Informing Ontario’s response to COVID-19

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Absolute risk difference

The absolute difference in the risk of an outcome, such SARS-CoV-2 infection or death between two groups. If an intervention reduces the absolute risk of death from 25% to 10%, the absolute risk difference is 15%. This is synonymous with risk difference and absolute risk reduction/increase. This is different from relative risk and relative risk reduction/increase.


A protein found on the surface of various cell types. It is considered to be a cellular entry receptor for the SARS-CoV-2 virus.

Acute respiratory distress syndrome (ARDS)

A serious form of lung injury that occurs in people who are critically ill, with a buildup of fluid in the lungs that prevents the effective exchange of oxygen. ARDS can be caused by the SARS-CoV-2 virus.

Adaptive immune system

A second line of defense in the immune system that specifically responds to the initial exposure to a pathogen or vaccine. This is synonymous with acquired immune system.

Adverse event

An unintended event that is harmful or otherwise unwanted in a participant of a clinical study, which does not necessarily have a causal relationship with the studied treatment.

Adverse reaction

A response to a drug, treatment or medical intervention that is harmful or otherwise unwanted in a participant of a clinical study and suspected to be related to, or caused by, the studied drug, treatment or intervention.

Airborne disease

A disease that is caused by a pathogen that can be transmitted through the air by very small particles called aerosols, which have the ability to remain suspended in the air over considerable time and distance.

Alternate level of care (ALC)

A type of hospital care that occurs when a patient is occupying a hospital bed but does not require typical hospital-level care such as acute care.


A large Y-shaped protein produced by the immune system to help in the identification and elimination of viruses and bacteria.

Antibody-mediated immunity

An immune response that involves the activation of B lymphocytes, as well as the production of antibodies in response to a pathogen. This is synonymous with humoral immunity.


A specific molecule or structure found on a pathogen that allows it to be identified as foreign by the immune system.


An individual who is infected with a pathogen but does not have symptoms.

Attack rate

The total number of new cases with an infection divided by the total population that was observed.

Atypical symptoms

Typical clinical symptoms of COVID-19 are fever, cough, and respiratory distress. In the context of SARS-CoV-2 and COVID-19, atypical symptoms of COVID-19 can include confusion, weakness, vomiting and other symptoms not considered typical of an upper respiratory tract infection by clinicians.


B cell

A white blood cell that produces antibodies. B cells are part of the adaptive immune response. This is synonymous with B lymphocyte.

B.1.1.7 variant

A variant of concern of the SARS-CoV-2 virus that was first detected in the United Kingdom. This variant is also commonly known as the UK variant.

B.1.351 variant

A variant of concern of the SARS-CoV-2 virus that was first detected in South Africa.This variant is also commonly known as the South African variant.

B.1.617 variant

A variant of concern of the SARS-CoV-2 virus that was first detected in India. This variant has two lineages: Kappa variant (B.1.617.1 variant) and Delta variant (B.1617.2 variant) This variant is also commonly known as the Indian variant.


The set of measurements at the beginning of a study.*

Basic reproduction number (R0)

An indication of how contagious an infectious disease is, defined as the
average number of cases generated by one case in a population in which all individuals are susceptible to infection in the absence of public health interventions and vaccines.


A measurable substance in an organism whose presence is indicative of a disease, infection, environmental exposure or other phenomenon.

Booster dose

A subsequent dose of a vaccine that is administered once or periodically to boost immunity against the antigen, when memory against the antigen declines.


C-reactive protein (CRP)

A protein produced by the liver that is a marker of inflammation in the body.

Case fatality rate

The proportion of people who die from a disease among those who are infected with it over a defined period of time. This is similar to case fatality ratio.

Case fatality ratio

The proportion of people who die from a disease among those who are infected with it. For example, in the context of SARS-CoV-2 and COVID-19, this would be the proportion of individuals who died of COVID-19 compared with the total number of individuals infected with SARS-CoV-2. This is synonymous with case fatality risk and is similar to case fatality rate.

Case management

A collaborative process of analysis, planning, support, evaluation, and advocacy for services to meet an individual’s comprehensive health needs. In the context of SARS-CoV-2 and COVID-19, it means ensuring that cases are reached in a timely and effective way to prevent the spread of SARS-CoV-2.

