses seem to have originated from China. The first case of SARS-CoV-1 was reported in the Guangdong province of Southern China whereas the first case of COVID-19 was reported in the Hubei province of Central China. Both viruses seem to have originated from an animal, and it is suggested that the SARS-CoV-1 virus evolved from cave-dwelling horseshoe bats through masked palm civets before infecting humans (21).  

These two coronaviruses share a very similar range of symptoms, from dry cough to severe acute respiratory distress. While the SARS-CoV-1 virus infection was reported to have killed ~1/10 people infected, case-fatality ratio for SARS-CoV-2 tends to vary globally, with some regions reporting values over 15% (e.g. Chiapas, Mexico, Yemen) while other regions having reported values below 1% (e.g. Singapore) (4). While such indices may provide general insights into the behaviour of SARS CoV-2, considerable regional differences in the applied methodologies used to evaluate case-fatality ratios have been observed by the WHO, thus making regional comparisons difficult (5). It must be noted, however, that the SARS-CoV-2 virus seems to be propagating more extensively. In fact, there were only 8,098 reported cases of SARD-CoV-1 infection worldwide during the 2003 pandemic, whereas there are over 50 million confirmed cases of COVID-19 around the world as of November 9th, 2020 (6,7).

How is it different from the flu?

Influenza and COVID-19 are diseases caused by viral infections of the respiratory tract that can vary from asymptomatic or mild disease to severe pneumonia, acute respiratory distress and possibly death (see Symptoms of COVID-19 and disease evolution for more details). Since they can be transmitted in a similar fashion, a lot of the prevention strategies recommended by Public Health are appropriate in both cases. While these diseases share many similarities, we have listed some of the notable differences between both illnesses, as illustrated by data from the CDC and the World Health Organization (WHO) (7, 8, 9):

  • COVID-19 appears to proliferate more easily than influenza
  • Time between infection and apparition of symptoms is typically shorter in Influenza
  • People infected with COVID-19 may be contagious for a longer period of time compared to influenza
  • Children are at a lower risk of health complications when infected with COVID-19 compared to when infected with the flu
    • It must be noted, however, that Multisystem Inflammatory Syndrome in Children is a possible but rare complication affecting school-aged children affected with COVID-19
  • While annually there are an estimated 290000-650000 influenza-related respiratory deaths, there have already been over 1 million deaths due to COVID-19 in 2020

Although these differences are very interesting to note, one important question remains to be answered: how can you tell if you have COVID-19 or the flu? As mentioned before, an individual infected with either of these two viruses can present with very similar symptoms. So how can we differentiate them clinically? Fortunately, the following chart, created by the Alberta Health Services, helps us compare the symptoms of COVID-19 from those of the flu and the common cold (see Symptoms of COVID-19 and disease evolution for more details on the symptoms of COVID-19) (10).

Alberta Health Services


How can I get infected with COVID-19?

After coming in contact with COVID-19, the virus tends to make its way to the nose, throat and lungs and causes an infection in those areas. The NY times offers a brief summary on how the SARS-CoV-2 virus infects our cells and replicates itself.

COVID-19 is mostly spread from person to person (11). There have been some rare case reports of pets infected with COVID-19, but present data shows that the risk of pets transmitting COVID-19 to people is pretty low. However, there is a need for more research in that area, and it is recommended that the same protective measures applied between members of the same family be applied to household pets as well (12, 13).

COVID-19 is most commonly known to spread from an infected person to someone else through :

  • Respiratory droplets that spread when you cough, sneeze or talk within 6 feet of another person (14)
    • Droplet transmission is reported to be the most prevalent mode of transmission
  • Airborne transmission via droplets that persist in the air over a longer period of time (14)
  • Contact with surfaces with harboring respiratory droplets containing the virus (15)
    • Personal items such as cellphones, tablets, glasses, and remote controls and surfaces like doorknobs, toilet seats, tables, window ledges, and floors were shown to be possible sources of COVID-19 contamination (16,17).
    • It is important to take note that cleaning counters and surfaces with common household assists with control of the virus (18,19). The Government of Canada has issued a list of hard-surfaces disinfectant and hand sanitizers that are safe and likely to be effective in eliminating COVID-19 (20).
  • While the potential risk of oral-fecal transmission of SARS-CoV-2 has been considered, no transmission via feces or urine has been found as of July 2020 (21,22).
  • Some studies have demonstrated blood samples that test positive for SARS-CoV-2, however the WHO has stated that transmission of the virus via the blood is low (21).

The length of time during which COVID-19 can survive on surfaces depends on the material. The following chart illustrates how long the virus survives on different materials, according to the WHO.

In summary, while the SARS-Cov-2 virus can be transmitted in various ways, it is important to remember that transmission via respiratory droplets when near other persons remains the most prevalent mode of transmission (14).

Who can transmit COVID-19?

Individuals tested positive for COVID-19 can be divided in three different categories : asymptomatic, pre-symptomatic, and symptomatic carriers. Individuals tested positive for COVID-19 can be divided in three different categories : asymptomatic, pre-symptomatic, and symptomatic carriers. By selecting any of the icons on the following chart, you can have access to additional information about that specific type of carrier. Individuals tested positive for COVID-19 can be divided in three different categories : asymptomatic, pre-symptomatic, and symptomatic carriers.

