Seasonal respiratory virus circulation was diminished during the COVID‐19 pandemic

Abstract Background Measures introduced during the COVID‐19 pandemic intended to address the spread of SARS‐CoV‐2 may also influence the incidence of other common seasonal respiratory viruses (SRV). This evaluation reports laboratory‐confirmed cases of common SRV in a well‐defined region of central Canada to address this issue. Methods Surveillance data for common non‐SARS‐CoV‐2 SRV in Ottawa, Canada, was provided by the Eastern Ontario Regional Laboratory Association (EORLA) reference virology lab. Weekly reports of the number of positive tests and the proportion that yielded positive results were analyzed from August 26, 2018, to January 2, 2022. Results A drastic reduction in influenza and other common SRV was observed during the 2020–2021 influenza season in the Ottawa region. Influenza was virtually undetected post‐SARS‐CoV‐2 emergence. Rhinoviruses and enteroviruses were the only viruses that remained relatively unaffected during this period. Conclusions We speculated that the introduction of nonpharmaceutical measures including masking to prevent SARS‐CoV‐2 transmission contributed to the near absence of SRV in the Ottawa region. These measures should remain a key component in addressing spikes in SRV activity and future pandemics.


| INTRODUCTION
Since emerging in late 2019, SARS-CoV-2 has caused unprecedented morbidity and mortality worldwide. Public health policymakers have implemented nonpharmaceutical public health measures to mitigate SARS-CoV-2 spread including the use of face coverings, targeted lockdowns and stay-at-home orders, physical distancing, school closures, restricted international travel, mandated quarantine, and reduced close contacts. In Ontario, most of these measures were put into place from March 12 to 30, 2020. Testing regulations and access to testing varied by stage of the pandemic and availability of tests.
Influenza, RSV, and other common respiratory viruses typically follow predictable seasonal patterns with high activity levels in the winter months. Australia and New Zealand saw a 98% decrease in influenza incidence during the 2020 peak influenza or "flu" season.
Reduced activity was also reported for common respiratory illnesses such as the respiratory syncytial virus (RSV); seasonal human coronaviruses (HCoV); parainfluenza virus types 1, 2, 3, and 4 (PIVs); and human metapneumovirus (HMPV). Irregular seasonal behavior was observed in enteroviruses (EV), adenoviruses (AV), and rhinoviruses (RV) throughout the Southern Hemisphere. 1,2 Similar patterns were observed throughout the winter influenza season in the Northern Hemisphere. Minimal influenza and RSV activity were reported during peak seasons in the UK and Europe, where most respiratory illnessrelated healthcare visits were accredited to COVID-19. 3 Surveillance data from the United States showed that influenza and RSV were circulating at historically low levels from March 2020 to May 2021.
HCoV, PIV, and HMPV activity decreased in March 2020 and remained low through May 2021, after which HCoV and PIV rose to prepandemic levels. The increase in HCoV, RSV, and PIV in the spring of 2021 was inconsistent with historical seasonal peaks usually observed in the winter months. RV and EV activity decreased in March 2020 but returned to prepandemic levels in May 2020. 4

| Data sources
Specimens were collected from the Champlain region population and submitted to the Eastern Ontario Regional Laboratory Association (EORLA) regional reference virology lab based at CHEO. These data are in the public domain and circulated weekly to several sources including the Public Health Agency of Canada (PHAC) as part of the Respiratory Virus Detection Surveillance System and FluWatch report. 7 All data included in this analysis were obtained from publicly available de-identified datasets. As such, ethics board and laboratory approval were not required.  T A B L E 1 Types of tests used to measure the seroprevalence of each respiratory virus. Location of where testing was performed within the catchment area and contextual factors and restrictions in place that may have impacted the measured seroprevalence of SARS-CoV-2 and common respiratory viruses in the Ottawa region.  ; 2c) ; 2d) ; 2e) ; 2f) ; 2g) ; 2h) years to come. Early exposure and frequent stimulations early in life train the immune system to effectively respond to threats and have been linked to a fortified immune response in adulthood. Lack of immune stimulation caused by nonpharmaceutical public health measures over the past 2 years may result in immune deficits in children with negative consequences when the pandemic resolves. 30 Opening international borders and increasing human mobility may also stimulate viral circulation as influenza transmission and seasonality have been partially attributed to international travel. 31  More detailed information on contextual factors that may have influenced the number of tests completed can be found in Table 1. Others may have been hesitant or incapable of accessing usual care or testing services amid lockdowns and COVID-19 precautions. CHEO received 34% more specimens in the 2020-2021 influenza season compared to the 2018-2019 influenza season, so it is unlikely that testing capacity largely underestimated viral activity for influenza A, influenza B, or RSV. No seasonal spikes in influenza incidence, RSV hospital, and ICU admissions were observed, which also supports the proposal that findings truly represent reduced community circulation.

| Statistical methods
Relatedly, Australia had more liberal testing criteria than Canada and still found only 33 positive influenza tests out of 60,031 tested specimens during peak season. 2 Another limitation was that the influenza vaccination rate was not measured in this evaluation. Findings from the 2020-2021 Canadian Seasonal Influenza Vaccination Coverage Survey indicated that influenza vaccination coverage in the 2020-2021 season (40%) was slightly reduced but comparable to the 2019-2020 season (42%) and 2018-2019 season (42%). 32 It is likely the rates were similar in this evaluation. However, as this variable was not measured, the exact proportion of the population vulnerable to infection is unknown and may have affected reported influenza infection rates. Finally, the surveillance data used in this analysis may not be fully representative of the Ottawa population at risk for RV infection as not all viral testing occurred at the CHEO virology laboratory.
Individuals tested for SRVs through the Public Health Ontario Laboratory were not captured in these data.

| CONCLUSION
This study describes a near absence of seasonal respiratory viruses in the Ottawa region for the 2020/2021 respiratory virus season.
Findings from this study add to a growing body of evidence that suggests that public health measures introduced to prevent the spread of SARS-CoV-2 have also reduced laboratory-confirmed seasonal respiratory virus infections. These measures should continue to be strategically utilized to prevent, or at least diminish, the burden of future seasonal respiratory virus spikes and in response to further pandemics.

CONFLICTS OF INTEREST
The authors declare no conflicts of interest.

ETHICAL APPROVAL
All data included in this analysis were obtained from publicly available de-identified datasets; therefore, ethical approval or lab approval was not required. writing-review and editing.

PEER REVIEW
The peer review history for this article is available at https://publons. com/publon/10.1111/irv.13065.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available on request from the corresponding author.