Case study

An examination and analysis of a particular instance (case), such as a person, population or event.

Case-control study

An observational study to find out the possible cause(s) of a disease or condition. This is done by comparing a group of patients who have the disease or condition (cases) with a group of people who do not have it (controls) but who are otherwise as similar as possible (in characteristics thought to be unrelated to the causes of the disease or condition).*

Cell-mediated immunity

A cellular immune response that involves the activation of phagocytes, antigen-specific cytotoxic T lymphocytes, but does not involve the production of antibodies. This is synonymous with cellular immunity.

Clinical effect

The effect that something (for example, a public health intervention or a drug) is likely to have on a particular group of people. This is synonymous with clinical impact.*

Clinical test

The collection of information on an individual, usually through the performance of medical tests or physical exams.

Clinical trial

A clinical study that examines and assesses the effects of a novel treatment, if it is safe, and if it works in people. Clinical trials are classified in 4 phases:

Phase I: The first time a new treatment or vaccine is tested in humans, it will usually be given to a small group of healthy volunteers, usually less than 50 participants. The principal objectives in Phase I are to make sure that the new medicine or vaccine presents no major safety issues, clarify that it can reach the targeted body area, remains there long enough to deliver its benefits, and gain preliminary evidence that it could offer therapeutic value, or prevent the disease or condition.

Phase II: If Phase I is successful, approval will be sought for a trial involving a larger group of people, usually several hundreds of participants. Phase II trials will typically include patients who have the disease the potential medicine is targeting and aim to establish the efficacy of the treatment or vaccine in treating or preventing the disease, and identify the optimal dose.

Phase III: If the results from Phase II are encouraging, a Phase III trial is conducted, which involves several hundreds to several thousands of participants coming from different countries. The objectives in Phase III are to evaluate the safety and effectiveness of the new treatment or vaccine in the typical patient likely to use it in clinical practice and identify side effects. If a new treatment or vaccine completes Phase III with positive results, regulatory approval might be sought.

Phase IV trials evaluate the treatment or vaccine after approval and frequently evaluates longer-term effects or compares effectiveness and safety of the new treatment with effectiveness and safety of established treatments. It typically involves several hundreds to several thousands of participants.

Cohort study

A longitudinal research study with two or more groups (cohorts) of people with similar characteristics.*


Separating a population into smaller groups (cohorts) in order to minimize their risk of infection, to contain an infection, and to easily trace close contacts in the case of infection. Members within each group must always remain the same, with no, or limited, contact with members from other cohorts.

Compartmental model

A mathematical model that categorizes a population into groups, or “compartments”, where individuals may move between the model’s compartments, mapping the population’s epidemiological state. A SIR model is an example of a compartmental model.

Confidence interval (CI)

A way of expressing how certain we are statistically about the findings from a study. It gives a range of results that is likely to include the ‘true’ value for the population. A wide confidence interval indicates a lack of certainty about the true effect of the test or treatment – often because a small group of patients has been studied. A narrow confidence interval indicates a more precise estimate (for example, if a large number of patients have been studied). The confidence interval is usually stated as ‘95% CI’, which means that the range of values has a 95 in a 100 chance of including the ‘true’ value. For example, a study may state that ‘based on our sample findings, we are 95% certain that the ‘true’ population blood pressure is not higher than 150 and not lower than 110′. In such a case the 95% CI would be 110 to 150.*

Contact tracing

A method used to keep track of individuals who have, or may have, come in contact with an infected individual.

Control group

A group of people in a study who do not have the intervention or test being studied. Instead, they may have the standard intervention or a placebo intervention. The results for the control group are compared with those a group receiving the intervention being tested. The aim is to check for any differences. Ideally, the people in the control group should be as similar as possible to those in the intervention group, to make it as easy as possible to detect any effects due to the intervention.*


A class of medications that mimics the effect of cortisol, a naturally occurring hormone, in the human body. Corticosteroids have potent anti-inflammatory effects and suppress the immune system.

