Overview of influenza monitoring in Canada

FluWatch is Canada's national surveillance system that monitors the spread of the influenza and influenza-like illnesses (ILI) year-round. A weekly surveillance report on influenza activity in Canada is posted online every Friday during active influenza season.

FluWatch program objectives

The FluWatch program is a surveillance system consisting of virological surveillance, influenza and ILI activity level surveillance, syndromic surveillance, outbreak surveillance, severe outcome surveillance and vaccine monitoring. Data are provided by sentinel laboratories, provincial and territorial ministries of health, hospitals, primary care providers and volunteer Canadians. Program objectives are:

Detect: Identify signals for timely identification of, and coordinated assessment and response to epidemics and other events of urgent public health concern
Inform: Contribute to the evidence base necessary for planning, development and implementation of health programs and healthy public policies
Enable: A robust surveillance infrastructure to enable the timely and relevant response and research necessary to mitigate the impacts of viral respiratory pathogens

Seven parts of flu monitoring in Canada

1. Geographic spread of influenza and influenza-like illness (ILI) activity

All provincial and territorial public health departments provide an assessment of the intensity and geographic spread of influenza for surveillance regions within their jurisdictions. For each surveillance region, the number of reported influenza detections, health care visits due to ILI and influenza outbreaks are assessed and assigned a level of activity (no activity, sporadic, localized and widespread).

Influenza/ILI activity level definitions

No activity:

No laboratory-confirmed influenza detections in the reporting week; however, sporadically occurring ILI may be reported

Sporadic:

Sporadically occurring ILI and laboratory-confirmed influenza detection(s) with no outbreaks detected within the influenza surveillance region

Localized:

Evidence of increased ILI and laboratory-confirmed influenza detection(s) and outbreaks in facilities under surveillance occurring in less than 50% of the influenza surveillance region

Widespread:

Evidence of increased ILI and laboratory-confirmed influenza detection(s) and outbreaks in facilities under surveillance occurring in greater than or equal to 50% of the influenza surveillance region

2. Laboratory-confirmed influenza detections

Data on laboratory-confirmed influenza detections are provided to FluWatch through the Respiratory Virus Detection Surveillance System (RVDSS). RVDSS is a sentinel laboratory surveillance system that consists of provincial, territorial and regional public health laboratories and some hospital laboratories that report the number of tests and the number of positive tests for respiratory viruses (influenza, SARS-CoV-2 (COVID-19), respiratory syncytial virus (RSV), human parainfluenza virus, adenovirus, human metapneumovirus, enterovirus/rhinovirus, human coronavirus).

Primary surveillance populations are acute respiratory infection cases at emergency departments, hospitalized severe acute respiratory virus infection cases, and influenza outbreak cases. Outpatient ILI cases may be targeted, but testing is typically limited to higher-risk individuals and respiratory testing algorithms vary by province or territory.

Additionally, several provincial and territorial public health laboratories provide the age and sex of influenza detections in order to monitor the impact of the influenza in different age-groups. This case-level data represents a portion of laboratory-confirmed influenza detections.

FluWatch uses laboratory data to monitor and analyse patterns of influenza circulation in Canada in order to:

Assess influenza activity including signals for the start, progression and end of seasonal epidemics.
Assess patterns of transmission of influenza by person, place and time.
Assesses the impact and burden of influenza epidemics.

Laboratory-confirmed influenza detection definition:

isolation of influenza virus from an appropriate clinical specimen

demonstration of influenza virus antigen in an appropriate clinical specimen

significant rise (e.g. fourfold or greater) in influenza IgG titre between acute and convalescent sera

detection of influenza RNA

3. Syndromic surveillance

Syndromic surveillance uses the reports of symptoms such as cough and fever to detect early signals of influenza-like illness (ILI) activity in the community (before influenza cases get confirmed and reported to public health agencies). Data collected through syndromic surveillance are not specific to any one respiratory pathogen. Syndromic surveillance provides an important indicator of overall respiratory illness activity in the community. Data from syndromic surveillance programs are used with other surveillance data to obtain a more complete and accurate picture of influenza activity in Canada.

