Informing Ontario’s response to COVID-19

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Absolute risk difference

The absolute different 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. Synonymous with acquired immune system.

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.


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. 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.


B cell

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

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.


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. 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. Synonymous with clinical impact.*

Clinical test

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

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.

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.


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).*


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

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.


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 (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.


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.*


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.*


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.**


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.

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.



A patient with COVID-19 patient who experiences lasting symptoms for an extended period of time after infection. Long-haul cases of COVID-19 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.

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.**


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.

Moving average

A statistical way of smoothing data by taking an average over a period of time, for example 7 days.

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.


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 antibody

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


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).*

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.*



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.


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.*

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.


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.


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.*

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. 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 see how effective the experimental intervention was. 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.*

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.

Relative risk

Estimate of the ratio of the probability of an outcome in one group to the probability of an outcome in another group.

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

Ratio of the risk of an outcome in one group to the risk of an outcome in another group.



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.*

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.

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.

Social distancing

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


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.


The systematic collection and analysis of data in a population.

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

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. 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.


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.**

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.


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.