Cost-effectiveness analysis

An analysis that assesses the cost of achieving a benefit by different means. The benefits are expressed in non-monetary terms related to health, such as symptom-free days, heart attacks avoided, deaths avoided or life years gained (that is, the number of years by which life is extended as a result of the intervention). Options are often compared on the cost incurred to achieve one outcome (for example, cost per death avoided).*


The provincial system to record COVID-19 vaccination in Ontario. This is often referred to as COVax in Ontario. This is different from COVAX, the global collaboration for equitable access to COVID-19 vaccines.


A respiratory illness, formerly known as the 2019 novel coronavirus, caused by the virus SARS-CoV-2.

Credo box

A packaging container for storing and transporting medical materials at cooled temperatures. In the context of SARS-CoV-2 and COVID-19, COVID-19 vaccines that need to be stored and transported at cooled temperatures may sometimes be held in Credo box. This is synonymous with Credo container and Credo cube.

Cross-sectional study

An observation of a set of people, or interventions, at one specific point in time. This contrasts with a longitudinal study, which follows a set of people over a period of time.*

Cumulative incidence

A type of genetic material that is found in all organisms and many viruses, and is usually double-stranded.

Cytokine release syndrome

A multi-system inflammatory response in the body due to overactivation of the immune system. Cytokine release syndrome is a form of cytokine storm that is associated with chimeric antigen receptor T-cell (CAR-T) cancer therapy.

Cytokine storm

A multi-system inflammatory response in the body due to overactivation of the

immune system. Cytokine storm can occur in response to infection, medications, or organ transplantation.


Data modelling

A conceptual representation of data and various associations based on a set of parameters. In the context of SARS-CoV-2 and COVID-19, data modelling may be used to estimate how COVID-19, health systems changes, or other health-related variables may impact population outcomes. Mathematical modelling is a type of data modelling.

Decision analytic model

A mathematical model that is used to evaluate and compare competing public health interventions and medical interventions. The main goal is to assist with decision-making.

Deoxyribonucleic acid (DNA)

A type of genetic material that is found in all organisms and many viruses, and is usually double-stranded.


A type of corticosteroid medication.

Doubling time

In the context of SARS-CoV-2 and COVID-19, this refers to the amount of time it takes for cases to double in a particular population.


Ecological study

An observational study where data is analyzed at a group, country or state level, rather than at an individual level.

Effective reproduction number (Rt, Re)

An indication of how contagious an infectious disease is in a specific population at a specific time, defined as the actual average number of cases generated by one case in the presence or absence of partial immunity, public health interventions and/or vaccines.


How beneficial a test or treatment is under usual or everyday conditions, compared with doing nothing or opting for another type of care.*


How beneficial a test, treatment, or public health intervention is under ideal conditions (for example, in a laboratory), compared with doing nothing or opting for another type of care.*


A disease that recurs in a particular area or population. The disease may also be considered native to that particular area or population.


The set of measurements at the end of a study.


The rapid spread of a disease to a large number of people in a population.


The study of the causes, distribution, control and prevention of disease. Epidemiologists collect and examine medical data and spot health trends to establish which diseases are on the increase and where, which treatments and other activities work and which do not. (This includes activities to prevent disease and to improve health and well-being.) In other words, they consider the possible risk factors for a whole population or area, not just for individual patients.*

Essential work

Work that is considered critical to protecting life, health, or public functioning. In Ontario, essential work in the context of SARS-CoV-2 and COVID-19 includes, but is not limited to, grocery store workers, gas station workers, truck drivers, long-term care home staff, trades workers, health care workers, first responders, warehouse and distribution workers, and manufacturing workers. This is synonymous with essential services.

Excess mortality

The number of deaths from all causes during a crisis, above and beyond what would have been expected under normal conditions.

Extra-corporeal membrane oxygenation (ECMO)

A machine used to oxygenate the blood of a person with severe heart or lung injury. With ECMO, blood is transferred from the body to a heart-lung machine that removes carbon dioxide and adds oxygen and the blood is then returned to the body.


Gastrointestinal symptoms

In the context of SARS-CoV-2 and COVID-19, gastrointestinal symptoms include nausea, vomiting, abdominal pain and diarrhea.


