RNA viruses have high mutation rates that result in several slightly different versions of the viral genome being made each time the viral genome is replicated (Figure 1). This creates a viral population with diverse genomes, known as a quasispecies. With each viral replication cycle, the differences accumulate between the original viral genome and the progeny viral genomes. This may contribute to differences in clinical outcomes between patients, as the viral populations that are infecting them are slightly different. The fact that there are multiple versions of
the viruses also makes it challenging to categorize viruses as different species. Instead, the genomes will have varying levels
of relatedness to one another. This is why the CSG assessed the relative levels of relatedness between and within SARS-CoV and SARS-CoV-2 and compared it to the level of relatedness between other and within other coronavirus species.

How COVID-19 Mutations Compare to Other RNA Viruses
All viruses mutate at different rates. More than 25 RNA viruses have been measured in labs, with individual viruses counted for mutations. Scientists have found that anywhere from one in ten thousand to one in a million RNA virus particles contains a mutation, depending on the type.

Here are some mutation rate estimates for other known viruses. This number tells you roughly how many viral particle mutants will exist in a given viral population:

Influenza A: 319/10,000,000, (.00319%)
Human Immunodeficiency Virus (HIV): 9,927/100,000,000 (.009927%)
Measles: 9/100,000, (.009%)
Poliovirus 1: 3/10,000 (.03%)
This does not tell the whole story about a virus' potential to mutate, but it offers a rough idea of how easily and often they do.

COVID-19 is not as well-studied as these other diseases. But an estimated range of its mutation rate has emerged from a study of 30 publicly available samples of SARS-CoV-2. Researchers at Johns Hopkins estimate the mutation rate of COVID-19 at:

COVID 19: 21/20,000 (.00105%) to 63/50,000 (.00126%)