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Most likely is human travel, but it's technically possible for the same variant to evolve more than once.
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Viruses can mutate in different places in similar ways, but there are generally ways to determine the particular history of a given variant by studying its genetic sequences. As I understand it, any given variant won't be 100% homogeneous in composition, and the variability present in a sample of a virus variant can tell you a lot about its particular history.
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It would be remarkably rare for a new variant to spontaneously appear with the same exact mutations in the same places as one of the previously identified variants, but it's within the realm of possibility. (On the order of falling out of an airplane and surviving twice I'd imagine.)
Least untenable hypothesis would be that any matching strain that appeared first in South Africa and then a month later in Maryland was carried to Maryland rather than spontaneously arising in Maryland.
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sekker has it right. The most likely explanation is -- unfortunately -- previously-undetected community spread.
That being said, it definitely IS possible for the same mutation to occur simultaneously and independently.
And... and this is a BIG "and"... there has been very little discussion so far of the possibility of viral swapping of mutations by way of simultaneous co-infection with different strains either of SARS-CoV-2 or of other coronaviruses which are quite common in the environment (typically giving more typical common cold/flu symptoms).
There HAS been some discussion of the fact that we may be inadvertently selecting for mutations with people who are infected for an extended period and then undergo various treatments which, for various reasons, may not be fully effective ... so then you are effectively selecting for mutations in the virus which help it evade the treatments. Somewhat akin to what happens when you don't use the full dose of an antibiotic on a bacterial infection and you end up selecting for some mutant form of the bacterium that has partial or full resistance to the antibiotic.
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PeterB wrote:
sekker has it right. The most likely explanation is -- unfortunately -- previously-undetected community spread.
That being said, it definitely IS possible for the same mutation to occur simultaneously and independently.
And... and this is a BIG "and"... there has been very little discussion so far of the possibility of viral swapping of mutations by way of simultaneous co-infection with different strains either of SARS-CoV-2 or of other coronaviruses which are quite common in the environment (typically giving more typical common cold/flu symptoms).
There HAS been some discussion of the fact that we may be inadvertently selecting for mutations with people who are infected for an extended period and then undergo various treatments which, for various reasons, may not be fully effective ... so then you are effectively selecting for mutations in the virus which help it evade the treatments. Somewhat akin to what happens when you don't use the full dose of an antibiotic on a bacterial infection and you end up selecting for some mutant form of the bacterium that has partial or full resistance to the antibiotic.
It seems like the volume (figuratively) of infected individuals would be big factor in the likelihood of a nastier strain entering into our communities. Hopefully, the vaccines will deflate the volume of infections quickly enough to prevent some really nasty strain from evolving.
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Interesting. Thanks to all for your responses. So much yet to discover.