Hazard ratio

The hazard or chance of an event occurring in the treatment arm of a study as a ratio of the chance of an event occurring in the control arm over time.*


A substance or toxin that has the potential to be damaging to the liver.

High-flow nasal cannula

A device used to provide oxygen to a patient with moderate oxygen requirements or moderate respiratory difficulty. The flow of oxygen is up to 60 L/min. A high flow nasal cannula can increase the oxygen concentration to 24-100% (for reference, the percentage of inspired oxygen in the environment is 21%). A high flow nasal cannula provides a fixed oxygen concentration.


Immune system

The organ system that is involved in protecting people from infection, infestation, and other potential harm from infections by pathogens.**


The number of new cases of a disease among a certain group of people during a specific period of time. To be distinguished from prevalence.*

Incubation period

The period between the initial infection and the onset of signs and/or symptoms.**

Infection prevention and control (IPAC)

A multidisciplinary field that applies evidence-based practices and procedures to health care settings to reduce the transmission of infection.


A family of viruses that cause the respiratory illness commonly known as the “flu”. The severity of infection with influenza can be mild, with symptoms such as fever and cough, or severe, manifesting as injury to the lungs that require mechanical ventilation.

Interleukin (IL-6)

Interleukins are proteins that can act as signalling molecules for the immune system and regulate the immune response and inflammation. Interleukin-6 is released in response to infections and acts as a signal to increase the inflammatory response to infections with microbes such as SARS-CoV-2.

Interrupted time series

A method of statistical analysis that involves tracking a long period of time before and after a specific identified point, in order to determine the impact and effect of that point.

Intramuscular injection

A procedure used to administer a drug into the muscle tissue, enabling the drug to be more easily absorbed into the bloodstream. Flu shots are given this way.



A patient with COVID-19 experiences lasting symptoms for an extended period of time after infection. Long COVID cases can last for months; the maximum duration is not yet known.

Longitudinal study

A study of the same group of people at different times. This contrasts with a cross-sectional study, which observes a group of people at a point in time.*


Mechanical ventilation

The use of a machine to help a person with breathing when they are unable to breathe effectively on their own.


A protein that is bound to the wall of a cell.

Memory B cell

A small, long-lived B lymphocyte that was previously exposed to a particular antigen, and can proliferate and produce large amounts of antibodies during re-exposure to the same antigen.**

Memory T cell

A long-lived T cell that bears receptors for an antigen during its encounter with a prior infection or vaccination.**

Messenger ribonucleic acid (mRNA)

A type of RNA that carries the genetic code for a specific protein, and conveys this code from the DNA to the ribosome, which produces the protein. mRNA is the intermediate step between the protein-encoding DNA and the production of proteins.


A method often used in systematic reviews to combine results from several studies of the same test, treatment or other intervention to estimate the overall effect of the treatment.*

Middle East Respiratory Syndrome (MERS)

A respiratory illness caused by the Middle East Respiratory Syndrome Coronavirus (MERS-CoV). The virus was first identified in 2012. MERS may present as a mild respiratory illness with fever and cough or as a severe respiratory illness causing injury to the lungs.

Monoclonal antibody

Monoclonal antibodies are laboratory-made proteins that mimic the immune system’s ability to fight off harmful antigens such as viruses.

Moral distress

Moral distress occurs when one feels unable to take what they believe to be an ethically appropriate or right course of action. It is especially likely to occur during public health emergencies and in other situations with extreme resource limitations.


A prolonged suspension of an action or regulation, which is to be repealed by a subsequent consideration at a later date. For example, a COVID-19 eviction moratorium may suspend the eviction of individuals or households who have lost income due to the pandemic and cannot pay rent.

Morbidity rate

The number of cases of an illness, injury, or condition within a given time (usually a year). It can also refer to the percentage of people with a particular illness, injury or condition within a defined population.*

Mortality rate

The proportion of a population that dies within a particular period of time. The rate is often given as a certain number per 1,000 people.*

Moving average

A statistical way of smoothing data by taking an average over a period of time, for example 7 days. This is synonymous with rolling average and running average.

mRNA vaccine

A vaccine that works by introducing an mRNA sequence into its host. The mRNA “teaches” the host cells to build a protein specific to the virus that the vaccine is intended to protect against. The protein is then recognized as non-self and the immune system begins generating antibodies, protecting the host from future infection.

