National Influenza Annual Report, Canada, 2022–2023: Canada’s first fall epidemic since the 2019–2020 season

Coinciding with the beginning of the coronavirus disease 2019 (COVID-19) pandemic in March 2020, Canadian seasonal influenza circulation was suppressed, which was a trend reported globally. Canada saw a brief and delayed return of community influenza circulation during the spring of the 2021–2022 influenza season. Surveillance for Canada’s 2022–2023 seasonal influenza epidemic began in epidemiological week 35 (week starting August 28, 2022) and ended in epidemiological week 34 (week ending August 26, 2023). The 2022–2023 season marked the return to pre-pandemic-like influenza circulation. The epidemic began in epidemiological week 43 (week ending October 29, 2022) and lasted 10 weeks. Driven by influenza A(H3N2), the epidemic was relatively early, extraordinary in intensity, and short in length. This season, a total of 74,344 laboratory-confirmed influenza detections were reported out of 1,188,962 total laboratory tests. A total of 93% of detections were influenza A (n=68,923). Influenza A(H3N2) accounted for 89% of the subtyped specimens (n=17,638/19,876). Late-season, Canada saw community circulation of influenza B for the first time since the 2019–2020 season. The 2022–2023 influenza season in Canada had an extraordinary impact on children and youth; nearly half (n=6,194/13,729, 45%) of reported influenza A(H3N2) detections were in the paediatric (younger than 19 years) population. Weekly paediatric influenza-associated hospital admissions were persistently above historical peak levels for several weeks. The total number of influenza-associated paediatric hospitalizations (n=1,792) far exceeded historical averages (n=1,091). With the return of seasonal influenza circulation and endemic co-circulation of multiple high burden respiratory viruses, sustained vigilance is warranted. Annual seasonal influenza vaccination is a key public health intervention available to protect Canadians.

Suppressed seasonal influenza activity in recent years, and resulting growing population susceptibility, has raised concerns about the timing, impact, and severity of re-emerging post-pandemic seasonal influenza epidemics (3,9,11)

Methods
FluWatch is Canada's long-standing influenza surveillance system, which monitors the spread of influenza and influenzalike illness (ILI) through core surveillance indicators based on global epidemiological standards (12).FluWatch is a composite surveillance system that consists of eight key areas: virological surveillance; geographic spread; syndromic surveillance; severe outcome surveillance; outbreak surveillance; influenza strain characterization; vaccination coverage; and vaccine effectiveness (13).Annually, influenza surveillance is conducted across Canada from epidemiological week 35 to week 34 of the following year.For the 2022-2023 Canadian influenza season, this surveillance period began on August 28, 2022, and ended on August 26, 2023.Detailed methods, including surveillance indicator definitions, data sources and statistical analyses, can be found on the Public Health Agency of Canada's FluWatch website (13).

