A new study revealed that if all the SARS-CoV-2 particles currently circulating in humans around the world were grouped together in one place, they would range from the weight of an apple to a small child.
A group of researchers recently calculated that each infected individual carried approximately 10 billion to 100 billion SARS-CoV-2 particles at the peak of infection.
This suggests that all of the SARS-CoV-2 viruses currently infecting people around the world – which amounted to about 1 million to 10 million infections at any time during the pandemic – have a mass mass somewhere between 0.1 and 10 kg.
“When taking a look from a larger historical context,” senior author Ron Mello, professor in the Department of Plant and Environmental Sciences at the Weizmann Institute of Science in Israel, and co-lead author Ron Sender, a PhD student in Mello’s lab, said in an email to Live Science. “From the point of view of influence, an atomic bomb is less than 100 kg of fissile material. However, look at the devastation.”
Likewise, they added, “We are talking about a very small mass of viruses, which are wreaking havoc on the world.”
The virus has now infected more than 173 million people, and killed more than 3.7 million, according to the Johns Hopkins Coronavirus Dashboard.
To calculate how much virus each infected person could carry, the researchers used previous measurements from rhesus monkeys about how much SARS-CoV-2 they carried during peak infection in various tissues exposed to the virus, including the lungs, tonsils, lymph nodes and gastrointestinal tract.
They then multiplied the number of virus particles present per gram of tissue in rhesus monkeys, with the mass of human tissue, to estimate the number of virus particles in human tissue.
From previous calculations based on the diameter of the virus, they already knew that each viral particle had a mass of 1 femtogram (10 raised to minus 15 grams). Using the mass of each particle and the number of estimated particles, they calculated that each person, at peak infection, carried about 1 µg to 10 µg of virus particles.
Analyzing these numbers allowed the team to better understand what happens in the body during infection, such as the number of infected cells and how the number of virus particles in the body compares to how quickly the virus develops, according to Milo and Sunder.
They then calculated how many mutations the virus would collect, on average, while infecting one person and also across the entire population. To do this, they used a previous estimate, from a similar coronavirus, of how many times a single nucleotide had mutated, multiplied it by the number of nucleotides in the SARS-CoV-2 genome, and then took into account how many times the virus made copies of itself inside the body during infection.
They found that during infection in a single host, the virus may accumulate about 0.1 to a single mutation across its entire genome. Because there are 4 to 5 days of time between infections, the virus will collect about three mutations per month, which corresponds to the known evolution rate of SARS-CoV-2, the researchers wrote.
But they also found great variation in the number of viral particles across infected humans. In fact, it can vary from five to six orders of magnitude, which means that some affected people may have millions of times more of these particles than others.
Milo and Sunder said: “We know that people who have a low percentage of the virus, they actually have less chances of transmitting the infection to others,” according to the “rt” website.
They added that it is not yet clear whether the spread of the virus, for example, spreads the virus more than others for biological reasons, such as high viral loads, or for social reasons such as the presence of many close encounters with people at large events held in closed places.
“We hope this research will start with fresh ideas and new experiments,” they said.
The results were published June 3 in the Proceedings of the National Academy of Sciences.