Multisystem inflammatory syndrome

A condition where different organ systems can become inflamed, including the heart, lungs, kidneys, brain, skin, eyes, or gastrointestinal organs.

Multivariable analysis

An analysis of one dependent variable measuring an outcome and multiple independent variables meant to predict the outcome.


N501Y mutation

A mutation of the SARS-CoV-2 virus.

Variants with the N501Y mutation are referred to as N501Y+ variants. This mutation has allowed the variants to be more transmissible by permitting the SARS-CoV-2 virus to more readily bind to ACE2 receptors. This includes the Alpha variant (B.1.1.7 variant), the Beta variant (B.1.351 variant), and Gamma variant (P.1 variant).

Variants without the N501Y mutation are referred to as N501Y- variants. This includes the Delta variant (B.1.617.2 variant).

Nasal prongs

A device used to provide oxygen to a patient with low oxygen requirements and minimal respiratory difficulty. It is a plastic tube with two ports that fit into the nose. Nasal prongs can increase the oxygen concentration (FiO2) to 25-40% (for reference, the percentage of inspired oxygen in the environment is 21%). However, the actual delivered concentration of oxygen through nasal prongs can be variable between people, particularly in the context of respiratory difficulty.

Nasopharyngeal swab

A method of collecting a clinical test sample which involves a swab of the secretions located in the back of the nose and throat.

Neutralizing antibodies

Antibodies that protect a cell from a pathogen by neutralizing the virus. Neutralizing antibodies not only bind to a virus, they bind in a manner that blocks infection.

Neutralizing antibody

An antibody that is capable of keeping a virus from infecting a cell by neutralizing or completely inhibiting its biological effect.**


A low neutrophil cell count. Neutrophils are a type of white blood cell and part of the immune response to infection.


The average number of patients who need to receive the treatment or other intervention for one of them to get the positive outcome in the time specified. The closer the NNT is to 1, the more effective the treatment. For example, if the NNT for drug A compared with drug B for pain relief after a tooth extraction is 4, on average, for every four people who get drug A instead of drug B, one person will have pain relief after tooth extraction who would not have done if all four had got drug B. The other three people out of the four will have or not have pain relief, just as if they had taken drug B. The NNT is 100 divided by the absolute risk reduction (ARR) expressed as a percentage. For example, if the ARR is 5%, the NNT is 100/5=20.*


Observational study

A retrospective or prospective study in which the investigator observes the natural course of events with or without control groups (for example, cohort studies and case-control studies).*

Occupational health

A health care field that involves the well-being and safety of workers in the workplace. In the context of SARS-CoV-2 and COVID-19, occupational health includes the provision of necessary personal protective equipment and the implementation of appropriate health and safety measures in the workplace to prevent the transmission of SARS-CoV-2.

Odds ratio

Compares the odds (probability) of something happening in one group with the odds of it happening in another. An odds ratio of 1, shows that the odds of the event happening (for example, a person developing a disease or a treatment working) is the same for both groups. An odds ratio of greater than 1 means that the event is more likely in the first group than the second. An odds ratio of less than 1 means that the event is less likely in the first group than in the second group.*

Off-label use

The use of a drug in a manner other than the one defined on the drug label, and not as approved by Health Canada.

On-label use

The use of a drug in the manner defined on the drug label, as approved by Health Canada.



A measure of the strength of the evidence against the null hypothesis; the smaller the p-value, the stronger the evidence against the null hypothesis. A p-value of less than 0.05 is typically considered to indicate statistical significance. A p-value of less than 0.005 can be considered strong evidence against the null hypothesis. This is synonymous with significance levels.

P.1 variant

A variant of concern of the SARS-CoV-2 virus that was first detected in travellers from Brazil. This variant is also commonly known as the Brazilian variant.


An epidemic of an infectious disease that has spread across a large region, for instance multiple continents.