Virological
The 2022-2023 national influenza epidemic began early in the season, exceeding the seasonal epidemic threshold (5% or more positive tests and 15 or more detections) in week 43 (late-October).For the second consecutive season, the Canadian influenza epidemic was brief in duration, lasting only 10 weeks, ending week 1 (early-January; Figure 1).Compared to pre-pandemic seasons, this tied the earliest start of an epidemic with the 2018-2019 season.The end of the season was unprecedentedly early, as pre-pandemic epidemics consistently ended around week 22 (late-May).
During the 2022-2023 Canadian influenza epidemic, influenza activity peaked in week 47 (late-November) at 24.3% tests positive.This was the first time since the declaration of the COVID-19 pandemic that peak activity approached peak levels observed in pre-pandemic seasons (average 31.3%).
During the 2022-2023 influenza season, a total of 74,344 laboratory-confirmed influenza detections were reported out of 1,188,962 total laboratory tests (Table 1).This is both the most detections and most tests ever recorded in a single season, as test counts have increased dramatically from pre-pandemic seasons (average of 276,592 tests and 47,018 detections from seasons 2014-2015 to 2018-2019).3).
Similar to last season, the age distribution of influenza A(H3N2) cases was much younger than pre-pandemic seasons.Nearly half of influenza A(H3N2) detections (45%) were among individuals aged 0-19 years compared to an average of 17% in pre-pandemic seasons (Table 2).
Conversely, influenza B detections were least common among individuals aged 65 years and older (5%; n=196) and 45-64 years (8%; n=327; Table 3).A similar case age distribution was observed in pre-pandemic seasons where influenza B Victoria lineage predominated over Yamagata lineage.In each of these seasons, cases occurred least frequently in these older age groups.
Influenza/influenza-like illness activity levels Sporadic influenza activity was reported by at least 10 reporting regions in each week of the 2022-2023 season.Localized activity was also reported by at least one reporting region in each week of the 2022-2023 influenza season.Coinciding with peak percent positivity observed in FluWatch's virological data, national influenza activity levels peaked between weeks 45 and 52 (early-November to late-December), where widespread activity was reported every week (Figure 4).No widespread influenza activity was reported after week 52 (late-December).The sharp decline at weeks 50 and 51 coincided with the holiday season, where data was not reported by many regions.Syndromic-sentinel primary healthcare provider influenza-like illness surveillance During the 2022-2023 influenza season, a weekly average of 3,144 patients were seen by a weekly average of 42 sentinel primary care providers.Both metrics were lower than the 2021-2022 season, where an average of 50 sentinel primary care providers saw a weekly average of 3,769 patients.
During that season, the weekly percentage of visits to primary care providers for ILI followed expected trends, ranging between 0.2% and 3.5% (Figure 5).The percentage of weekly visits for ILI remained within historical levels until week 45 (early-November), peaked in week 47 (late-November) at 3.5%, and remained above historical levels until week 51 (late-December).Influenza-like illness visits remained within or below historical levels for the remainder of the 2022-2023 season.Influenza-like illness visit trends coincided with increases in influenza activity and ultimately reflected the timing of the 2022-2023 influenza season.6).Levels gradually decreased and remained below expected levels until the end of the 2022-2023 season.Self-reported ILI did not increase significantly over the period of influenza B circulation.
The reports of ILI are not specific to any one respiratory pathogen and can be due to influenza or other respiratory viruses, including SARS-CoV-2.This makes the proportion of FluWatchers reporting ILI an important indicator of overall respiratory illness activity in the community.The percentage of FluWatchers reporting ILI captured trends in laboratory-confirmed respiratory virus detections, notably of SARS-CoV-2 and influenza.Increases in self-reported ILI tend to mirror increases in both SARS-CoV-2 percent positivity as well as influenza percent positivity (Figure 7).
The number of laboratory-confirmed outbreaks reported in a week peaked in week 49 (early-December; n=84), which coincided with the peak of the influenza season.
Severe outcomes-provincial/territorial severe outcome surveillance During the 2022-2023 season, 4,216 influenza-associated hospitalizations were reported by participating provinces and territories.Most hospitalizations were associated with influenza A (97%), and among hospitalizations with subtype information, 85% (n=1,804) were associated with influenza A(H3N2).
The annual seasonal hospitalization incidence for the 2022-2023 season was 49 hospitalizations per 100,000 population, which is within values recorded in previous seasons (Table 5).Among hospitalizations, heterogeneity existed between age groups.
The highest cumulative hospitalization rates were among children aged 0-4 years (131 per 100,000 population) and adults aged 65 years and older (131 per 100,000 population).These rates significantly exceeded both the cumulative rates among remaining age groups, a trend observed in last  season's predominant influenza A(H3N2) epidemic.However, in pre-pandemic seasons of predominant influenza A(H3N2) circulation, hospitalization rates were much higher among adults aged 65 years and older, relative to younger age groups.Influenza A accounted for the vast majority of hospitalizations.
When hospitalizations are broken down by type, the paediatric population (19 years and younger) accounted for 49% of hospitalizations associated with influenza B compared to 22% of hospitalizations associated with influenza A (Figure 8).
This season, 362 intensive care unit (ICU) admissions and 275 deaths were reported by participating provinces and territories.Intensive care unit admissions were most common among adults aged 65 years and older (32%) and 45-64 years of age (28%).Deaths were most common among adults aged 65 years and older (76%).The percentage of hospitalizations that resulted in ICU admissions was comparable to values reported in historical seasons (Table 6).

