How much COVID-19 lurks on planet Earth?
It’s a seemingly simple question: just how much COVID-19 (SARS-CoV-2, or coronavirus) is there on planet Earth at any given moment? And, as one Twitter user asked, how much does it weigh?
To find out, I first tried to google the question, but didn’t see any obvious answers. Then I tried some related questions: how many virus particles are inside a human being? According to wikipedia, there is currently no estimate of the number of virus particles in a healthy human being, but like the friendly bacteria that live inside our stomachs, there should be friendly viruses too. Of course, there will be far more inside a sick person.
To make a guess for coronavirus, I googled another virus that has been well-studied: the flu. According to a study published in Science, there are up to 100 trillion flu particles (or virions) inside a person’s body during the peak of a flu infection. That’s a lot of virus!
Next, we need to know how much mass a virion has. There don’t seem to be a whole lot of studies of this yet (that I’ve run into), but one research group tested this for the vaccinia virus, which is a member of the Poxviridae family which includes smallpox. They created a tiny cantilever beam, which they used as a type of scale. Virus particles could bind to the end of their cantilever beam, changing its natural vibrations in a measurable way. The takeaway: the vaccinia virus weighs about 9.5 femptograms, or 0.0000000000000095 grams. Very light!
Now, we’re playing a bit fast and loose with numbers here, but maybe that’s OK. After all, viruses in our bodies attach to cell and proteins, or stick to water droplets in our breath, and viruses themselves all vary a bit in structure, and individual virus particles might have small quirks or defects of their own. There are also multiple strains of coronavirus. So we move bravely on and do the best we can!
We have a guess of the number of viruses in a sick person’s body and a rough guess of the mass. Now, we need to multiply by the number of cases. Not all victims will have the full 100 trillion particles. Some will be freshly sick, while others are recovering. We could multiply by ½ here to account for this, but we also note that some victims are asymptomatic and won’t be counted at all, and that some virions will be out in the environment, degrading due to exposure to air, light, cleaning chemicals, or other effects. We may be underestimating. We may be overestimating. Let’s go for it anyway:
So, we multiply like this: (virions in a victim)×(mass of virion)×(#victims), or (100 trillion virions)×(9.50 fg per virion)×(19,400,000 victims).
The result: 1.94×10²¹ particles, for a total of 18,400 kg (or 40,500 lbs).
That’s more than the mass of a fully grown Tyrannosaurus rex (14,000 kg) — or 206 Jeff Goldblums (89 kg). We’ll call that 206 Blumens of coronavirus on planet Earth at any given time. What a monster disease!
