Thanks to Brilliant for supporting  this episode of SciShow.

Go to Brilliant.org/SciShow if you’d  like to join their community of 8 million  learners. [♪ INTRO]. Gearing up for flu season in 2020, some  experts feared we were in for a double whammy of both influenza and COVID-19.

Not only would there be a so-called “twindemic” of two harmful respiratory viruses, but people with symptoms would have to  be tested for not one but two diseases. But some experts were surprised when  the 2020-2021 flu season came and went with far fewer cases than usual in the world. One big reason that the twindemic didn’t  happen is that social safety interventions to control the spread of  COVID-19 also appear to have reduced influenza transmission.

But whatever the reason for  the flu's no-show in 2020, experts say that it poses challenges  for flu vaccine development. In other words, we could be in for a bad season. See, ever since the mid-20th century  when scientists started to realize that influenza mutates regularly, global  influenza preparedness has worked like a well-oiled machine.

In over a hundred countries, government  officials and scientists share data year-round on emerging mutations  in the influenza virus and coordinate the next steps. Officials keep an eye on what’s happening  in the northern hemisphere to predict and manage what will happen in the  southern hemisphere, and vice versa, to optimize vaccine strategies. Unlike other viruses such as  measles, influenza mutates quickly.

That’s because of influenza  viruses’ segmented genome structure, and when two or more different  influenza viruses infect the same host, they can swap genome segments  and create new mutants. And this swap happens more  often in hosts like pigs, so researchers call them  mixing vessels for the flu. Basically, these animals can  catch multiple flu viruses from different species at the same time.

When the viruses infect  the same individual animal, the viruses can trade their  DNA to create new mutations. Part of influenza preparedness includes  constant monitoring of flu circulating in animals to catch any new strains  with the potential to spill over, moving from one species to  another, like humans nearby. And for decades, researchers have had about a six-month head start ahead of each flu season.

They’ve also had plenty of  virus samples to work with from people who tested positive for the flu. Those samples help officials  determine which version of the virus will dominate the season. This past nonstarter flu season basically throws a wrench in that well-oiled machine.

See, just because there weren't a lot of  flu cases in humans doesn’t necessarily mean influenza viruses haven’t been  doing their usual mutations in animals. Which doesn’t mean it’ll get bad now, but it could be a problem in the future  if the viruses are transmitted to humans. And now, there’s a hole in the usual  year-round samples of the circulating strains.

But the lack of surveillance and  influenza sequence data isn’t the only problem for the upcoming flu season. Even though influenza mutates, scientists  think that people who have been infected or vaccinated may contribute to  the herd immunity for the next flu season - or maybe even the one after. So even though people still received  the flu vaccine in 2020-2021, the fact that not as many people got  sick with the flu last year means there are likely more people who are  susceptible to getting sick this year.

For the optimists in the house, there are some potential silver  linings to the influenza no-show. For one, it appears that combining  social safety interventions and yearly vaccines can stave off the flu. All of this isn’t to say that we’re doomed  to an unavoidable killer flu season!

It just means things will be trickier. But something that isn’t tricky is  learning with today’s sponsor Brilliant! You can access courses like  Knowledge and Uncertainty to learn tools for managing life’s uncertainty.

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