Severe outcomes-Canadian Immunization Monitoring Program, ACTive
The Canadian Immunization Monitoring Program, ACTive (IMPACT) network preliminarily reported 1,792 influenza-associated paediatric hospitalizations during the 2022-2023 influenza season, which was greater than historical seasons.From 2014-2015 to 2019-2020, an average of paediatric hospitalizations were reported, with 1,354 hospitalizations being the highest reported in a single season (2018-2019).
Weekly preliminary paediatric hospitalizations rapidly increased as of week 42 (mid-October) before reaching a peak in week 48 (early-December; n=242; Figure 9).This peak was early and of extraordinary intensity.Pre-pandemic (seasons 2014-2015 to 2019-2020), paediatric hospitalizations peaked no earlier than at week 52, at an average of 66 hospitalizations.Most hospitalizations (n=1,612, 90%) were associated with influenza A. Among hospitalizations for which influenza subtype was available, 94% (n=643) were associated with influenza A(H3N2).The overall age distribution of paediatric hospitalizations was not vastly different compared to previous seasons (Figure 10).However, for the first time over the last seven influenza epidemics, the proportion of hospitalized cases aged 2-4 years was highest, rather than their younger cohort younger than 2 years of age.The total number and the age distribution of paediatric influenza B-associated hospitalizations were within ranges seen in pre-pandemic seasons (Table 7).
This season, 283 ICU admissions and 10 deaths were reported.The highest proportion of ICU admissions was reported among cases aged 2-4 years (29%) and 10-16 years (22%).The percentage of paediatric hospitalizations that resulted in ICU admissions was comparable to values reported in historical seasons (Table 8).

Genetic characterization influenza A(H3N2)
Ten influenza A(H3N2) viruses did not grow to sufficient hemagglutination titers for antigenic characterization by hemagglutination inhibition (HI) assays.Therefore, the National Microbiology Laboratory performed genetic characterization to determine the genetic group identity of these viruses.Sequence analysis of the hemagglutinin (HA) genes of the viruses showed that they belonged to genetic group 3C.2a1b.2a2.The A/Darwin/6/2021 (H3N2)-like virus is an influenza A(H3N2) component of the 2022-2023 Northern Hemisphere influenza vaccine and belongs to genetic group 3C.2a1b.2a2.

Antiviral resistance
The 604 influenza viruses (383 A(H3N2), 106 A(H1N1) and 115 influenza B) were tested for antiviral resistance, with 100% of viruses sensitive to oseltamivir and zanamivir.

Vaccination coverage
Influenza vaccination coverage among all adults for the 2022-2023 influenza season (43%) was slightly higher than the previous season (39%).Among those at higher risk of complications from influenza (adults aged 65 years and older and adults aged 18-64 years with chronic medical conditions), vaccination coverage was 74% and 43% respectively, both similar to the previous season and below Canada's influenza vaccination coverage goal of 80% for those at higher risk (14).

Vaccine effectiveness
The Canadian Sentinel Practitioner Surveillance Network provides estimates of the effectiveness of the seasonal influenza vaccine in preventing medically attended illness due to laboratory-confirmed influenza among Canadians (15).Based on data collected between November 1, 2022, and January 6, 2023, vaccine effectiveness was estimated to be 54% against influenza A(H3N2