FluWatch syndromic surveillance consists of two programs:

The Sentinel Primary Care ILI Surveillance program consists of primary care providers across Canada who report the proportion of patients seen in their clinic each week with influenza-like illness

The FluWatchers program consists of volunteers across Canada who report each week if they had cough or fever via an online questionnaire. Anyone in Canada can be a volunteer FluWatcher. Visit our FluWatchers page for more info and to sign up.

Influenza-like-illness (ILI) definition:

Acute onset of respiratory illness with fever and cough and one or more of the following:

sore throat, arthralgia, myalgia or prostration, which could be due to influenza virus.

In children under 5, gastrointestinal symptoms may also be present.
In patients under 5 or 65 and older, fever may not be prominent.

4. Outbreak surveillance

Outbreaks of laboratory-confirmed influenza or influenza-like illness (ILI) in different settings (long-term care facilities, nursing homes, acute care facilities, schools, daycares, remote and/or isolated communities and other settings) are reported from provinces and territories.

Influenza outbreak surveillance data provides a timely, sensitive measure of early influenza activity in at-risk populations that is scalable from local to national level and an indicator of the burden of influenza within certain at-risk groups and settings.

Outbreak definitions:

Setting	Setting definition	Outbreak definition
Long-term care facilities, nursing homes	Facilities that provide living accommodation for people who require on-site delivery of 24 hour, 7 days a week supervised care, including professional health services, personal care and services such as meals, laundry and housekeeping or other residential care facilities. Provincial/territorial public health is responsible for outbreak management under provincial legislation for these facilities.	Laboratory-confirmed influenza outbreak: 2 or more cases of ILI within 7 days, and at least 1 lab-confirmed case of influenza in the same setting (on the same floor, or in the same unity or ward).
Acute care facilities	Publicly funded facilities providing medical and/or surgical treatment and acute nursing care for sick or injured people, through inpatient services. (i.e. hospitals including inpatient rehabilitation and mental facilities).	Laboratory-confirmed influenza outbreak: 2 or more cases of ILI within 7 days, and at least 1 laboratory-confirmed case of influenza in the same setting (on the same floor, or in the same unity or ward).
Schools and daycares	Facilities that provide educational and/or childcare programs for children under the age of 18 years.	ILI outbreak: Greater than 10% absenteeism that is likely due to ILI.
Remote and/ or Isolated communities	A community that is physically and/or socially separated from the surrounding population For example, communities that are geographically isolated due to limited transportation links.	Laboratory-confirmed influenza outbreak: Unusual or unexpected number of ILI cases within 7 days and at least 1 laboratory-confirmed case of influenza.
Other	Any other locations/facilities not previously identified in which an outbreak of the influenza or ILI occurs. Example facilities: retirement homes assisted living or hospice settings private hospitals/clinics correctional facilities colleges/universities adult education centres shelters group homes workplaces	Note: Specific to setting as determined by reporting province Laboratory-confirmed influenza outbreak: 2 or more cases of ILI within 7 days, and at least 1 laboratory-confirmed case of influenza. ILI outbreak: 2 or more cases of ILI within 7 days. ILI outbreak in workplaces: Greater than 10% absenteeism that is likely due to ILI.

5. Severe outcomes surveillance

Severe outcome surveillance identifies and characterizes influenza-associated hospital admissions, intensive care unit (ICU) admissions and deaths. Data received on influenza-associated severe outcomes is used to:

Assess patterns of intensity in the transmission and severity of influenza by person and time
Understand which population groups are at the highest risk for severe influenza outcomes during each surveillance year
Assess the impact and burden of influenza epidemics in Canada
Understand and monitor factors that contribute to an increased risk influenza-associated severe outcomes in the population

Influenza serious outcomes are reported by participating provincial and territorial ministries of health as well as two hospital networks

Provincial/Territorial influenza hospitalizations and deaths

Influenza-associated hospitalizations, ICU admissions and deaths by age group and influenza type/subtype are reported by nine provincial/territorial ministries of health (excluding British Columbia, Nunavut, Ontario and Québec).