Asymptomatic: There is some evidence that people who are positively tested for COVID-19 can present without any symptoms for the entire course of their infection. In a recent review reported in June 2020, Byambasuren et al. found the rate of asymptomatic cases to be ~16%. It is also important to consider that asymptomatic persons were found to be 42% less likely to cause transmission when compared to symptomatic persons (23).

Pre-symptomatic: When individuals with COVID-19 is asymptomatic, but will develop symptoms in the following days, they are considered pre-symptomatic. This usually means that the person was recently infected and is still in the incubation period of the disease  (see Symptoms of COVID-19 and disease evolution). There is evidence that shows pre-symptomatic transmission of COVID-19 (24).

Symptomatic: Symptomatic means that an infected individual shows signs or symptoms associated with COVID-19. According to studies, people in this category are the most at risk of transmitting COVID-19. Some preliminary data also suggests that people are generally more contagious at the onset of symptoms or briefly before onset (25).

Symptoms of COVID-19 and disease evolution

After coming in contact with COVID-19, people will initially go through a phase called incubation period. The incubation period or latent phase represents the time between exposure and symptoms. At that time, the virus is busy replicating itself inside its host’s body, and the affected person will develop symptoms after there is enough virus to cause damage. The typical incubation period for COVID-19 is from 1 to 14 days, but the average period is between 5-6 days (26). 

After the incubation period, people tend to present a variety of different symptoms with varying frequencies, including but not limited to (27): 

  • Most frequent
    • Fever
    • Cough
    • Shortness of breath
    • Fatigue
    • Loss of appetite
    • Loss of smell/taste
  • Less frequent
    • Muscle aches
    • Chest pain
    • Diarrhea
    • Vomiting
    • headache/dizziness
  • Rare
    • Confusion
    • Runny nose

People who are more fragile can develop shortness of breath from the start (28). Early studies conducted by the Centre for Disease Control in China found that in 81% of cases, people only experienced mild to moderate symptoms for the entire length of the disease and recovered at home after about 14 days (29)

For the minority that will evolve into a more severe disease, they will typically develop shortness of breath days after onset of symptoms. It has been observed that older patients with an already impaired lung function typically develop this symptom after about 5-7 days, but it can take longer in young and healthy individuals (28). This seems to be a major event in the course of the disease, since those people tend to rapidly deteriorate afterwards. In fact, according to studies in China and the US, most people who have developed a severe pneumonia and who have required hospitalization have been admitted on average after about a week of showing symptoms (15,16). 

In some rare cases, the disease can evolve into severe acute respiratory distress, in which case the person will require mechanical ventilation to breathe and will be admitted to the intensive care unit (ICU). In an epidemiological report of Canadian data in June 2020, 7.9% of persons with COVID-19 were hospitalized, with ~20% of hospitalized patients requiring intensive care, and 4.7% of patients requiring ventilation (30).

It is important to note that even though COVID-19 can quickly develop into a severe disease, as of November 2020, less than 5% of reported cases of COVID-19 in Canada have resulted in death (7). 

What are some signs and symptoms of a severe infection?

If you show any of the symptoms linked to COVID-19 (see Symptoms of COVID-19 and disease evolution) and suspect you may have the illness, the Government of Canada instructs to contact your local public health authority, and isolate for 14 days. Self-assessment tools are also available which, once completed, provide guidelines as to whether getting tested for COVID-19 is necessary. Most people who are infected and present with mild symptoms will be able to recover at home (31).

Make sure to stay in touch with your doctor during the course of your illness. It is recommended to get plenty of rest and to stay hydrated. You should not leave your home, unless there is an emergency or you need medical attention (see When to seek urgent medical attention? for more details). It’s a good idea to first call your healthcare provider before seeking medical care. It is also possible that your local public health authority may have additional guidelines/procedures to follow if you have symptoms and/or are confirmed to have COVID-19. Consulting your local public health authority is also highly recommended.

Who’s more at risk?

If you have any of the following conditions, you are more at risk of severe complications of  COVID-19 (26):

  • Age 60 years or older
  • Heart diseases
  • Lung conditions
  • Kidney disease 
  • High blood pressure
  • Diabetes 
  • Weak immune system
  • Cancer
  • Obesity

Are you immunized from COVID-19 once you are recovered?

There is some research suggesting that COVID-19 specific antibodies formed by the immune system following infection may have a protective role from future infection. However, more research needs to be done before in this area. It is also important to note that immunity varies from one individual to another and the factors that play a role in the risk of re-infection haven’t been established yet. In short, it is unclear at the moment whether recovering from COVID-19 infection can lead to immunity (12).

References

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  2.  Andersen KG, Rambaut A, Lipkin WI, Holmes EC, Garry RF. The proximal origin of SARS-CoV-2. Nature Medicine [Online]. March 17 2020 [cited on May 5 2020]; 26:450-452. Available from: [link]
  3. Cyranoski D. Bat cave solves mystery of deadly SARS virus – and suggests new outbreak could occur. Nature [Online]. December 1 2017 [cited May 5 2020]. Available from: [link]
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