By Yasemin Saplakoglu - Staff Writer Live Science
Scientists crunched some numbers.
If all the SARS-CoV-2 particles currently circulating in humans around the globe were gathered together into one place, they would weigh somewhere between the weight of an apple and that of a young toddler, according to a new study.
A group of researchers recently calculated that each infected individual carries about 10 billion to 100 billion individual SARS-CoV-2 particles at the peak of their infection. That suggests that all of the SARS-CoV-2 viruses currently infecting people around the world — which has been about 1 million to 10 million infections at any given time during the course of the pandemic — have a collective mass of somewhere between 0.22 and 22 pounds (0.1 and 10 kilograms).
Small doesn't mean insignificant, however.
"Taking a view from a larger historical context, from the standpoint of leverage, an atomic bomb is less than 100 kg [220 Ibs] of fissile material," senior author Ron Milo, a professor in the Department of Plant and Environmental Sciences at the Weizmann Institute of Science in Israel, and co-lead author Ron Sender, a doctorate student in Milo's lab, said in an email to Live Science. "And yet, look at the destruction that is wrought."
Similarly, "here we are talking about a super-tiny mass of viruses, and they are completely wreaking havoc on the world," they added. The virus has now infected more than 173 million people and killed over 3.7 million, according to the Johns Hopkins coronavirus dashboard.
To calculate how much virus each infected person may carry, the researchers used previous measurements taken from rhesus monkeys on how much SARS-CoV-2 they carried during peak infection in various tissues that are known to be susceptible to the virus, including in the lungs, tonsils, lymph nodes and the digestive system. They then multiplied the number of virus particles present per gram of tissue in rhesus monkeys with the mass of human tissues, to estimate the number of virus particles in human tissues.
From previous calculations based on the virus's diameter, they already knew that each viral particle has a mass of 1 femtogram (10 raised to the minus 15 grams). Using the mass of each particle and the number of estimated particles, they calculated that each person, at peak infection, carries about 1 microgram to 10 micrograms of virus particles.
Crunching these numbers allowed the team to better understand what's going on in the body throughout an infection, such as how many cells are being infected and how the number of virus particles made in the body compares with how fast the virus can evolve, Milo and Sender said.
They then calculated how many mutations the virus would gather, on average, during the course of an infection of a single person and also across the entire population. To do this, they used a previous estimate, from a similar coronavirus, for how often a single nucleotide mutates, multiplied it by the number of nucleotides in the SARS-CoV-2 genome, and then factored in how many times the virus made copies of itself inside the body during infection.
They found that during an infection in a single host, the virus would accumulate about 0.1 to one mutation across its entire genome. Given there are 4 to 5 days of time between infections, the virus would therefore gather about three mutations per month, which is consistent with the known evolution rate of SARS-CoV-2, the researchers wrote.
But they also found a large variation in the number of viral particles across infected humans; in fact, it can differ by five to six orders of magnitude, meaning that some infected people may have millions of times more of these particles than others.
"We know that people with low viral load indeed have lower chances of infecting others," Milo and Sender said. But it's not yet clear if superspreaders, for example, spread the virus more than others due to biological reasons, such as high viral loads, or sociological reasons such as having many close encounters with people in large events held in closed spaces, they added.
"We hope this research will initiate new thoughts and new experiments," they said.