An agent causing disease or illness, such as a virus or bacterium capable of producing a disease.**

Peer review

A review of a study, service or recommendation by those with similar interests and expertise to the people who produced it to make sure a study and the report describing it are accurate and valid.*

Period of viral replication

The time period during which a virus is actively replicating within a host.


The quantification of the amount of follow-up that accumulated in a group of people or in a study, which takes into account both the number of individuals in the study and the amount of time spent each individual spent in the study. For example, 1,000 individual years of data will be used in an analysis that tracked 1,000 individuals for 1 year.

Personal protective equipment (PPE)

In the context of SARS-CoV-2 and COVID-19, this is equipment that provides protection to the body and face in order to avoid exposure to SARS-CoV-2 infection. For example, medical or N95 masks, goggles, gowns, and face shields.

Phylogenetic study

A study that develops branching diagrams (phylogenetic trees) that represent the relationship between different mutations of SARS-CoV-2. A phylogenetic tree is characterised by a series of branching points, expanding from the most recent common ancestor. The branches represent the passage of genetic information between generations, and the length of the branches represents genetic changes.


An inert treatment, such as a sugar pill or saline injection, given to patients in the control group of a clinical trial. It is indistinguishable from the actual treatment, which is given to patients in the experimental group. The aim is to determine the effect of the experimental treatment over and above any perceived benefit someone has because of participating in the clinical study and receiving the inert treatment (placebo effect).


A highly contagious virus that targets the central nervous system, causing paralysis. Infected individuals are often asymptomatic.

Polymerase chain reaction (PCR)

A technique used to amplify a specific segment of DNA or RNA as part of a laboratory test. PCR allows investigators to make millions of copies of a DNA or RNA segment, from a small sample.

Population-based study

A study that is representative for the entire population addressed in the study question.


The state after infection, but before the development of symptoms.

Pre-test probability

The prevalence of a disease in a population, either at a point in time or over a period of time, which indicates the probability that an individual has the disease prior to a diagnostic test.


How common a disease or condition is within a population, either at a point in time or over a given period of time, including new and existing cases. This is different from incidence.*

Prospective study

A research study in which the health or other characteristic of patients is monitored (or ‘followed up’) for a period of time, with events recorded as they happen. This contrasts with retrospective studies.*

Psychoactive drug

A medication that alters brain functions (for example, vision, consciousness, awareness). Examples include antidepressants, benzodiazepines, and antipsychotic medications. This is synonymous with psychotropic drug.

Public health intervention

In the context of SARS-CoV-2 and COVID-19, a measure taken to contain the pandemic, such restricting mass gatherings, social distancing, or closing schools. This is synonymous with public health measures.


Randomized controlled trial (RCT)

A study in which a number of similar people are randomly assigned to two (or more) groups to test a specific drug, treatment, or other intervention. One group (the experimental group) has the intervention being tested, the other (the comparison or control group) has an alternative intervention, a dummy intervention (placebo) or no intervention at all. The groups are followed up to examine the effectiveness of the experimental intervention. Outcomes are measured at specific times and any difference in response between the groups is assessed statistically. This method is also used to reduce bias. A randomized controlled trial is a type of clinical trial.*

Rapid antibody test

In the context of SARS-CoV-2 and COVID-19, it is a rapid diagnostic test that is used to detect the presence of SARS-CoV-2 antibodies.

Rapid antigen-based test

In the context of SARS-CoV-2 and COVID-19, it is a rapid diagnostic test that is used to detect the presence of SARS-CoV-2 antigens.

Rapid PCR test

In the context of SARS-CoV-2 and COVID-19, it is a rapid diagnostic test that is used to detect the presence of SARS-CoV-2 molecules through polymerase chain reaction.

Rapid review

A form of evidence synthesis in which components of the systematic review process are simplified or taken out in order to provide information to reach conclusions in a timely manner.

Rate ratio

A form of a relative risk. It is a way to express the probability of an event occurring in the study group during a specific time compared with the probability of the same event occurring in the control group during the same time. If both groups face the same level of risk, the rate ratio is 1. If the first group had a rate ratio of 2, subjects in that group would be twice as likely to have the event happen. A rate ratio of less than 1 means the outcome is less likely in the first group.*


An element or substance that is added to a compound in order to create chemical a reaction that will allow the user to detect or measure other substances.