Discussion
The 2022-2023 influenza epidemic in Canada, driven by influenza A(H3N2), was early, intense, and had an extraordinary impact on children and adolescents (10).The national influenza epidemic began in week 43 (late-October), peaked rapidly in week 47 (late-November), and ended unprecedentedly early in week 1 (early-January).Early and intense activity with influenza A(H3N2) predominance was also seen in the United States and Europe this season and in regions of the Southern Hemisphere during their 2022 influenza season (16)(17)(18)(19).The intensity of this season's influenza epidemic coincided with unusually early respiratory syncytial virus (RSV) activity and ongoing SARS-CoV-2 circulation, which posed a threat to public health and increased pressures on the Canadian healthcare system.
The dominance of influenza A(H3N2) seen during the 2021-2022 Canadian influenza season continued into the 2022-2023 season.Similar to last season, several FluWatch indicators demonstrated that the paediatric population was atypically afflicted.For the second straight season, nearly half of influenza A(H3N2) cases were aged 0-19 years, more than double the average recorded in pre-pandemic years.Additionally, hospitalization rates were once again similar among children aged 0-4 years and adults aged 65 years and older, a distribution not observed during pre-pandemic influenza A(H3N2) predominant epidemics, where burden is typically highest in older adults.Perhaps most notable, the total number of influenza-associated paediatric hospitalizations preliminarily reported by IMPACT during the 2022-2023 influenza season greatly exceeded the total reported in any pre-pandemic season.Weekly paediatric influenza-associated hospitalization admissions were persistently higher than historical peak levels for several weeks during the 2022-2023 season.As was previously hypothesized, the atypical age distribution may reflect immunologic factors (9,10).A large, unexposed cohort of young children may have been more vulnerable to infection following the suppression of seasonal respiratory virus transmission across Canada in recent years.The percentage of hospitalizations in both paediatrics and adults that resulted in ICU admissions was within values previously reported, suggesting that despite the high number of hospitalizations this season, they were not necessarily more severe.
As the 2022-2023 influenza epidemic waned, so did the dominance of influenza A(H3N2), as increased detections of influenza A(H1N1) and influenza B were observed, which was a trend also seen in other Northern Hemisphere regions (17,20).The small wave of influenza B that occurred later in the season mirrored pre-pandemic patterns with its timing.The National Microbiology Laboratory characterized and classified all influenza B viruses as belonging to B/Victoria lineage.As of February 2023, it was reported by the World Health Organization that there had been no confirmed detections of circulating B/Yamagata lineage viruses since before April 2020 (21).
Historically, in Canada, the age distribution of influenza B cases has differed between influenza B/Victoria and influenza B/Yamagata predominant seasons.In pre-pandemic seasons, where influenza B/Victoria predominated, the majority of influenza B cases were younger than 45 years of age, while the opposite was true of influenza B/Yamagata predominant seasons.This trend has been reported elsewhere and was notable through the 2022-2023 influenza season in Canada, with 87% of influenza B cases younger than 45 years of age (22)(23)(24)(25).If influenza B/Yamagata community circulation does not return, there may be future implications for how populations are affected by influenza B.
Canada has not observed widespread circulation of influenza A(H1N1) since the 2019-2020 season, leaving a large unexposed cohort of the general population, especially new cohorts of children younger than four years.The 2023 summer saw waning dominance of influenza A(H3N2) globally, and a resurgence of influenza A(H1N1) activity in the upcoming season is possible.However, an abundance of factors can influence influenza activity and severity: antigenic drift, co-circulation of other respiratory viruses, vaccination coverage, vaccine effectiveness, antiviral use, population imprinting, cohort effects, and contextual factors (25)(26)(27)(28)(29)(30)(31)(32)(33).
Though the younger cohort was unusually impacted during the past two influenza epidemics in Canada (9,10), adults with chronic health conditions and older adults remain at high risk of severe outcomes.With endemic co-circulation of multiple high burden respiratory viruses impacting all age groups (influenza, SARS-CoV-2, RSV), and potential emergence of non-seasonal respiratory viruses, the importance of respiratory virus surveillance in Canada is highlighted.Predicting influenza activity is notoriously difficult, and this can be mitigated with comprehensive surveillance activities and the use of historical data and trends to determine likely outcomes to in-season observations.Sustained vigilance and integrated planning approaches for upcoming predictably unpredictable respiratory virus seasons, in the context of a strained healthcare system, are essential (3,29).
Influenza can cause severe illness across all age groups, with or without chronic health conditions (25).Certain populations, such as young children, older adults, individuals with chronic health conditions, residents of LTCF and chronic care facilities, pregnant individuals, and Indigenous peoples are at greater risk of serious complications or worsening of underlying health conditions (34).Annual influenza vaccination remains a critical tool for the prevention of influenza and its complications, and reduced transmissibility to others.
Ongoing and timely surveillance plays a critical role in the Public Health Agency of Canada's ability to respond to influenza and other respiratory virus trends, monitor changes in circulation patterns, CCDR • October 2023 • Vol.49 No. 10 and effectively prepare and plan for mitigation measures within the influenza season.This surveillance report summarizes trends observed during the 2022-2023 influenza season in Canada through analysis of FluWatch core indicators reported by the Public Health Agency of Canada from August 28, 2022 (epidemiological week 35) to August 26, 2023 (epidemiological week 34).

