Mapping the Triple Threat: How COVID-19, Influenza and RSV Swept Through Alberta’s Emergency Departments

When three respiratory viruses circulate at once, emergency departments become the front line where the collision becomes visible. A new retrospective population-based study from Alberta, Canada, has now mapped exactly when and where patients arrived at hospital emergency departments with COVID-19, influenza, and respiratory syncytial virus (RSV) across two full years of the pandemic era, revealing striking patterns in both time and space that could reshape how public health officials prepare for future viral surges. The research, led by Rhonda J. Rosychuk of the University of Alberta and Brian H. Rowe of the University of Alberta’s Department of Emergency Medicine, was published in BMC Infectious Diseases and draws on comprehensive administrative health data covering the entire province.

The study period ran from April 2021 to March 2023, a window that captured a remarkable epidemiological transition. During the first year, COVID-19 overwhelmingly dominated emergency department presentations, while influenza and RSV were nearly non-existent. This suppression of the classic seasonal viruses was a well-documented side effect of the aggressive public health measures deployed during the COVID-19 pandemic, including physical distancing, masking requirements, and travel restrictions, which interrupted the usual transmission cycles of influenza and RSV along with the novel coronavirus. But the second year of the study told a very different story: COVID-19 presentations declined while influenza and RSV returned, creating new patterns of viral pressure on hospital services.

In total, the researchers identified 74,277 emergency department presentations for COVID-19, dwarfing the 1,914 presentations for influenza and 2,038 for RSV recorded over the same two-year span. These numbers, extracted from the National Ambulatory Care Reporting System using diagnostic codes from the Canadian modification of the International Statistical Classification of Diseases, represent every patient in Alberta who arrived at an emergency department with one of these three viral infections as the reason for the visit. The sheer scale of the COVID-19 burden relative to the other two viruses underscores how profoundly the pandemic reshaped emergency care demand, even in a province with a relatively dispersed and suburban population distribution.

The methodological heart of the study lies in its use of retrospective space-time scan statistics, a sophisticated analytical technique originally developed for cancer cluster detection and now widely applied to infectious disease surveillance. Rather than simply counting cases by calendar month or by health region, the space-time scan statistic moves a cylindrical window across the province, varying both its geographic footprint and its temporal duration, and asks whether the number of cases inside the window exceeds what would be expected by chance. The analysis was repeated for each virus individually and then for all three viruses collectively in a multivariate framework, allowing the researchers to detect not only where and when each virus surged, but where multiple viruses surged together.

Geographically, the study divided Alberta into sub-Regional Health Authority areas, providing a finer spatial resolution than provincial or even regional-level reporting typically offers. This granularity matters because respiratory virus transmission is rarely uniform across a large jurisdiction. Urban centers such as Calgary and Edmonton, with their dense populations and major hospitals, can experience different epidemic dynamics than smaller communities in the province’s rural south and north. By aggregating emergency department visits to these sub-regional units and analyzing them with the scan statistic, the researchers could pinpoint localized hotspots of viral activity that would otherwise be masked in province-wide averages, offering a template for other jurisdictions seeking to understand the local texture of respiratory disease burden.

The multivariate analysis identified four statistically significant space-time clusters of emergency department presentations across the three viruses combined. Most notably, three of these clusters exhibited elevated numbers of presentations for both COVID-19 and RSV simultaneously, a pattern the authors describe as “twin-demic” clusters. The term twin-demic entered popular usage during the pandemic to describe the feared scenario in which COVID-19 and influenza would surge together and overwhelm hospitals. In Alberta’s emergency departments, however, the actual pairing that materialized most often was COVID-19 alongside RSV, the virus best known for causing bronchiolitis in infants but capable of severe disease in older adults as well. This finding suggests that the practical co-circulation threat to emergency services may differ from the one that dominated headlines.

The temporal structure of the clusters also carries important lessons. The early study period, when COVID-19 presentations were abundant and the other two viruses were nearly absent, reflects the tail end of the period in which non-pharmaceutical interventions suppressed seasonal respiratory pathogens. As those measures eased and population immunity evolved, influenza and RSV resumed their circulation, and their return did not simply fill the gap left by declining COVID-19 presentations. Instead, the viruses overlapped in specific places and specific windows of time, producing concentrated bursts of demand on emergency care. For hospital planners, the implication is that respiratory season can no longer be modeled as a single-virus phenomenon; capacity planning must account for the possibility of simultaneous, geographically concentrated surges of multiple pathogens.

The study’s reliance on administrative data brings both strengths and limitations worth understanding. On the strength side, the dataset captures the full population of Alberta rather than a sample, and emergency department presentations are a robust, consistently recorded indicator of severe respiratory illness that does not depend on testing availability or voluntary reporting. Diagnostic coding in the emergency setting, however, depends on the clinician’s documentation and the coding practices applied afterward, and patients with mild illness who never sought emergency care are invisible to this approach. The researchers also note that the study period, beginning in April 2021, does not capture the earliest and most disruptive waves of the COVID-19 pandemic, meaning the observed COVID-19 burden likely understates the full pandemic impact on Alberta’s emergency departments.

Beyond its specific findings, the research demonstrates the value of space-time surveillance as a public health tool. Traditional surveillance systems report case counts by jurisdiction and week, which is useful for tracking trends but can miss the localized, time-limited clustering that actually stresses a particular hospital or group of hospitals. Scan statistic methods, applied routinely to emergency department data, could give public health officials earlier and more precise warnings about where to deploy staff, open overflow capacity, or target vaccination and communication campaigns. The authors emphasize that their findings, combined with ongoing surveillance programs, can help inform public health planning, and the approach is readily transferable to other provinces and countries with comparable administrative data infrastructure.

As respiratory viruses continue to evolve and circulate in increasingly unpredictable patterns, the Alberta study offers a clear-eyed picture of what the post-pandemic normal may look like: not a single dominant pathogen, but an overlapping mosaic of COVID-19, influenza, and RSV, each with its own seasonality and geography, converging on emergency departments in ways that vary from one community to the next. The identification of COVID-19 and RSV twin-demic clusters is a reminder that the viruses that matter most for hospital capacity may not always be the ones that attract the most attention. For emergency physicians, epidemiologists, and the officials who allocate health resources, the message of this research is that watching the map, not just the calendar, may be the key to staying ahead of the next respiratory season.

Subject of Research: Spatiotemporal clustering of COVID-19, influenza, and RSV emergency department presentations in Alberta, Canada

Article Title: Spatiotemporal variations of COVID-19, influenza, and respiratory syncytial virus (RSV) presentations to emergency departments in Alberta, Canada: A space-time retrospective population-based administrative data study

Article References: Rosychuk, R. J., & Rowe, B. H. (2026). Spatiotemporal variations of COVID-19, influenza, and respiratory syncytial virus (RSV) presentations to emergency departments in Alberta, Canada: A space-time retrospective population-based administrative data study. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14514-w

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14514-w

Keywords: COVID-19, influenza, RSV, emergency departments, space-time scan statistics, spatiotemporal analysis, Alberta, surveillance, twin-demic, public health, epidemiology, administrative data