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

Abstract 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 circulation of influenza, respiratory syncytial virus and other seasonal respiratory viruses in Canada. Methods Epidemiologic data were obtained from the Canadian Respiratory Virus Detection Surveillance System. Weekly data from the week ending 30 th August 2014 until the week ending the 13 th February 2021 were analysed. We compared trends in laboratory detection and test volumes during the 2020/2021 influenza season with baseline 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 baseline. For influenza A and B the percent positive decreased to 0·0017 and 0·0061 times that of baseline respectively and for RSV, the percent positive dropped to 0·0145 times that of baseline. 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. Research in context Evidence before this study We searched PubMed, preprint servers and country-specific public health rapid communications to identify surveillance and epidemiological studies on influenza, respiratory syncytial virus and other seasonal respiratory virus detection during the COVID-19 pandemic. A number of regional and national studies were identified worldwide. The majority of these studies focus on influenza epidemiology and all studies show consistent decreases in circulation of seasonal non-SARS-CoV-2 respiratory viruses. One previous study on the impact of non-pharmaceutical interventions on laboratory detections of influenza A and B in Canada included data for the 2019/2020 influenza season. Another recent study examined the effect of seasonal respiratory virus transmission on COVID-19 syndromic surveillance in the province of Ontario, Canada. No previous Canada-wide study has described the epidemiology of influenza, respiratory syncytial virus and other seasonal respiratory virus detection during the 2020/2021 influenza season. Added value of this study The Canadian Respiratory Virus Detection Surveillance System provides weekly respiratory virus detection reports from sentinel laboratories across Canada for influenza, respiratory syncytial virus, parainfluenza viruses, adenovirus, human metapneumovirus, enterovirus/rhinovirus and seasonal coronaviruses. Data have been collected continuously since 2004. Analysis of this dataset provides a comprehensive assessment of the impact of the COVID-19 pandemic on circulation of seasonal respiratory viruses in Canada and analysis of data from the Canadian Public Health Infobase on COVID-19 allowed comparison of SARS-CoV-2 epidemiology. This is the first country-wide study in the Northern hemisphere to describe the concurrent epidemiology of all major seasonal respiratory viruses and SARS-CoV-2 during the 2020/2021 influenza season. Implications of all the available evidence The effective absence of the annual seasonal epidemic for most non-SARS-CoV-2 respiratory viruses in 2020/2021 has important public health implications for informing ongoing and future responses to respiratory virus epidemics and pandemics.