Hospital admissions and/or deaths don't have to be directly caused by the influenza; a positive influenza test is sufficient for reporting.

Adult and pediatric influenza hospitalizations and deaths

Detailed case-level information such as age, influenza type/subtype, gender, underlying medical conditions, vaccination status and treatment, etc. for influenza-associated hospitalizations, ICU admissions and in-patient deaths are reported on a weekly basis by sentinel hospital networks to the FluWatch program.

Reported hospital admissions and/or deaths are laboratory confirmed and are attributable to influenza infections; cases in which influenza is incidental, unrelated to the real reason for admission or nosocomial (hospital acquired) are excluded.

6. Influenza strain characterization and antiviral resistance testing

The Public Health Agency of Canada's National Microbiology Laboratory (NML) conducts genetic and antigenic characterization on a proportion of laboratory-confirmed detections. A temporal distribution of isolates are required to accurately characterize the season including those from the early part of the season, 10% of isolates during the period of active influenza circulation, and late-season isolates from specimens collected after the majority of influenza activity has passed.

The purpose of virus characterization is to:

Summarize the specific strains of influenza circulating in the Canadian population each season
Compare how similar the current circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines
Monitor evolutionary changes that continually occur in influenza viruses circulating in humans.

Influenza virus isolates are characterized by hemagglutination inhibition (H.I.) assay for comparison to the four WHO-recommended strains for the current season's vaccine. Genetic analysis is also conducted, comparing the sequence of the hemaggultinin (HA) gene to the WHO-recommended vaccine strains.

The NML also tests a proportion of influenza viruses for susceptibility to antivirals, specifically amantadine, oseltamivir and zanamivir.

7. Influenza vaccine monitoring

Influenza vaccine monitoring provides evidence to support public health interventions and their evaluation. Results of monitoring activities related to vaccine coverage and effectiveness are included in the FluWatch reports.

The Public Health Agency of Canada also keeps track of adverse events following immunization for all vaccines, including the influenza vaccine. These data are not included in the weekly FluWatch report; however, reports with this data can be found on the Canadian Adverse Events Following Immunization Surveillance System page.

Influenza vaccine coverage

The Public Health Agency of Canada conducts a National Influenza Immunization Coverage Survey to collect information on uptake of the influenza vaccine each season.

Vaccine effectiveness

Vaccine effectiveness is a measure of how well the influenza vaccine is able to prevent the influenza infections. Throughout the influenza season, surveillance networks estimate how well the influenza vaccine is working. Estimates can vary depending on the study methods used; the population, setting and outcomes that are being studied; the type and mix of vaccine products; the stage of the season and the kinds of influenza viruses that are circulating when the study is conducted. Estimates are usually available in February/March.

FluWatch surveillance indicators and interpretation notes

The FluWatch program reports on a core set of surveillance indicators in the weekly FluWatch report to meet FluWatch's three main objectives. The table below summarizes and defines the surveillance indicators reported by the FluWatch program.

