How much of the coronavirus does it take to make a person sick? This may sound like a strange question, but growing evidence suggests that viral dosage determines our illness severity.
Sick of Social Distancing? Hang in There — It's Making a Difference
Over the last six months, all of our lives have dramatically changed. To see the stark difference, answer this question: How many people outside of your household have you engaged with in a long face-to-face conversation with since the pandemic started?
If you've been taking every coronavirus protection measure you can, then it's probably not many at all. Maybe you've had socially distant meetups with friends in which you talked to each other from a distance or ate a meal together outside. But when was the last time you found yourself in a crowded indoor environment surrounded by strangers without any face masks on? You'll probably recall an event in early February before the whole world shut down.
Fortunately, it turns out our deliberate change in behavior over the last six months or so has not been for naught. In fact, it not only reduced our chances of coming into contact with COVID-19, but it also decreased the probability of exposure to large amounts of the virus. We may be wearing masks everywhere and may be having fewer social interactions these days, but science hints that it's helping the world combat this pandemic.
Coronavirus Load May Be Key to Understanding Sickness Severity
Scientists firmly believe that all the measures we've taken — social distancing, wearing face masks, sanitizing our hands repeatedly throughout the day — have played a huge role in reducing the infectious dose levels that people are exposed to. And now, they're wondering if these efforts can also help explain why hospitalization and death tracking have deviated from confirmed active cases since last spring.
If this is correct, it has crucial implications for epidemiological modeling and how we assess our risks and behaviors. It could inform important questions such as when we should wear masks, when we should go out, and when we could potentially return to our offices.
"The amount of virus exposure at the start of infection — the infectious dose — may increase the severity of the illness and is also linked to a higher viral load [in infected patients]," explained Oxford COVID-19 Evidence Service Team members in March. Hard evidence, both old and new, supports this.
Studies from China have shown that the amount of coronavirus that patients have in their systems, also known as the viral load, is linked to the severity of the illness. A large American study recently published in The Lancet discovered that a patient's viral load at diagnosis was an "independent predictor of mortality." Essentially, this means the higher the viral load, the greater the possibility of dying.
But What About the Initial Infectious Dose?
By this point, you're probably wondering: Does the initial amount of coronavirus you're exposed to make a difference in your chances of catching it or surviving it? While the evidence is less clear for this question, it's still mounting.
In May, scientists in the UK gave ferrets varying doses of SARS-COV-2. There was a clear difference in the animals' outcomes. Those given medium and high doses contracted the illness and suffered similar ailments to what humans experience. In contrast, only one ferret from the "low dose" group was infected. It also avoided the worst effects of the disease and recovered with no lingering fatigue or lung scarring.
If this doesn't surprise you, then your views align with several other studies. "There are loads of examples in the literature for symptom severity being dose-dependent for plenty of [other] bugs," explains Francois Balloux, an epidemiologist and geneticist.
Perhaps the most obvious example is the flu. In 2010, an influenza A study concluded there was definitely a clear-cut relationship between viral dose and patient outcomes. The authors stated, "Low dose exposure may lead to infection, due to the high infectivity of the virus, but of those infected only a small proportion may become ill." On the other hand, "Exposure to high doses of virus results in most of the infected subjects also becoming ill."
A 1918 Spanish Flu retrospective study also concluded that the higher fatalities that occurred during the second and third waves could be due to people being exposed to larger infectious dosages instead of a virus mutation, as was previously believed.
Key Takeaways From All of This Research
Since the pandemic is still active, it's best to keep interactions short and brief instead of intense and prolonged. If someone in your home is COVID-19 positive, isolate the infected household member as much as possible.
And don't let up on the mask-wearing and hand-washing, especially if you often find yourself in environments that are too crowded for your liking. As the Spanish Flu study predicted, "When exposure to airborne virus is reduced, for instance by population-wide use of face masks, the relative decrease in numbers of illnesses is expected to be greater than the relative decrease in [viral] transmission."
Stay tuned. We'll be following developments in this story closely.
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