This Year’s Flu Is Especially Miserable. Here’s Why.

It’s a chicken-egg problem—but not what you think.


If you haven’t gotten it, thank your lucky stars and brace for it. This year’s influenza is a violent storm of body aches, hacking, fevers, and feeling pummeled. Emergency rooms in California are reporting crowds and pharmacies unable to keep up with demand. And death rates are worrisome.

The culprit? A tiny monster named H3N2 (sometimes called the Aussie flu, where it was first reported this summer) that’s gotten more voracious over time, mutating its form unpredictably.

“It’s been a bad flu year,” Scott Hensley, an associate professor of microbiology at the University of Pennsylvania, told The Daily Beast. In October, Hensley co-authored a paper in PNAS about why the flu vaccine this year didn’t protect us as well as we’d hoped.

Blame chicken eggs.

The flu shot can be thought of as a weather vane for the upcoming influenza season, containing a cocktail of influenza strains that scientists believe will be attacking the country. That usually contains forms of H1N1, influenza B, and this year’s H3N2.

But that H3N2 strain in this year’s flu shot hasn’t been able to combat the flu circling your office. That’s because it’s not the same strain at all.

“The problem with flu viruses is that they’re constantly changing,” Hensley explained. “We need to update our flu vaccines annually, but the viruses are constantly changing as they are circulating among humans. It’s a constant [game of] cat and mouse.”

That cat-and-mouse game means that while viruses are evolving faster and faster, scientists developing the flu shot are struggling to keep up. This year’s vaccine was a “mismatch,” according to Hensley, of what the H3N2 was expected to look like and how it actually turned out. “There are different H3N2 viruses, and if you’re looking at the sequences, there is diversity in what is circulating,” he said.

That brings us back to the chicken egg problem. Flu vaccine development involves choosing the predicted human virus strains, which are then propagated. The viral bits have proteins that are isolated from the viruses and injected into a fertilized chicken egg; the virus then replicates again in the chicken egg. After the virus has had a couple days to “amplify,” it’s isolated, with viral proteins from the chicken egg being activated to create what will then become the flu shot being jabbed into your arm at the local clinic, in hopes of helping you fight the flu.

The problem here is that chicken eggs are the mainstay in this process, and perhaps not the most practical way to create a flu shot, especially for the H3N2 strain, which Hensley said does not grow well in chicken eggs. “When the H3N2 strain is used, it actually changes to adapt to the environment of the chicken cell and changes its antigenic structure”—or the abrupt change in the virus that means humans aren’t prepared for, leading to a fast spread.

In plain English: The replication process in a chicken egg’s cell becomes one more chicken-like than human-like, and that change inhibits our ability to fight the flu, fundamentally altering the location of the antibodies used to target the virus.

There are other reasons why the flu is so terrible this year, some of which can be traced to other factors. For one, there’s climate change. A 2013 study in PLOS: Epidemiology showed that warmer than average winters in one year are followed by earlier, more severe flu seasons the next. That pattern certainly holds true here: The 2016-17 winter season was mild, while this year’s has been brutal across the country, marking those who weren’t sick last year as susceptible to this year’s flu.

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The solution to this chicken egg conundrum might seem simple: Why don’t we just inject the human strains into some other cells, like insect or canine cells?

It’s not that easy, Hensley said. “They [chicken eggs] are the majority [of vaccine development] in the U.S.,” he said. “Most vaccine manufacturers rely on chicken eggs. It’s difficult to just switch, it’s a different production process.”

Despite this, Hensley strongly encourages Americans to get the flu shot despite its shortcomings. “The vaccine might not work well because of the egg adaptation, but people should get vaccinated,” he emphasized. “It’s not ideal but it will prevent some infections and almost certainly severe disease.”

That said, Hensley said the H3N2 fiasco is not new—he said it happened last year as well—and that it’s a strong case for the universal flu vaccine. “That would not have to be updated every year, and we wouldn’t have to be at the mercy of egg adaptations and antigenic drift events,” he said, referring to the idea that once we’re exposed to a strain, we’re able to fight it, but that the immune system fails when it comes to recognizing related antigens—what is happening with the H3N2 strain. “We need to fund the basic science to move it [research] along and come up with a different approach.”

Until then, though, get yourself a flu shot—and wash your hands.