If you’re feeling under the weather—tired, achy, with a cough or runny nose; possibly feverish—well, first of all, I hope you feel better soon because that sucks.

But if it’s more than a cold, you might have influenza, or the flu. And even if you’re feeling fine, there’s a good chance you know someone who isn’t.

Flu season is picking up steam, and it’s turning out to be a pretty bad year for North America— especially in the US. And as of the first week of January, flu was widespread in virtually every part of the US, along with some parts of southern Canada. In the United States, some 23 out of every 100,000 people had been hospitalized for the disease.

That’s nearly twice what it was the week before, and three-quarters of the way to the number of cases three years ago, which was one of the worst flu seasons in recent memory. But why is this year any worse than normal? The simple answer is that more than 80% of the flu viruses making the rounds, at least in the US, are a subtype called H3N2.

And in the flu world, H3N2 is a baddie. These viruses tend to not only make more people sick, but to make more of them very sick, which means more hospitalizations and deaths. Researchers don’t have everything entirely figured out, but they do know that there are at least a couple of reasons for this.

The first has to do with the fact that H3N2 viruses are tricky to vaccinate against. You might know that flu viruses come in a few different main flavors, named A, B, C, or D. ‘A’ and ‘B’ are the ones we encounter with seasonal flu, and influenza A viruses are further broken down by the kind of proteins on their surface. The ‘H’ stands for hemagglutinin, and the N is for neuraminidase.

Each type of these proteins gets a number, like H1 or N2. Since the proteins are on the outside of the virus, they’re what your immune cells see and respond to if you get infected. But for that very same reason, if flu viruses mutate, changing what these proteins look like, the virus can escape your immune response and get off scot-free.

This is called antigenic drift, and it’s why you need a new vaccine every year. Since flu viruses are changing all the time, scientists have to make an educated guess as to which specific viruses are likely to circulate in a given season, and then make a seasonal flu shot that will protect against those three or four strains. In the case of H3N2, this is especially hard to do because those viruses mutate even more quickly than their cousins.

That means there’s a higher chance that in the six months or so between when the scientists choose the strains in their vaccine and when flu season starts, the virus could have changed. It mutates so quickly that the virus can even shift during the flu season. A second complication is that in recent years, the H3N2 viruses we’ve wanted to vaccinate against are ones that don’t grow very well in chicken eggs.

That becomes a problem because almost all flu shots are made by growing lots of virus in eggs. And to grow well, the virus can mutate to adapt to chicken cells. That’s all fine and dandy if it’s not a part of the virus that matters.

But if the mutation changes one of those main surface proteins, like hemagglutinin, it could make the vaccine much less effective. This appears to have been what happened last season, and it’s almost certainly the case this year as well. Researchers were puzzled because last season’s H3N2 strain was well-matched, but that part of the vaccine was only about 30-35% effective.

Virologists looking at the issue realized that the hemagglutinin protein that ended up in the vaccine was missing an important spot on it that had a sugar attached. That change made it easier for the virus to infect chicken cells, but it no longer looked as much like the virus that was infecting people. So the antibodies people made in response to the vaccine weren’t as good at clearing out the real virus.

The other unfortunate aspect of H3N2 viruses is that they take an especially heavy toll on the elderly. Flu is usually worse for older people. They’re often frail, or sick with other things, and their immune systems simply aren’t as robust.

But H3N2 viruses take this to the extreme. And researchers have a pretty interesting idea for why that is, although it hasn’t been proven. It’s called original antigenic sin, or imprinting, and the idea is that how well you respond to the flu depends on which flu viruses your immune system sees first.

Whatever you were first exposed to as a kid, your immune system knows super well and can handle with relative ease. Less so with later strains. They might look enough like the ones you were exposed to earlier in life to trigger the same response, but because the virus is different, it’s not as good a defense.

The first H3N2 virus reared its ugly head around 1968, so people over the age of about 50 didn’t grow up seeing these viruses. That could mean their immune systems are really better suited to fight off different types of flu, and explain why they’re more susceptible to H3N2 viruses now. So, with H3N2, you have a flu subtype that’s potentially more deadly, simply because of timing and age, and a vaccine that’s less effective.

But the takeaway here should not be to skip your flu shot if you haven’t gotten it yet, especially since flu season is nowhere near over. It usually peaks around February and doesn’t end until May. The vaccine still protects against other types of flu, which are also circulating.

And even reduced effectiveness is better than no effectiveness, especially if you’re at risk, because it can mean if you do get sick, it won’t be as bad. By getting vaccinated, you will also protect those around you through what’s known as herd immunity. If enough people have even partial protection, that can cut down on the total amount of flu going around and make it less likely that people who can’t get the shot will catch it, like infants and those with weakened immune systems.

So, even if you feel like you particularly don’t need the flu shot for yourself, think about the fact that you will become a vector who may infect someone who can’t get that protection. So go get jabbed! And you know who else does good things for people around them?

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