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

Abstract

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.

Keywords: : influenza; Canada; human/epidemiology; human/virology; influenza A virus; influenza B virus; non-pharmaceutical interventions; seasons; surveillance.

Figure 1

Number of regions reporting sporadic, localized or widespread influenza activity, by epidemiological week, Canada, weeks 35-2021 to 52-2021^{a a} Total number of regions reporting: 52

Figure 2

Percentage of influenza tests positive, by report week, Canada, 2021–2022 influenza season to date (weeks 35 to 52) compared with historical average and minimum-maximum (seasons 2014–2015 to 2019–2020)

Figure 3

Spatial distribution of (sporadic) influenza detections (n=429) by influenza type/subtype, Canada, weeks 35-2021 to 52-2021

Figure 4

Seasonal epidemics of influenza by type or subtype before, during and following the 2009 H1N1 pandemic, Canada, weeks 35-2007 to 52-2021 Note: Grey area denotes the period of the 2009 H1N1 pandemic (June 11, 2009 to August 11, 2010), and of the COVID-19 pandemic (March 11, 2020 and ongoing as of time of publication), as declared by the World Health Organization (16,17)