THE SPREAD OF THE UNDEAD: ZOMBIE VIRUS

Nearly every zombie film tells us that the zombie pathogen will spread rapidly throughout the human population. One single zombie can wreak havoc on an entire city, quickly leading to a world-wide pandemic. But just how fast would this zombie pathogen actually spread?

One way that scientists try to understand how quickly a disease spreads is by calculating the R0 (pronounced “R-naught”) value. This number is a calculation of the average number of people who will contract a disease from one infected person. For example, if zombies bite an average of 2 people, then the R0 is 2. If the R0 is less than 1, then the outbreak is expected to peter out on its own. However, the disease can still spread, and people can still get sick or die.

Figure 1. The zombie outbreak with various R0 values. Each green dot represents a new zombie, with the original zombie shown at the beginning of the branches. In the top panel, each zombie infects two new zombies, so the R0 is 2. After three generations of infections, there are a total of eight zombies. In the bottom panel, each zombie infects three new zombies, so the R0 is three, creating a total of 27 zombies. A small difference in R0 can have a large impact on the total number of zombies.

An R0 of more than 1 represents the possibility for continuous spread of the zombie disease, potentially leading to an epidemic or pandemic. A higher R0 doesn’t necessarily mean that a given pathogen will cause a massive outbreak. But it does have that potential, and measures need to be taken to prevent its spread. While this concept is fairly straightforward, calculating the R0 value can be very complicated and the implications of R0 are varied. Measles has one of the highest R0 values that we know of, clocking in somewhere between 12 and 18 (though it can range widely). So, what could the R0 of the zombie virus look like?

Well, there are many things that can change the value of R0, such as the length of time that a person is actually contagious and how many people an infected person interacts with during this time. According to the CDC, people infected with the influenza virus can be contagious up to 7 days, measles is contagious for up to 8 days, and the novel coronavirus may spread from an infected person for 10 to 20 days. On the other hand, the living dead are contagious throughout their entire afterlife, which could last anywhere from a few months to many years. This is a massive window of time for a single zombie to infect hordes of people!

While many illnesses typically cause people to slow down, isolate, and rest, zombies actively and relentlessly seek out new targets to infect. Though classical depictions of zombies show them as slow and shambling, newer iterations of zombies such as those portrayed in 28 Days Later (2002) are inhumanly fast and persistent. A lengthy contagious period coupled with this prey-seeking behavior could make for a large R0 and a quickly-spreading zombie pandemic.

The R0 value is also specific to geography since the population density and local measures against infection can alter how many people one zombie can get to. As Katarina Zimmer writes for The Scientist, “How many people one person comes into contact with can differ dramatically depending on their activities and the populations and structures of their towns and cities.” While zombies may quickly eat their way through a densely-populated urban area, they might have a harder time finding a new meal in more sparsely-populated, rural areas.

In this case, the zombie R0 may likely be higher in crowded cities than in less inhabited areas. Quarantining and sheltering in zombie-proof areas may help slow the spread and lower the R0. A recent study of an outbreak of SARS-CoV-2 in 2020 on the Diamond Princess cruise ship shows that the R0 started at about 14.8, but isolation and quarantine reduced the R0 to about 1.78. Unfortunately, any zombie film can tell us that these measures ultimately fall flat in the face of the relentless zombie horde.

One major limitation of calculating the R0 is the occurrence of super-spreading events (SSEs). Author Pien Huang writes that an SSE may occur when “one person or gathering leads to an unusually high number of new infections.” Events like this may drive the majority of the 2020 SARS-CoV-2 pandemic through mass gatherings. One single SSE can increase the calculated R0, even if most of the transmission events occur at a lower number. Circumstances leading to an SSE can include pathogen, host, environmental, behavioral, and response factors. In the case of the zombie pathogen, a particularly quick and strong zombie may be able to infect more than the usual number of people. Alternatively, one lucky zombie that finds a gathering of defenseless humans may infect them all, creating a super-spread of the zombie pathogen!

Figure 2. Super-spreading events complicate R0 calculations. The spread of the zombie virus may typically only be two new infections per zombie, but a super-spreading event (SSE) where ten new infections occur can significantly increase the R0 value.

Hopefully, we’ll never need to calculate the R0 for an actual zombie outbreak, but that hasn’t stopped mathematicians from trying. One study calculated the spread of zombies following various mathematical models, and ultimately showed that humans have 10 days to stop the zombie outbreak with aggressive counter-measures before an inevitable apocalypse ensues.

Other groups have used the inevitable zombie apocalypse as a way to teach mathematical modeling, or have created interactive maps to model how the zombie pathogen might spread given different variables. We can only assume that once zombies do begin to roam the Earth, the R0 could be one of the highest we have ever experienced. So, please take a look at all of the links and research above to learn even more; after all what you don’t know could eat you!

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Guest post by Ashley Knox – Ashley Knox is a Colorado native living in Denver. Her love of zombie movies started at a young age and resulted in such antics as a zombie-themed “Sweet 16” birthday party, acting in a haunted house attraction, numerous zombie crawls, and a college thesis on post-humanist philosophy in zombie films. Fascinated by the virology of a potential zombie pathogen and curious about how such a disease could spread, Ashley chose to pursue a career in microbiology. She is currently a doctoral candidate at the University of Colorado, where she is working on a PhD dissertation based on her studies of gammaherpesviruses.