Surveillance indicators reported by the FluWatch program
Component	Indicator	Operational definition
Virological	Weekly percentage of RT-PCR influenza tests positive	Numerator: weekly number of influenza detections. Denominator: total weekly number of influenza tests reported.
Virological	Counts of weekly and cumulative influenza detections by age group, type, and/or subtype.	N/A
Influenza/ILI activity levels	Weekly influenza/ILI activity level, based on activity within each influenza surveillance region over the preceding week.	No activity: no laboratory-confirmed influenza detections during reporting week. Sporadic: sporadic ILI cases and influenza detections, but no outbreaks. Localized: increased ILI cases, influenza detections, and outbreaks occurring in less than 50% of the surveillance region. Widespread: increased ILI cases, influenza detections, and outbreaks occurring in 50% or more of the surveillance region.
Syndromic	Weekly percentage of patients seen by primary healthcare providers with ILI.	ILI: acute onset of respiratory illness with fever and cough and one or more of sore throat, arthralgia, myalgia or prostration. In children under 5, gastrointestinal symptoms may also be present. In patients under 5 or 65 and older, fever may not be prominent. Numerator: weekly number of patients seen with ILI. Denominator: total weekly number of patients seen.
Syndromic	Weekly percentage of FluWatchers participants reporting cough and fever.	Numerator: weekly number of participants reporting cough and fever. Denominator: total weekly participants reporting.
Outbreaks	Number of weekly and cumulative laboratory-confirmed influenza outbreaks by setting.	Laboratory Confirmed Outbreak: two or more cases of ILI reported in the setting during a seven-day period with at least one case laboratory-confirmed as influenza. ILI Outbreak: 2 or more cases of ILI within 7 days. ILI Outbreak (Workplace, School/Daycare): Greater than 10% absenteeism that is likely due to ILI.
Severe outcomes	Weekly and cumulative influenza-associated hospitalization rates per 100,000 population.	Numerator: number of influenza-associated hospitalizations. Denominator: sum of population of reporting provinces and territories
	Counts of weekly and cumulative influenza-associated hospitalizations, ICU admissions and deaths.	N/A
	Counts of weekly and cumulative influenza-associated hospitalizations, ICU admissions, and deaths by age group, type and/or subtype.	N/A
Viral characterization	Cumulative counts and proportions of influenza isolates antigenically similar to the vaccine strains.	N/A
Viral characterization	Cumulative counts and proportions of influenza isolates susceptible to antivirals.	N/A
Vaccine monitoring	Estimated percentage of Canadian adults who received the seasonal influenza vaccine during the current influenza season. Sub analyses are performed by age group, with and without chronic conditions	Coverage is estimated via an annual survey. Numerator: number of surveyed adults who received the influenza vaccine Denominator: total number of adults surveyed
Vaccine monitoring	Effectiveness of seasonal influenza vaccination against laboratory-confirmed influenza-associated medically attended illness and hospitalization.	VE estimates are estimated by test negative design and calculated as: VE=100%×(1-O_pos/O_neg) where O_pos is the odds of vaccination among those testing positive for influenza and O_neg is the odds of vaccination among those testing negative.

Interpretation notes

Surveillance data reported to FluWatch is used to monitor trends in influenza activity throughout the surveillance season namely where, when and what influenza viruses are circulating as well as whether activity is increasing or decreasing. These data are an underestimate and are not intended to reflect the true number of influenza cases in Canada.
The data in the Fluwatch report represent surveillance data that are available at the time of writing. Surveillance partners are encouraged to update previously submitted data throughout the surveillance season. All data are preliminary and may change as more reports are received throughout the surveillance season.
The FluWatch program refers to time in surveillance weeks. This allows for the comparison of historical data (e.g. comparing data from week 45 in the 2021-22 season to week 45 of the 2019-2020 season). Data are aggregated by surveillance weeks that run from Sunday through Saturday.
The surveillance period for each influenza season begins in week 35 of a calendar year (end of August) and ends in week 34 of the following calendar year (end of August). A calendar of surveillance weeks for the current season is available online for reference.
The "influenza season" or the "seasonal influenza epidemic" is a period where there is elevated influenza activity. The start of influenza season is signaled when the percentage of tests positive for influenza crosses above 5%. During this period, usually from November to April, we expect to see the majority of influenza activity and the greatest impact of influenza on the health and wellbeing of Canadians.
Laboratory testing and surveillance practices have been impacted by the COVID-19 pandemic. Changes, especially to laboratory testing practices, may affect the comparability of the data from week 11 from the 2019-2020 season onwards to data from previous seasons.

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Date modified:: 2023-09-20

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