National influenza mid-season report, 2022-2023: A rapid and early epidemic onset.

Canada's 2022-2023 national influenza epidemic was declared in epidemiological week 43 (week ending October 29, 2022), relatively early in comparison to historical seasons. This year marks the return to pre-pandemic-like influenza circulation, following the brief and delayed influenza epidemic declared in the spring of the 2021-2022 season. To date this season, 59,459 detections of influenza have been reported out of 456,536 tests; both values exceeding historical averages. This epidemic is being fundamentally driven by influenza A, with influenza A(H3N2) accounting for 94% of subtyped detections. This season to date has had a significant impact on adolescents and young children, with a high proportion of detections occurring in those aged 0-19 years (42%). Provinces and territories have reported higher than usual influenza-associated hospitalizations, intensive care unit admissions, and deaths in comparison with previous seasons; in particular, paediatric hospitalization incidence was persistently far above historical peak levels for several weeks. The return of seasonal influenza circulation highlights the importance of sustained vigilance with regard to influenza and employment of available mitigation measures, especially of annual seasonal influenza vaccination.

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.

National FluWatch mid-season report, 2021-2022: Sporadic influenza activity returns.

Surveillance for Canada's 2021-2022 seasonal influenza epidemic began in epidemiological week 35 (the week starting August 29, 2021) during the ongoing coronavirus disease 2019 (COVID-19) global public health emergency. In the 2021-2022 surveillance season to date, there has been a return of persistent sporadic influenza activity, and the first influenza-associated hospitalizations since mid-2020 have been reported. However, as of week 52 (week ending 01/01/2022) activity has remained sporadic, and no influenza-confirmed outbreaks or epidemic activity have been detected. There has been a delay or absence in several traditional seasonal influenza milestones, including the declared start of the influenza season, marked by a threshold of 5% positivity, which historically has occurred on average in week 47. The 429 sporadic detections reported in Canada to date have occurred in 31 regions across seven provinces/territories. Nearly half (n=155/335, 46.3%) of reported cases have been in the paediatric (younger than 19 years) population. Three-quarters of the cases were influenza A detections (n=323/429, 75.3%). Of the subtyped influenza A detections, A(H3N2) predominated (n=83/86, 96.5%). Of the 12 viruses characterized by the National Microbiology Laboratory, 11 were seasonal strains. Among the seasonal strains characterized, only one was antigenically similar to the strains recommended for the 2021-2022 Northern Hemisphere vaccine, though all were sensitive to the antivirals, oseltamivir and zanamivir. Until very recently, seasonal influenza epidemics had not been reported since March 2020. Evidence on the re-emergence of seasonal influenza strains in Canada following the A(H1N1)pdm09 pandemic shows that influenza A(H3N2) and B epidemics ceased through the 2009-2010 season and second wave of A(H1N1)pdm09, but then re-emerged in subsequent seasons to predominate causing epidemics of higher intensity than in the pre-pandemic seasons. When and where seasonal influenza epidemic activity resumes cannot be predicted, but model-based estimates and historical post-pandemic patterns of intensified epidemics warrant continued vigilance through the usual season and for out-of-season re-emergence. In addition, ongoing population preparedness measures, such as annual influenza vaccination to mitigate the intensity and burden of future seasonal influenza epidemic waves, should continue.

Implications of the unexpected persistence of human rhinovirus/enterovirus during the COVID-19 pandemic in Canada.

Stringent public health measures imposed across Canada to control the COVID-19 pandemic have nearly suppressed most seasonal respiratory viruses, with the notable exception of human rhinovirus/enterovirus (hRV/EV). Thanks to this unexpected persistence, we highlight that hRV/EV could serve as a sentinel for levels of contact rate in populations to inform on the efficiency, or the need of, public health measures to control the subsequent COVID-19 epidemic, but also for future epidemics from other seasonal or emerging respiratory pathogens.

National Influenza Annual Report, Canada, 2021-2022: A brief, late influenza epidemic.

Canadian seasonal influenza circulation had been suppressed since the beginning of the coronavirus disease 2019 (COVID-19) pandemic. This suppression was reported globally and generated concern that the return of community influenza circulation could be intense and that co-circulation of influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was possible and potentially severe. Community circulation of influenza returned to Canada during the 2021-2022 influenza season. The influenza epidemic began in week 16 (mid-April 2022) and lasted only nine weeks. This epidemic was driven by influenza A(H3N2) and was exceptionally late in the season, low in intensity and short in length. Community co-circulation of influenza and SARS-CoV-2 was observed in Canada for the first time during the 2021-2022 seasonal influenza epidemic. The unusual characteristics of the 2021-2022 influenza epidemic suggest that a breadth of factors moderate transmission dynamics of the two viruses. Concerns of an intense seasonal influenza epidemic did not come to fruition during the 2021-2022 season; therefore, high influenza susceptibility remains, as does predisposition to larger influenza epidemics. Ongoing circulation of SARS-CoV-2 creates uncertainty about dynamics of future influenza epidemics, but influenza vaccination remains a key public health intervention available to protect Canadians. Public health authorities need to remain vigilant, maintain surveillance and continue to plan for both heightened seasonal influenza circulation and for the potential for endemic co-circulation of influenza and SARS-CoV-2.

