Beijing International Committee on Taxonomy of viruses, novel coronavirus pneumonia, was officially named "COVID-19" in 2019, when the International Classification Committee of viruses declared that it was "SARS-CoV-2" in February 25th. The International Committee on Taxonomy of viruses is responsible for the designation of viruses, and the naming of diseases is carried out by WHO. The name of sars-cov-2 also reflects the similarity between the new coronavirus and the coronal SARS CoV that caused the SARS epidemic.
When someone asked me to calculate the total volume of global sars-cov-2 virus particles, frankly, I didn't know the answer myself. My wife thinks that the total volume should be the size of an Olympic swimming pool. Then she added, "or just a teaspoon."
So, how can we calculate the approximate volume of global sars-cov-2 virus particles?
Fortunately, I have some experience with such large-scale rough estimates, and I have made many similar estimates in my book "the maths of life and death". Before we embark on this special digital journey, I would like to make it clear that this is an approximation based on the most reasonable assumption. Of course, I do not deny that some areas may be further improved.
We'd better start by calculating how many sars-cov-2 virus particles there are on earth. To do this, we need to know the total number of people infected with the virus. (here, we assume that humans are the most significant carriers of the virus, not animals.)
According to the Statistics website "see the world with data", about 500000 people test positive for new coronavirus every day. However, we also know that there are many people who are not included because they are either asymptomatic, or do not do nucleic acid testing, or nucleic acid testing is not yet universal in some countries.
Using statistical and epidemiological modeling, the Institute for health indicators and assessment (IHME) estimates that the actual number of infections per day is closer to 3 million.
The amount of virus (viral load) currently carried by each infected person depends on the time they have been infected. On average, the viral load increased gradually after infection, peaked on the sixth day, and then decreased steadily.
Among all the people who have been infected at present, those who were just infected yesterday will not affect the total amount of the virus. Those who were infected one or two days ago had a more significant impact on the total amount of virus, while those who were infected three days ago had a more significant impact on the total amount of virus. On average, people infected six days ago will show the highest viral load. Then, people who were infected seven days, eight days, nine days or even longer would have a decreasing effect on the total amount of virus.
Finally, we need to know the number of virus particles that people carry at any time during infection. Given that we have a general idea of how viral load changes over time, we can estimate the peak viral load. An unpublished study analyzed the number of virus particles per gram in a range of different tissues of infected monkeys and scaled up to represent human tissues. Their rough estimates show that the peak viral load contains between 1 billion and 100 billion virus particles.
We might as well take one of the geometric averages, which is 1 billion. If we add up the number of virus particles carried by the 3 million people who were infected every day in the past few days (assuming that the infection rate of 3 million people per day remains unchanged), we will get such a result: at any time, there are about 2 billion virus particles (2x10 to the 17th power) in the world.
Two billion, that sounds like an astronomical number. Indeed, there are only so many grains of sand on earth. However, what we have to calculate is the volume, and the sars-cov-2 virus particles are extremely small. The diameter of sars-cov-2 virus particles is about 80 to 120 nm. One nanometer is one billionth of a meter. Specifically, the radius of sars-cov-2 virus particles is about one thousandth of that of human hair. Similarly, in the next calculation, we also take an average of the diameter of the virus particles, that is 100 nm.
Assuming that the radius of sars-cov-2 virus particles is 50 nm, the size of a single spherical virus particle is 523000 cubic nanometers.
If we multiply this super mini volume by the number of large particles we got before, and then convert it into the volume unit we are familiar with, we can get the total volume of all sars-cov-2 virus particles is about 120 ml! But if we want to put all these virus particles in one place, then we need to know that there is no way for spheres to pile up tightly.
Like the orange pile in the supermarket. As you can see, there are many gaps between the oranges. In fact, the best way to minimize voids is a scheme called "ball packing". Under this scheme, voids account for 26% of the total volume. As a result, the total volume of our sars-cov-2 virus particles increased to 160 ml, which can be easily divided into six bullet cups. Even if we take the upper limit of particle diameter (120 nm) and the size of spike protein, all sars-cov-2 virus particles may not be able to fill a coke can.
It turns out that the total volume of sars-cov-2 virus particles is between one teaspoon and the size of a swimming pool predicted by my wife. Looking back last year, these little viruses once made the world into chaos and made countless people lose their loved ones. However, all these viruses add up to less than the size of a can of coke, which is really impressive.