Tracking down the sneeze that started seasonal flu

October 17, 2019

On these humble pages, scientific inquiries
stack one upon the next to form a magnificent castle of human knowledge.
Trevor Bedford (TB): We’re looking at how flu moves about the world and we’re trying
to trace back where viruses come from. In a new study, Trevor Bedford and his colleagues
investigate the lifestyles and travel habits of flu, to understand the origin of outbreaks. Think of it: when millions come down with
a strain of flu, TB: someone existed in the world that had
the flu and that now everyone who had the flu descends from that one sneeze. We don’t
know that person, obviously, but we can establish the region that they lived in. Trevor says, the person who sneezed the sneeze
that would give rise to flu season, TB: that person was most likely and usually
in south China, India, or Southeast Asia. You might be thinking, ‘wow..that’s crazy.’
  TB: Yes, yes it it is. Trevor compared the genetic material – the
RNA – of flu samples from all over the world and put together a giant family tree of flu.
Which Trevor agreed to recreate using paper and pipe-cleaners. TB: I mean, this is my level of abstraction
anyway. it’s fine. Think of each circle as a flu sample. TB: Here’s the virus…Both of these viruses
share a common ancestor here… This virus actually seeded kind of two infections…
Flu mutates…So mutation has occurred… Subsequent people infected. And this goes on … TB: How’s that looking? And on… Now imagine almost 10,000 circles. That’s
the size of Trevor’s tree, making it perhaps the largest study of flu evolution in history. TB: Yeah, depending on how you parse it. i
think this, i think this is it. What did this flu family tree reveal? That
the life of flu has more twists than a pretzel factory. TB: So, the biggest surprise is that Influenza
B behaves very differently from flu A. You see, there are different kinds of flu.
Flu A… includes the strain H3N2. That was the most common and deadly strain of seasonal
flu last year. And H3N2, Trevor found, leads a very different lifestyle from Flu B. TB: So, H3N2 evolves very quickly. So your immunity to H3N2 doesn’t last long.
The flu evolves, escaping our immune system. TB: Then you’ll get it every 5-10 years
for the rest of your life.   Flu B lives a quieter life. It evolves more
slowly. It infects a higher proportion of children, who haven’t been exposed to flu
before. TB: Everyone get it as a kid. but then you’ll
have immunity to it for the next 20, 15 years. These flus also travel differently, Trevor
found.  Flu B can settle for years in one place. TB: We can see one variant of B that sticks
around Southeast Asia for 6 years and doesn’t go anywhere else. And H32N, on the other hand, travels faster
than small town gossip. Which was kind of mysterious. Why would H3N2 travel while Flu
B is stuck in one place? The answer? Airplanes. And the people who ride ‘em. TB: Adults get on airplanes and travel around
the world and spread flu. Whereas  Flu B infects primarily children and kids don’t
get on airplanes as much and don’t spread the virus about the world as, as quickly. Because Flu B evolves more slowly and infects
mostly kids, it travels when kids travel. And kids don’t travel that much. H3N2 evolves more quickly, infects more adults,
and hitches a ride with them all over the world. Trevor’s data showed H3N2 often migrates
from Southeast Asia, where flu circulates all year long. It heads to North America,
where it deplanes, outcompetes local strains, and takes over. Knowing how different flus operate can help
us make better vaccines. We could tailor B vaccines to specific regions. And H3N2’s
migration patterns tell us where to look for strains that dominate seasonal flu. TB: All of this geographic work points you
towards where in the world we should be picking our vaccines from. Enigmas about this world are a dime a dozen.
But, with every study published, our view of the world gets a little bit more interesting.
Until next time.

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