Regression model

A statistical method used to investigate the relationship between two or more variables and estimate one variable dependent on the others. In the context of COVID-19, it helps to predict COVID-19 outbreaks.

Relative risk

An estimate of the ratio of the probability of an outcome in one group compared to the probability of an outcome in another group. A relative risk of 1 indicates that there is no difference between the two compared groups. A risk ratio is an estimate of the relative risk.

Respiratory droplets

Saliva and respiratory secretions which are expelled when a person exhales (for example, coughs, sneezes, speaks). Droplets are defined by their size (they are usually greater than 5 to 10 micrometers in diameter) and characteristic pattern of quickly falling to the ground from gravitational forces. Pathogens that are transmitted via respiratory droplets are suspended within the respiratory droplets and, thus, do not tend to travel for large distances or time. However, smaller droplets can dry out and stay airborne.

Retrospective study

A research study that focuses on the past and present. The study examines past exposure to suspected risk factors for the disease or condition. Unlike prospective studies, it does not cover events that occur after the study group is selected.*

Ribonucleic acid (RNA)

A type of genetic material that is found in all organisms and many viruses, and is usually single-stranded.

Risk ratio

A ratio of the risk of an outcome in one group compared to the risk of an outcome in another group. This is an estimate of the relative risk.



Severe Acute Respiratory Syndrome Coronavirus 2; the novel strain of coronavirus that causes coronavirus disease 2019 (COVID-19).

Second wave

A second significant and sustained surge in SARS-CoV-2 infections after a sustained decrease in SARS-CoV-2 infections in a particular region. This is synonymous with wave two.


How well a test detects what it is testing for. It is the proportion of people with the disease or condition that are correctly identified by the study test. For example, a test with a sensitivity of 96% will, on average, correctly identify 96 people in every 100 who truly have the condition, but incorrectly identify as not having the condition 4 people in every 100 who truly have it. This is different from positive predictive value.*

Serious adverse event

An unintended event in a participant of a clinical study that results in death, is life-threatening, requires a hospitalization or causes prolongation of an existing hospitalization, results in persistent or significant disability/incapacity, may have caused a congenital anomaly/birth defect, or requires a medical intervention to prevent permanent impairment or damage.

Serological test

A test that detects the presence of antibodies in blood serum, which are usually produced in response to an infection. In a serological test, antibodies are detected in the serum irrespective of the presence of symptoms.


Having a negative serological test result, with no detection of antibodies against a specific virus or bacterium.


Having a positive serological test result, with detection of antibodies against a specific virus or bacterium.


The proportion of people in a population who test positive for a specific disease based on serological tests.

Severe Acute Respiratory Syndrome (SARS)

An viral respiratory disease caused by SARS-CoV-1, a coronavirus genetically related to SARS-CoV-2.


A process of developing a mathematical representation of an actual or proposed product in a computer model.

Single-centre study

A study that is conducted at a single site (for example, a hospital or clinic) and in accordance with a single protocol or set of parameters.

SIR model

A type of compartmental model that has three compartments: S, denoting the number of susceptible individuals; I, denoting the number of infected individuals; and R, denoting the number of recovered individuals. In the context of SARS-CoV-2 and COVID-19, this model can be used to analyze and predict the spread of the SAR-CoV-2 virus.

Social distancing

Maintaining a physical distance between people or groups of people to prevent the spread of an infectious disease. This is synonymous with physical distancing.


A substance or protein that dissolve in blood or other body or cellular fluids.


How well a test correctly identifies people who do not have what it is testing for. It is the proportion of people without the disease or condition that are correctly identified by the study test. For example, a test with a specificity of 96% will, on average, correctly identify 96 people in every 100 who truly do not have the condition, but incorrectly identify as having the condition 4 people in every 100 who truly do not have it.*


A piece or portion of a sample selected for examination.**

Spike protein

A protein with the shape of spike that studs the surface of SARS-CoV-2 and other coronaviruses, giving them the appearance of a crown.

Structural inequities

Social inequities that are theoretically remediable and are unequal or unreasonable towards a particular population. This is synonymous with systemic inequities.

