Tom Koch is a medical geographer at the University of British Columbia and the author of Cartographies of Disease and Disease Maps: Epidemics on the Ground.
Thinking of the current surge of COVID-related cases as a new wave of ongoing viral epidemic activity makes it seem like the current increase in cases is just more of the same. The problem is that epidemics are not waves and the new cases are quite different.
What we are experiencing instead is a new epidemic caused by “variants of concern” whose different demographics (younger persons), greater transmissibility and increased virulence mark it as distinct. Understanding that explains a great deal about the surge in hospital admissions and the burden they place on intensive care units in hospitals across the country.
The pandemic that began in December, 2019, was the great-grandchild of the 2003 SARS virus. That is why it was labelled SARS-CoV-2. It has been proven to be a more robust version of its ancestor. Today’s variants of concern are its offspring – and again more infectious and deadly than the parent.
Think of any movie about Mafia dons who get more conservative as they get older. Their younger lieutenants want to try to expand into new domains and activities. The previous generation is tired and the new, impatient generation takes over. Like those fictional characters, this microbial family acts similarly as it evolves.
Waves progress, one after another, in a predictable pattern defined by gravity and physics. Epidemics are non-constant events that advance outbreak by outbreak in one population or another. In 2020, the epidemic’s focus was primarily congregate settings of vulnerable adults living in assisted-care facilities. Cruise ships and large gatherings were the perfect medium for epidemic expansion.
In 2021, the new variants have targeted younger people and those working in congregate settings, including factories, warehouses and similar sites. And because it is primarily airborne, the likelihood of contagion in more general public settings is greater. The “kids” are exploring the venues the previous generation tended to overlook.
Mapping the epidemic at the international or national level hides the shotgun-spatter pattern of outbreaks that give an epidemic the appearance of regular spread. It is only when seen at the focussed resolution of the level of a city or local health district that the uneven incidence of infection becomes apparent. There is nothing wave-like here. Each outbreak sparks its own centre of expansion, its own “wave” of bacterial or viral influence.
We’ve used the wave analogy at least since the early 19th century – in early studies of epidemic cholera, for example. While the image worked well at the coarse level of a nationwide lens, it hid the specifics of disease dynamics.
Charts and tables of disease incidence originated in the late 18th-century attempts to correlate disease incidence (yellow fever) with temperature and other climatic variables in both the Caribbean and the United States. Over time, the idea of climatic causation led to increasingly sophisticated charts of weather variables and, for example, cholera cases.
In looking for a broad pattern of causation, the result gave the impression of a regular expansion and then decline of bacterial events such as cholera. When the charts showed a rising curve that fell and then rose again, the idea of first, second or third waves seemed natural.
As visual tools summarizing long tables of data, these were great and innovative tools. The wave analogy made sense – just as in this pandemic’s charts and maps, COVID-19 cases have looked wave-like, because that’s the way we’ve been taught to summarize this kind of event.
The reality is more complex. For the first time, advances in genomic testing can distinguish between earlier and later epidemics based on different viral generations. Then and now are different. And with data on local outbreaks, the wave image disappears as viral variants address different populations in different cities.
Because the pandemic is not a wave and geographic consistency is not a viral virtue, any given chart, map or table may be misleading. To understand disease incidence requires we see the irregularities. To truly profile these diseases over time, and thus predict their future occurrences, would require an atlas whose maps and data distinguished distinct viral profiles and different at-risk populations over time.
From this perspective, it is easy to understand how health officials appeared to be caught unaware by new viral activity that they should have expected. Were this simply a new wave of the same virus, they might be criticized for their lack of awareness. Given the early stretch of the pandemic, it was easy to assume existing measures would keep the disease under control. Alas, the children of SARS-CoV-2 took a different, unexpected turn.
Analogies are useful tools until they are not. General-population pictures smooth the irregularities that exist at finer resolutions. Seduced by the analogy, we forgot that the regularities of waves do not really serve to describe a disease outbreak – or predict the variants that map distinct health emergencies.
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