National Influenza Annual Report, Canada, 2020-2021, in the global context.

During the 2020-2021 Canadian influenza season, no community circulation of influenza occurred. Only 69 positive detections of influenza were reported, and influenza percent positivity did not exceed 0.1%. Influenza indicators were at historical lows compared with the previous six seasons, with no laboratory-confirmed influenza outbreaks or severe outcomes being reported by any of the provinces and territories. Globally, influenza circulation was at historically low levels in both the Northern and the Southern Hemispheres. The decreased influenza activity seen in Canada and globally is concurrent with the implementation of non-pharmaceutical public health measures to mitigate the spread of the coronavirus disease 2019 (COVID-19). Although it is difficult to predict when influenza will begin to re-circulate, given the increased COVID-19 vaccination and the relaxation of public health measures, an influenza resurgence can be expected and may be more severe or intense than recent seasons. Influenza vaccination, along with non-pharmaceutical public health measures, continues to remain the best method to prevent the spread and impact of influenza. Public health authorities need to remain vigilant, maintain surveillance and continue to plan for heightened seasonal influenza circulation.

The impact of the COVID-19 pandemic on influenza, respiratory syncytial virus, and other seasonal respiratory virus circulation in Canada: A population-based study.

BACKGROUND: The ongoing coronavirus disease 2019 (COVID-19) pandemic has resulted in implementation of public health measures worldwide to mitigate disease spread, including; travel restrictions, lockdowns, messaging on handwashing, use of face coverings and physical distancing. As the pandemic progresses, exceptional decreases in seasonal respiratory viruses are increasingly reported. We aimed to evaluate the impact of the pandemic on laboratory confirmed detection of seasonal non-SARS-CoV-2 respiratory viruses in Canada. METHODS: Epidemiologic data were obtained from the Canadian Respiratory Virus Detection Surveillance System. Weekly data from the week ending 30th August 2014 until the week ending the 13th March 2021 were analysed. We compared trends in laboratory detection and test volumes during the 2020/2021 season with pre-pandemic seasons from 2014 to 2019. FINDINGS: We observed a dramatically lower percentage of tests positive for all seasonal respiratory viruses during 2020-2021 compared to pre-pandemic seasons. For influenza A and B the percent positive decreased to 0•0015 and 0•0028 times that of pre-pandemic levels respectively and for RSV, the percent positive dropped to 0•0169 times that of pre-pandemic levels. Ongoing detection of enterovirus/rhinovirus occurred, with regional variation in the epidemic patterns and intensity. INTERPRETATION: We report an effective absence of the annual seasonal epidemic of most seasonal respiratory viruses in 2020/2021. This dramatic decrease is likely related to implementation of multi-layered public health measures during the pandemic. The impact of such measures may have relevance for public health practice in mitigating seasonal respiratory virus epidemics and for informing responses to future respiratory virus pandemics. FUNDING: No additional funding source was required for this study.

National influenza mid-season report, 2020-2021.

Canada's national influenza season typically starts in the latter half of November (week 47) and is defined as the week when at least 5% of influenza tests are positive and a minimum of 15 positive tests are observed. As of December 12, 2020 (week 50), the 2020-2021 influenza season had not begun. Only 47 laboratory-confirmed influenza detections were reported from August 23 to December 12, 2020; an unprecedentedly low number, despite higher than usual levels of influenza testing. Of this small number of detections, 64% were influenza A and 36% were influenza B. Influenza activity in Canada was at historically low levels compared with the previous five seasons. Provinces and territories reported no influenza-associated adult hospitalizations. Fewer than five hospitalizations were reported by the paediatric sentinel hospitalization network. With little influenza circulating, the National Microbiology Laboratory had not yet received samples of influenza viruses collected during the 2020-2021 season for strain characterization or antiviral resistance testing. The assessment of influenza vaccine effectiveness, typically available in mid-March, is expected to be similarly limited if low seasonal influenza circulation persists. Nevertheless, Canada's influenza surveillance system remains robust and has pivoted its syndromic, virologic and severe outcomes system components to support coronavirus disease 2019 (COVID-19) surveillance. Despite the COVID-19 pandemic, the threat of influenza epidemics and pandemics persists. It is imperative 1) to maintain surveillance of influenza, 2) to remain alert to unusual or unexpected events and 3) to be prepared to mitigate influenza epidemics when they resurge.