Welcome to Science of the ScareEvery month I will dissect a Big Science Question from a horror movie and talk through it in (mostly) easy-to-digest terms.

Science and horror have a wild, entangled history and have left us with loads of questions to ponder. Deep, important questions like: just how many ways could we have a zombie pandemic? Is genetic engineering always a slippery slope to monstrosity? This month’s Big Science Question:

Can a virus really be tailored to target people in a specific age bracket?

It happens all the time in apocalyptic science fiction: a novel virus sweeps the world and everyone over the age of 20 (or 30, if the virus is feeling generous) is gone, leaving the planet to a society of young people. This trope of a virus fatal only to adults hasn’t been explored as much in film as it has television series, such as Jeremiah (2002), Between (2015), and that one Star Trek: The Original Series (1966) episode “Miri.”

This month’s question may feel more than a little topical as we close out 2020 with the ever-looming awareness that the risks associated with COVID-19 disproportionately affect people over the age of 65, especially people over 80. But it’s not like this virus, or any real virus, has a refined sense for targeting people who are blowing out the candles on their cakes and crossing that (pretty much arbitrary) chronological threshold that defines them as elderly.



The reason why some viruses can affect older people so much worse is because our immune systems then to wind down and degrade as we age, leaving our bodies less able to fight off illness (and more likely to experience the worse symptoms). We see a decline in white blood cells, the agents responsible for seeking and destroying infection. As well, as we age, we tend to develop age-related chronic illnesses, like diabetes or heart disease, which further compromises the immune system’s capacity to fight off infection.


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On the flip side, consider a film like the The Children (2008), which is an interesting reversal of the trope — the youngest get infected first by a mysterious virus that a) makes them feverish and barfy; and then b) makes them murderous. This reversal, in terms of age and infection, is a little bit easier to predict than a virus that only infects people who are of legal voting age or older. Younger children, especially children under the age of six, are still developing their immune systems and are susceptible to picking up more viruses (especially colds) just because their bodies have no pre-existing immunity to protect them from new infections.



In The Children, it was suggested that older children and possibly even teenagers might pick up this virus and manifest symptoms, but the more developed immune systems of these older children offered some small protection and meant that the symptoms were slower to appear. So if you wanted to create a synthetic infection to obliterate kiddos and immunocompromised individuals, no DNA reading would be required. But sci-fi series always seem to be trying to get rid of the adults, which means that our evil virus would need to take a different approach.

Viruses work by inserting themselves into a healthy cell’s DNA and hijacking that cell’s natural replication mechanisms to make copies of itself. Some viruses can target specific kinds of cells, to get even more specific, know that we can alter viruses to deliver synthetic genes into a cell along with its viral DNA, all without getting the host sick — this is the basis for using viral vectors for gene therapy.

But gene therapy is a targeted personal treatment. There isn’t really a way to release a virus into a public space, for it to float around and get breathed into people’s lungs, and then for that virus to read their hosts’ DNA and mark only the ones who are past their 20th birthdays for death. The biggest stumbling block here is the difference between the chronological age of a body and its biological age.


“The reason why some viruses can affect older people so much worse is because our immune systems then to wind down and degrade as we age […]”


Your chronological age, of course, is the time you’ve spent alive, whether you count it in daylights, in sunsets, in midnights, or cups of coffee. You have an easy reference point in the form of your birthdate, and every other person born on the exact same day as you is the same age as you, plus or minus a few minutes or hours. But this is a reference point tailored to human behaviour and comprehension, not viral.

Although some signs of biological aging are easy to spot, like graying hair or wrinkles, these visible, physical signs are unreliable predictors. A virus programmed to kill people over a certain age by the basis of graying hair, for example, would wipe out more than a few teenagers who started going gray super early, and skip adults who aren’t going to begin graying until well into their 40s or 50s. When it comes to biological age, there are so many factors at play — even from the moment of birth — that might make age as read from your DNA different from someone born in the same year as you.

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Scientists used to think telomeres were a promising way to measure a person’s biological age. Telomeres are protective caps of redundant genetic information at the ends of each of our chromosomes, kind of like the plastic caps (called aiglets!) on the ends of our shoelaces. Every cell in the body has a limit to how many times it can divide, known as the Hayflick limit and generally accepted to be roughly 40 or 60 divisions for the average human cell. Telomeres play a role in this limit: every time a cell divides, the telomeres on the resultant cells’ chromosomes get a little bit shorter, until the so-called “protective caps” are worn away. If this rate of telomere shortening is so predictable, it would stand that this is a great way to estimate a person’s age based on the length of their telomeres, right?

Well, not quite. Measuring telomere length might give you a biological ballpark idea of a person’s chronological age, but it could never be an accurate predictor. For one, there’s a natural variation in the starting length of telomere’s — not everyone starts with the same amount of telomere. You would need to know how long a person’s telomeres were to begin with to get a reasonable estimate of how old they are based on their length at the time of testing. For another, it’s not just cell division that shrinks telomeres. Telomeres are extremely sensitive to environmental and lifestyle influences. Smoking, chronic stress, and lack of exercise, for example, can prematurely fray telomeres to give a bigger mismatch between your apparent biological age and the age on your driver’s license.

Therefore, creating a virus that targets people whose telomeres have reached a specific length wouldn’t allow for enough specificity to wipe out people a certain age or older.


“When it comes to biological age, there are so many factors at play — even from the moment of birth — that might make age as read from your DNA different from someone born in the same year as you.”


A more accurate biological measure has been referred to as the “epigenetic clock”. Epigenetics describe changes to your DNA that accumulate over time, like notes pencilled into the margins of your favourite book that change your reading of it over time, or markings for dynamics and articulations on sheet music that dictate which notes of a melody are played softly or with emphasis. In DNA terms, these accumulated markings (known as methylation, in the case where these accumulations are the addition of methyl groups) can change how certain genes are expressed.


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Some of these methylation sites change in steady and predictable ways. So predictable that some models have been able to accurately predict a person’s age within 2 or 3 years by looking at DNA samples. Like other methods, though, the epigenetic clock is limited by natural variations between different people’s epigenomes. So, as long as a hypothetical evil genocidal scientist isn’t too worried about killing off members of the world population who are older and younger than their target age by a handful of years, then I guess a virus that targets people based on the status of their epigenetic clock might be the most specific tool available. I mean, I don’t condone this form of apocalypse, but I guess if you’re going for a depopulation crisis, then I guess you’re probably not too picky about who gets to stick around.


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