by Matthew Magyar MD, Karam Ramotar PhD and Vincent Deslandes MD PhD FRCPC
May 31, 2024

COVID-19 is a respiratory disease caused by a virus called severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This virus is part of a large family of viruses known as coronaviruses. The coronavirus family includes many different types of viruses, some that cause disease in humans and some that cause disease in animals.  SARS-CoV-2 is closely related to the coronaviruses that cause Severe Acute Respiratory Syndrome (SARS) and Middle Eastern Respiratory Syndrome (MERS).


What are the symptoms of COVID-19, and how does it affect the body?

According to the evidence available today, most people who develop COVID-19 will experience mild symptoms such as cough, fever, low energy, muscle aches, and loss of taste or smell. Less common symptoms include sore throat, runny nose, and nasal congestion. Many people infected by SARS-CoV-2 will not experience any symptoms at all.

More severe symptoms, such as difficulty breathing, can develop in older patients and those with pre-existing medical conditions such as cardiovascular disease, diabetes, lung disease, cancer, chronic kidney disease, and a history of organ or bone marrow transplant. People who are obese or who smoke are also at higher risk for severe disease. Although rare, severe disease may also develop in some younger people without additional risk factors.

People with COVID-19 may develop a type of lung injury called pneumonia that may lead to respiratory failure. These people may need to be hospitalized until their condition improves. People with severe disease can die without medical attention. People with mild symptoms do not require any medical treatment and will recover fully within 7 to 14 days.

COVID-19 is also associated with an increased risk of developing blood clots in people with severe disease. When the clots involve the veins in the legs, they are called deep vein thrombosis. If the clot travels from the leg to the lungs, it can cause a life-threatening condition called a pulmonary embolism.

How do doctors test for coronavirus?

In order for a person to become infected, the virus needs to enter the body and get inside our cells. Once inside a cell, the virus uses the machinery of the cell to make new viruses. Similar to human cells, viruses have their own unique genetic material, which can be found inside an infected cell. Doctors test for COVID-19 by looking for pieces of genetic material that are only found in SARS-CoV-2.

SARS-CoV-2 typically infects cells at the back of the nose (nasopharynx), throat, and lungs. To see if a person has been infected by SARS-CoV-2, a doctor will use a swab to take a sample of cells from the back of the nose or throat (see picture below). The test takes approximately 5 seconds to complete, and while many people find it uncomfortable, it should not be painful. The sample will then be sent to a laboratory that will test for the virus.

NP and Throat Swab

How does the lab test work?

Most labs use a test called polymerase chain reaction (PCR) to look for SARS-CoV-2. The test looks for very specific pieces of viral genetic material called “nucleic acid sequences”. These sequences are part of a gene, a section of genetic material which is like a recipe used to build a specific protein. The test uses smaller pieces of genetic material called primers specially designed to stick to nucleic acid sequences unique to the virus. Primers that stick to a sequence are used to make more genetic material which tells the testing machine that the virus has been found.

How are the results of this test reported?

This kind of test can produce three possible results:

  • Not detected: The virus was not found in the sample. This is considered a negative test result.
  • Detected: The virus was found in the sample. This is considered a positive test result.
  • Invalid – The test could not be completed normally. This result does not mean the virus was not found in the sample. An invalid test should be repeated.

Variants of concern (VOCs)

When a virus multiplies within human cells, small mistakes occur, which cause changes in the genetic code of the virus. By random chance, some of these changes (termed “mutations”) may help the virus spread. When these changes or mutations occur, they need to be watched carefully because they may cause new strains, or “variants”, to become more infectious than the original virus. This concept is like that of natural evolution. There are now several variants of SARS-CoV-2 that can spread faster than the original virus.

Since the start of the pandemic, public health authorities have been monitoring these changes. When a new strain is shown to be more contagious, cause more severe disease, or potentially escape the protection conferred by vaccines, it is termed “variants of concern” (VOC), and patients are followed more closely.

Labs use the same equipment and perform the same tests to look for the original SARS-CoV-2 virus and its variants. This result will appear in your lab report as one of the following:

  • SARS-CoV-2 VOC S Gene Mutation “Detected”: This means that a variant of concern (VOC) was found in your sample.
  • SARS-CoV-2 VOC S Gene Mutation “Not Detected”: This means that a variant of concern (VOC) was not found in your sample.

This information is important for public health authorities as it will guide interventions at the population level to assess and prevent the spread of these variants. At this time, whether you have the original virus or a specific variant does not impact the care you will receive.

Testing positive for COVID-19

A person may test positive when they have been infected, and their body is producing new copies of the virus. For most people, this will happen early in the disease when they have symptoms. Other people will test positive before symptoms start. These people are still contagious and should take precautions not to spread the virus to others. People near the end of the disease who still have symptoms may test negative because the test looks for pieces of viral genetic material that will no longer be present after the virus has become inactive. For this reason, a person who tests positive does not need to be re-tested even if they continue to have symptoms.

Can a person have coronavirus and test negative?

Although uncommon, a person with COVID-19 can test negative for SARS-CoV-2. One possible reason is that the test was performed too early in the disease and the person was not making enough virus for it to be detected by the test.  Another possible reason is that the swab was performed incorrectly, and not enough cells from the back of the nose or throat were sampled.

How long will it take to receive my result?

This will depend on the type of machine used to perform your test and the number of people tested in your area. After the tissue sample has been received, most types of machines can produce a result in 24-48 hours. However, it may take longer depending on the number of tests that are being performed at any given time. Check with your doctor or local health authority to find out how long it will take to receive your result.

Video: What happens to my COVID-19 test?

Other COVID-19 resources

Government of Canada

Province of Ontario COVID-19 Self-Assessment Tool

World Health Organization

Centers for Disease Control

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