Subcutaneous hydration

An option of infusing fluid into subcutaneous tissue that requires only minimal equipment. This therapy is used to treat or prevent dehydration.

Suitable housing

According to Canada’s National Occupancy Standard (NOS), suitable housing refers to whether the building has a sufficient number of bedrooms for the size and composition of the household inhabiting the dwelling.

Supplemental oxygen

A care in which patients with low blood oxygen levels are provided with oxygen by a holding tank of oxygen or a compressor. Oxygen is then delivered through a nose tube or a mask.


The systematic collection and analysis of data in a population.


In the context of SARS-CoV-2 and COVID-19, this refers to a case of COVID-19 disease that presents with typical and/or atypical symptoms. Common symptoms include, but are not limited to, fever, loss of taste and/or smell, dry cough, chest tightness, fatigue, runny nose. COVID-19 may also exhibit gastrointestinal symptoms.

Syndromic surveillance

The systematic collection and analysis of health indicators, assisted by automated data acquisition and generation of statistical alerts, in real-time to detect disease outbreaks earlier than would be possible with traditional public health methods.

Systematic review

A review that summarizes the evidence on a clearly formulated review question according to a predefined protocol, using systematic and explicit methods to identify, select and appraise relevant studies, and to extract, analyze, collate and report their findings. It may or may not use statistical techniques, such as meta-analysis.*


T cell

A type of white blood cell developed in the lymphoid organ of the immune system. These cells are involved in cell-mediated immunity and play a major role when a foreign agent attacks the body. This is synonymous with T lymphocyte.

Test positivity

The percentage of SARS-CoV-2 PCR tests that come back with a positive result. This is synonymous with percent positivity.

Threshold analysis

Threshold analysis quantifies how much a parameter could change before conclusions change or the direction of an effect changes.


A low platelet count.

Time-to-event curve

A curve used to describe the length of time until the occurrence of defined clinical events in a group of people. The occurrence of events are typically indicated by steps in the curve.


Univariate analysis

A statistical analysis involving only one dependent and one independent variable.



A suspension containing live, attenuated, modified, or killed pathogens, genetic material (such as RNA), or proteins that encodes for part of a pathogen. When the vaccine is administered into the body stimulates the body’s immune system to produce antigen-specific antibodies.**

Vaccine efficacy

The percentage reduction of disease in a vaccinated group of people compared to an unvaccinated group, using the most favorable conditions. It is expressed as a percentage and calculated as (1-relative risk) x 100.

Variant of concern (VOC)

In the context of SARS-CoV-2 and COVID-19, it is a genetic variation of the SARS-CoV-2 virus that may potentially have an impact on SARS-CoV-2 transmissibility, COVID-19 severity, or effectiveness of COVID-19 vaccines or treatments.

Venturi mask

A device used to provide oxygen to a patient with moderate oxygen requirements or moderate respiratory difficulty. A venturi mask can increase the oxygen concentration to 24-60% (for reference, the percentage of inspired oxygen in the environment is 21%). A venturi mask provides a fixed oxygen concentration.

Viral load

The number of viral particles in a clinical specimen (for example, saliva, blood plasma).

Viral shedding

The act of spreading a virus through respiratory or other body fluids.

Virus variant

An evolved form of a virus that underwent a mutation or a number of mutations that changed part of the original virus’ genetic code. This evolution may impact the virus’ transmissibility, morbidity, or mortality.


Wastewater testing

The examination of sewage water to detect and monitor pollutants (for example, mercury) and biomarkers, to obtain data on the activities and health of the community living within the catchment area. Wastewater testing has been previously used to detect drug use (for example, opioid use), as well as the presence of viruses (for example, poliovirus) within a community, and can be used to detect components of SARS-CoV-2.

Wastewater treatment plant

A facility that uses physical, chemical, and biological processes to remove pollutants from sewage water.

Weighted average

An average of data points calculated after weighting each data point by its statistical precision or another measure of importance of the data point.

*The definitions for these terms were adapted from the National Institute for Health Care and Excellence (NICE) glossary.

**The definitions for these terms were adapted from the Biology Online dictionary.