This year’s awful flu season, mapped

By Julia Belluz

It’s still not too late to get a flu shot.

Flu is everywhere in America right now. If you’re not sick, there’s a good chance some of your friends, family members, or colleagues are.

Just look at how 2017 compares to the past two years in this GIF showing levels of flu activity — from sporadic to widespread — in the last week of December:

 Javier Zarracina/Vox
Flu activity at the end of December, in 2015, 2016, and 2017.

“This is the first year we’ve had the entire continental US be the same color on the [map],” Dan Jernigan, head of the CDC’s flu division, summed up today in a press briefing.

The season started early, and while we seem to be at peak flu right now, public health officials anticipate flu viruses will circulate for a few more moths — bringing with them a lot of illness, hospitalizations, and even death.

Around the country, there are already reports of doctors’ offices bursting at the seams with flu patients, and Alabama just declared a public health emergency over the flu. So why is this year’s flu season so wretched? And what can you do about it? Here’s what you need to know.

Why this flu season is so bad

Flu is a viral respiratory disease, and it leads to nasty fevers, headaches, coughs, muscle pains, and runny noses that make many people miserable in the fall and winter.

There are four species of influenza — A, B, C, and D — and seasonal flu is caused by influenza A and B viruses. Every year, different strains of these viruses circulate. The reason this year’s flu season is more severe and widespread is because it involves the dreaded H3N2, a strain of the influenza A virus that’s more difficult to prevent.

H3N2 is known to cause particularly nasty infections; it hits people harder than other seasonal flu strains and can be especially deadly among vulnerable groups like the elderly and children. So a flu season involving the H3 virus is generally nastier — with more hospitalizations and flu-related deaths — than seasons involving mostly H1N1 or influenza B viruses.

This year, more than 80 percent of flu cases involve H3N2 strain. And the CDC’s Jernigan reported that there were 22.7 hospitalizations related to flu per 100,000 population during the week of December 31, 2017, to January 6, 2018. That was up from 13.7 the week before. “We’re seeing a rapid rise in the number of people being hospitalized,” Jernigan said.

The highest rates of hospitalizations are occurring among those over age 65, but children younger than 5 years are also getting very sick.

It’s not yet clear what the final toll of this year’s flu season will look like, since it’s expected to continue for a few more months. But for context, in recent years, the CDC has found, mild flu seasons tend to kill about 12,000 Americans, and severe flu seasons kill about 56,000.

What happened in Australia, where the flu season peaks in August, might be a hint of what we’re in for here. H3N2 struck there as well — and contributed to more than two-and-a-half times the number of flu cases compared to the previous year. There were also 745 deaths, compared to the five-year average of 176 deaths, according to the Sydney Morning Herald.

H3N2 is especially difficult to prevent with the flu shot

Another reason having the H3N2 strain in circulation is so nasty is because it’s hard to prevent with the flu vaccine.

To understand why, you need to understand how the flu vaccine works — and why it is by no means perfect.

The vaccine is designed to protect people against three or four strains of the A and B viruses that researchers believe will be most common in a given year. So every year, public health agencies essentially make educated guesses on what strains and mutations will make the rounds. As you can imagine, this is no easy task — and in reviews of the research on flu vaccine effectiveness, researchers have found that years when H3N2 is circulating tend to be years when the vaccine is less effective.

Edward Belongia, a senior epidemiologist at the Marshfield Clinic Research Institute in Wisconsin who has studied flu vaccine effectiveness, found that the combined vaccine effectiveness during H3N2 seasons was 33 percent, meaning getting vaccinated reduced a person’s risk of having to go to the doctor by about a third. The flu shot’s effectiveness rose to 54 percent during influenza type B seasons, and 67 percent during H1N1 seasons.

There are a couple of reasons why H3N2 seems to be particularly resistant to the flu shot. H3N2 mutates as it moves through the population at a faster rate than other flu viruses — making it even harder to design a vaccine that can fend off the circulating virus. It’s also more difficult to grow H3N2 in eggs, where viruses for flu vaccines are produced. “In the process of adapting virus to grow in eggs, that seems to introduce further changes to the virus, which may impair the effectiveness of the vaccine, Belongia explained.

It’s not too late to get a flu shot

Still, if you haven’t done it yet, it’s a good idea to get the flu shot anyway. Flu season will last several more months, sometimes even stretching into May.

“Flu season is peaking now — but we know from past experience it’ll take many more weeks to slow down,” said CDC Director Brenda Fitzgerald. She also noted that other types of flu virus — like H1N1 or influenza B, which are more easily prevented with the vaccine — are on the rise now. Getting vaccinated now can reduce your risk of these other infections even if it won’t necessarily prevent H3N2.

Belongia noted that even 33 percent protection against H3N2 is better than no protection. “That still …read more

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