Researchers at the Weizmann Institute of Science (Israel) have calculated that each infected person, at the peak of infection, carries between 10 and 100 billion individual particles of SARS-CoV-2. This means that in total, all the SARS-CoV-2 viruses that currently infect people around the world - which is about 1 to 10 million infections at any given time during the pandemic - have a mass somewhere between 0, 1 and 10 kilograms, LiveScience writes about the research results.
For all its microscopic size, the new coronavirus is unusually deadly. Like an atomic bomb - 100 kilograms of fissile material is enough for the most devastating consequences. According to the latest data, the coronavirus has already infected more than 173 million people and killed more than 3.7 million.
To calculate how much viral load each infected person might carry, the researchers used data from rhesus monkeys, animals that are also susceptible to SARS-CoV-2. Previously, scientists measured their viral load at the peak of infection in various tissues: lungs, tonsils, lymph nodes and the digestive system. Then they multiplied the number of viral particles per gram of rhesus monkey tissue by the mass of human tissue, and as a result, they obtained the number of viral particles in humans.
Also from previous calculations based on the diameter of the virus, it was known that each viral particle has a mass of 1 femtogram (1 * 10-15 grams). Using the mass of each particle and the number of supposed particles, scientists estimate that each person at the peak of their illness carries between 1 and 10 micrograms of viral particles (1 microgram equals one millionth of a gram).
They then calculated how the virus mutated, infecting one person and the population as a whole. For this, data from a similar coronavirus were used, showing at what rate one nucleotide mutates. The researchers multiplied the available numbers by the number of nucleotides in the SARS-CoV-2 genome, and then took into account how many times the virus copied itself within the same organism during infection. It turned out that during infection of one host, the virus accumulates from 0.1 to one mutation in its genome. Given that an average of 4-5 days pass between infections, the virus accumulates about three mutations per month. This figure is consistent with the already known rate of evolution of SARS-CoV-2.
Interestingly, the researchers found large differences in the number of viral particles in infected people - it can differ by five to six orders of magnitude. That is, some sick people may have millions of times more of these particles than others.
People with low viral loads are known to infect others less. However, it is not yet clear why super-spreaders are transmitting the virus more actively - for biological reasons such as high viral loads or sociological reasons, such as numerous close contacts with other people.
The research results are published in the journal Proceedings of the National Academy of Sciences.