Global Diffusion of the SARS-CoV-2 Virus

In the spring of 2020, COVID-19 began spreading quickly across continents, countries, and state borders. Compared to past pandemics, this rate of diffusion was almost unprecedented. Our globalized society made this accelerated spread possible. With air travel reducing the time it takes to get from one place to another, people are able to move around the world at a rapid speed compared to decades past. Global networks increase the density of travel between specific locations. This interconnectedness increases the likelihood that local events can magnify to a global scale. In the case of the SARS-CoV-2 virus, genetic tracing has been utilized to determine just how the diffusion occurred. Through a combination of social and scientific factors, the SARS-CoV-2 virus was able to become a global pandemic within a matter of months.

Although the virus emerged in Wuhan, China in November of 2019, it wasn’t until late winter of 2020, around February and March, that it began rapidly spreading throughout the world (WHO 2020). The first confirmed case outside of China was on January 13 of 2020, from a Wuhan resident traveling to Thailand. The next three cases were confirmed in France on January 24, from citizens who had recently traveled to Wuhan. Russia, Germany, Finland, Italy, and Sweden soon followed after within just a few days. The vast majority of these cases were from Chinese tourists, or European residents who had recently traveled to China. Spain, Belgium and Italy quickly became some of the epicenters of the disease in Europe (Spiteri et al. 2020).

In the United States, COVID-19 arrived January 20th, in Washington state. This clade — a term for viruses grouped together based upon genetic similarities (Centers for Disease Control and Prevention 2019) — of the virus has been traced back to Asia, as the patient had recently returned from Wuhan, China. The pandemic truly spread through the US, however, once it hit New York City. Although the outbreak in New York City was originally thought to have traveled over from China, later genetic evidence suggested that it had come from a clade originating in Western Europe (Kaltwasser 2020; PBS Frontline 2020). The virus is thought to have traveled over from Europe and lay undetected for some time, allowing for rapid spread. There were many more people infected than ever got tested, and over 40% of people who did test positive reported not knowing how they got it — this suggested that many more people had COVID-19 than just who knew about it (Polan 2020).

However, recent findings suggest that SARS-CoV-2 may have been more widespread earlier than initially assumed. A recent analysis of blood samples collected across the United States from December 13, 2019 to January 17, 2020 found SARS-CoV-2 antibodies in 106 out of 7,389 samples (Diaz, 2020). Another study hypothesizes that traces of the virus existed in wastewater in Barcelona in March 2019 (Zimmer and Minder, 2020). Yet another found coronavirus antibodies in patients from a lung cancer screening trial collected from September 2019 and March 2020 in Italy (CNBC, 2020).

These emerging studies all lead us to believe that the SARS-CoV-2 virus spread much faster around the world than we previously thought. Globally interconnected networks have played the largest role in making this happen.

Reevaluating earlier pandemics, such as the SARS event of 2002-03, helps us see the connection between globalization and disease diffusion. Using a geographic lens, Ali and Keil (2006) explain how SARS diffused so widely and quickly from its point of origin in southern China. First, modes of transportation are considerably quicker, with international travel times reduced by several hours in most cases. Second, there are significantly greater numbers of people moving across international borders. Finally, the potential pathways for pathogens have become more concentrated and varied with the emergence of a “global city network” (Ali and Keil 2006). This is the idea that certain metropolitan areas around the world disproportionately receive the most amount of traffic through economic exchanges, and therefore the pathways in between them become the most used.

As globalization increases, so does "impact propensity" — that is, the probability for a local event to have global consequences (Ali and Keil 2006). This concept explains how globalization has had such a dramatic effect on the spread of SARS-CoV-2. Further analysis has also confirmed that spatial proximity has little influence on the pattern of viral spread, compared to the impact propensity of the global city network (Sigler et al. 2020). This helps explain nearly simultaneous spikes of COVID-19 in large but widely dispersed metropolitan areas such as New York City, São Paulo, Prague, and London during the early months of the spread (Jones and Kiley 2020). As viruses travel the world in a pattern of hierarchical diffusion — first, through highly interconnected global cities, later through small cities and towns — it is not surprising that later months of the pandemic witnessed a drastic increase in COVID-19 cases and deaths in rural areas (Jones and Kiley 2020). In fact, many maps of the COVID-19 pandemic, by focusing on the nation-state as the unit of analysis (including the one below from Science magazine), obscure the essential structure of the global cities network and the complex but ordered pattern of hierarchical diffusion.

Although travel restrictions were quickly put in place by national governments, many were too late to prevent infiltration of the virus (Woodward 2020). Individuals continued to move around for purposes of tourism, migration, business, and other purposes (Sigler et al. 2020). Had restrictions been placed earlier, the world may have stood a better chance at limiting the spread of the SARS-CoV-2 virus. One success story to closing borders early comes from Aotearoa (New Zealand). The government quickly closed the nation's borders on March 20, 2020 which successfully limited their coronavirus cases.

By analyzing the genome of the SARS-CoV-2 virus, experts are able to more accurately track the spread of the virus. This is done by comparing similarities between viral sequences. The virus, like all genetic material, periodically mutates. However, this specific virus mutates relatively slowly, with approximately two mutations per genome, per month. With this information scientists are able to track the diffusion of the virus by comparing the genome and the mutations within it. This helps determine both where a particular strain of the virus originated, as well as a timeframe for when the virus arrived, this can be seen in figure 3. showing the initial spread across continents with a timeline of when this occurred (Worobey et al. 2020). Tracking where a virus spreads, based on its genome and its mutations, can be seen in the image above. The left side shows the different mutations of the virus, and how they trace back to the original strain. The colors coordinate to the right side which shows the location from where a virus spread, based on its genome (“Coronavirus: The Science Explained” 2020). Within a COVID-19 cluster, there are only 1-3 nucleotides that vary across the entire genome. It is important to note that due to the slower mutation rate of COVID-19, the mutations can occur at a slower rate than the virus is transmitted. This can pose a limitation, but is still a great way to accurately test various hypotheses on the virus’ diffusion (Worobey et al. 2020).

Another big question when looking at the spread of Covid-19 cases is why some places are hit harder than others that have very similar population densities, weather, age distributions, and travel patterns. One explanation that has been presented is a value k, which measures the dispersion of a virus. In simpler words k measures weather a disease spreads in a speedy or clustural manner. In the case of Covid-19 it is thought that it spreads in clusters, meaning that not everyone is responsible for equal transmission of the disease. Evidence of “superspreading”, an incident where a single person infects 80 percent of the people in a single room, has been found for COVID-19 (Tufekci, 2020). Locations more at risk to these super-spreading events could account for higher case and death counts in communities that otherwise look rather similar.

The rapid diffusion of the COVID-19 pandemic has been unprecedented to say the least. This unparalleled diffusion is due to many different factors on a global and local scale. Global diffusion factors include international travel for vacations, through the global city network and hierarchical diffusion. The virus itself is more contagious than past infectious diseases, with high impact propensity and super-spreader events contributing to COVID-19’s intense spread. Delayed responses and irresponsible individual’s actions are also major contributors to COVID-19s severity. Through the sequencing of the viral genome, experts are able to track how the virus spread geographically, across the globe.

Image Credits

Header. "Will the Coronavirus Bring the End of Globalization? Don’t Count on It"

Figure 1. Daily new confirmed COVID-19 cases per million people

Figure 2. The changing Geography of Covid-19 in the US

Figure 3. Genomic sequencing data graphic

Figure 4. Map of SARS-CoV-2 introductions across the world