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Interviewer: Professor Katie Koelle, Thank you very
much for taking the time to do this interview with us today
to discuss SARS-CoV-2 evolution within and between individuals.
First of all, we've seen SARS-CoV-2 evolve
or change genetically over its last year of circulation.
Where have these genetic changes come from?
Prof. Koelle: Thank you for the invitation, Scott.
These mutations or genetic changes are known to
originate during viral replication within infected individuals.
If you look at an acute or short-term infection,
virus is transmitted from a donor or someone who's infected to a recipient,
so someone who's becoming infected.
And then that virus population expands rapidly in
that recipient host and reaches very high numbers and then declines again.
If you think about early stages of an infection and mutation can occur just by chance.
That mutation has the potential to reach really high frequencies, in that individual.
But that said, most mutations actually occur when the viral population
size is already very large with an infected individual.
Because that's when there's a lot of virus replicating.
and these mutations will likely stay at very low frequencies or low levels.
Just because at the time at which they are introduced,
there's already a lot of virus around so there's very little time for mutations,
even adaptive ones to actually get to very high frequencies or high levels.
Overall, what this results in is an expectation of actually
the viral population having
very low genetic diversity within an individual who's acutely infected.
and this is indeed what we see.
Interviewer: I see, and if we first think about the process of viral transmission between individuals,
how much virus is actually being transmitted?
Prof. Koelle: Studies are beginning to consistently show that very,