DETROIT – When it comes to tracking the way a virus circulates, researchers use many traditional methods, like contact tracing.
While that’s important, it only gives a narrow window into the transmission between a limited number of people. Local 4′s Dr. Frank McGeorge explains how epidemiologists can get the 10,000 foot view on spread.
June 26, 2020 update: Michigan coronavirus (COVID-19) cases up to 62,695, Death toll now at 5,888
Viruses naturally mutate, often in small unimportant ways. But every time a change occurs and the virus is spread to another person the newly created difference links those two individuals and by studying sequential changes it’s possible to follow a viruses travels across the country, or the globe.
The genetic code of the SARS-CoV-2 virus is made of roughly 30,000 base pairs. Like words in an instruction manual that the virus copies every time it spreads. When the virus copies itself, errors naturally occur. But these mutations are generally inconsequential, except for the trail they leave.
The value to these errors is that they act as a signature, unique to the virus at that time. That signature is passed onto every descendant of that virus. Researchers have been sequencing, or reconstructing, the genetic signatures of different SARS-CoV-2 viruses around the globe. By doing so, they have created an ancestral tree.
The tree shows how each successive change was passed on and where it branched with different mutations. Thinking of it like a family tree, when you see the same signature changes in different branches, you know they must be related.
By mapping the genome of the virus that infected different people, in different places, at different times -- and matching signatures -- we can infer relations.
Researchers believe after the viruses initial identification in China, it began to show differences as it traveled to different parts of the world. The signature of the virus identified in Washington State, suggests it was directly imported from China.
The viral genomes of the SARS-CoV-2 samples from people infected in Michigan months ago are more directly related to the signatures also found in Canada and Europe. Suggesting the virus didn’t come to Michigan directly from China, but rather was imported from Europe.
The Michigan signatures also have a relationship to viruses found in several other states. It’s important to mention that this only suggests a transmission relationship, it doesn’t say in what direction the virus traveled.