Figure 7 :
Figure 7: Percentage of influenza and SARS-CoV-2 laboratory tests positive and percentage of FluWatchers reporting cough and fever in Canada by surveillance week, 2022-2023 influenza season years 5−19 years 20−44 years 45−64 years 65+ years

Figure 8 :
Figure 8: Age distribution of hospitalizations by influenza type in Canada, 2022-2023 influenza season

Figure 10 :
Figure 10: Age distribution of paediatric hospitalizations in Canada reported by IMPACT, seasons 2014-2015 to 2022-2023

Table 1 :
Number of laboratory tests, detections, and percentage positivity by influenza season, seasons 2014-2015 to 2022-2023, Canada a The shaded area represents the maximum and minimum percentage of tests positive reported by week from seasons 2014-2015 to week 11 of 2019-2020.The epidemic threshold is 5% tests positive for influenza.When it is exceeded, and a minimum of 15 weekly influenza detections are reported, a seasonal influenza epidemic is declared Detailed information on age and influenza type/subtype was received for 54,096 laboratory-confirmed influenza detections.Influenza A detections were most common among individuals aged 65 years and older (27%; n=13,433), followed by individuals aged 5-19 years (22%; n=11,215).During the 2022-2023 epidemic, the increase in cases in this younger age group preceded increases in all other age groups (Figure

Table 2 :
Number and percentage of seasonal influenza A(H3N2) detections by influenza season, by age group, seasons 2014-2015 to 2022-2023, Canada The 2020-2021 season was excluded from Table 2 as <5 total influenza A(H3N2) cases with age information were reported b During the 2019-2020 season, case data from one jurisdiction used 20-64-year age group instead of 20-44 and 45-64.These cases have been omitted from the age group-specific case counts but are included in the total case counts Abbreviation: N/A, not applicable a

Table 3 :
Number and percentage of seasonal influenza B detections by influenza season, by age group, seasons 2014-2015 to 2022-2023, Canada Abbreviation: N/A, not applicable a The 2020-2021 season was excluded from Table3as <5 total influenza B cases with age information were reported b Predominant lineage was determined by influenza B specimen antigenic characterization performed by the National Microbiology Laboratory.In each season, >75% of characterized specimens belonged to either Victoria or Yamagata lineage with predominance attributed to the lineage exceeding this threshold c During the 2019-2020 season, case data from one jurisdiction used 20-64 years age group instead of 20-44 and 45-64.These cases have been omitted from the age group-specific case counts but are included in the total case counts d There were no influenza B specimens received and characterized by the National Microbiology Laboratory during the 2021-2022 season, therefore no predominant lineage could be assigned During the 2022-2023 season, an average of 10,142 FluWatchers reported each week, with a total of 15,755 FluWatchers participating over the season and a total of 527,363 questionnaires submitted.The percentage of FluWatchers reporting ILI symptoms (acute onset of cough and fever) surpassed historical levels in week 42 (mid-October), peaked in week 47 (late-November) at 3.1%, and remained above historical levels until week 48 (early-December; Figure Figure 5: Percentage of visits for influenza-like illness reported by sentinel primary care providers in Canada by season and surveillance week a Abbreviation: ILI, influenza-like illness a The shaded area represents the maximum and minimum percentage of visits for ILI reported by week from seasons 2014-2015 to week 11 of 2019-2020

Table 4 :
Number and percentage of laboratory-confirmed influenza outbreaks in Canada by setting and season, seasons 2018-2019 to 2020-2023

Table 5 :
Estimated annual seasonal incidence of influenza hospitalizations, per 100,000 population, in Canada by season and age group, seasons 2014-2015 to 2022-2023 a a The 2020-2021 season was excluded from Table5as no influenza hospitalizations were reported b The shaded cells highlight the age group with the highest estimated incidence of influenza hospitalizations for the respective season

Table 6 :
Percentage of hospitalizations that resulted in intensive care unit admissions in Canada by season and age group, seasons 2014-2015 to 2022-2023 a

Table 7 :
Number and percentage of paediatric influenza B-associated hospitalizations in Canada reported by IMPACT by age group, seasons 2014-2015 to 2022-2023 a Abbreviation: IMPACT, Canadian Immunization Monitoring Program, ACTive a The 2020-2021 and 2021-2022 seasons were excluded fromTable 7 comparison as no influenza hospitalizations and <5 influenza hospitalizations were reported, respectively CCDR • October 2023 • Vol.49 No. 10

Table 8 :
Percentage of paediatric hospitalizations that resulted in intensive care unit admissions in Canada reported by IMPACT by age group, seasons 2014-2015 to 2022-2023 a The 2020-2021 season was excluded from Table 8 as no influenza hospitalizations were reported a