The impact of seasonal respiratory virus transmission on syndromic surveillance for COVID-19 in Ontario, Canada

  1. Arjuna Maharaj
  2. Jennifer Parker
  3. Jessica P. Hopkins
  4. Effie Gournis
  5. Isaac I. Bogoch
  6. Benjamin Rader
  7. Christina M. Astley
  8. Noah Ivers
  9. Jared B. Hawkins
  10. Ashleigh R. Tuite
  11. David N. Fisman
  12. John S. Brownstein
  13. Lauren Lapointe-Shaw
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Abstract

Background

Syndromic surveillance systems for COVID-19 are being increasingly used to track and predict outbreaks of confirmed cases. Seasonal circulating respiratory viruses share syndromic overlap with COVID-19, and it is unknown how they will impact the performance of syndromic surveillance tools. Here we investigated the role of non-SARS-CoV-2 respiratory virus test positivity on COVID-19 two independent syndromic surveillance systems in Ontario, Canada.

Methods

We compared the weekly number of reported COVID-19 cases reported in the province of Ontario against two syndromic surveillance metrics: 1) the proportion of respondents with a self-reported COVID-like illness (CLI) from COVID Near You (CNY) and 2) the proportion of emergency department visits for upper respiratory conditions from the Acute Care Enhanced Surveillance (ACES) system. Separately, we plotted the percent positivity for other seasonal respiratory viruses over the same time period and reported Pearson’s correlation coefficients before and after the uncoupling of syndromic tools to COVID-19 cases.

Results

There were strong positive correlations of both CLI and ED visits for upper respiratory causes with COVID-19 cases up to and including a rise in entero/rhinovirus (r = 0.86 and 0.87, respectively). There was a strong negative correlation of both CLI and ED visits for upper respiratory causes with COVID-19 cases (r = −0.85 and −0.91, respectively) during a fall in entero/rhinovirus.

Interpretation

Two methods of syndromic surveillance showed strong positive correlations with COVID-19 confirmed case counts before and during a rise in circulating entero/rhinovirus. However, as positivity for enterovirus/rhinovirus fell in late September 2020, syndromic signals became uncoupled from COVID-19 cases and instead tracked the fall in entero/rhinovirus. This finding provides proof-of-principle that regional transmission of seasonal respiratory viruses may complicate the interpretation of COVID-19 surveillance data. It is imperative that surveillance systems incorporate other respiratory virus testing data in order to more accurately track and forecast COVID-19 disease activity.

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