The disturbing reason why so many baby boomers are being hit hard with this year's deadly flu

The Centers for Disease Control and Prevention (CDC) says adults aged 50-64 are alarmingly ill this flu season.
There are more older adults hospitalized with the flu than infants, which is very unusual.
Experts think the baby boomers might not have gotten exposed to a key virus strain when they were kids, and it's making it harder for them to fight off this year's flu.

Baby boomers across North America are really sick this flu season. In the US and in Canada, experts who track the flu say people between the ages of 50 and 64 are being hospitalized at alarming rates, second only to the elderly.

That's unusual. In a typical flu season, babies in the 0-4 age group tend to suffer more severe flu symptoms than adults, and tots ordinarily fill up more hospital beds than their parents and grandparents.

It makes sense that infants tend to get sicker with the flu than healthy, fully-grown adults: the babies' tiny systems haven't yet been exposed to as many viruses — a process that takes years — and boosts people's immunity over time. It's the one upside to feeling miserable: getting sick with a diverse swath of viruses over a lifetime makes us more resistant to future infections.

Boomers have decades of nasty flu seasons behind them, which should, in theory, help their very "exposed" immune systems better fight against this year's nasty flu epidemic. But not this year.

"Baby boomers have higher rates [of hospitalization] than their grandchildren right now," CDC flu director Dan Jernigan told reporters on a call Friday.

Matthew Miller, who studies the flu at McMaster University in Canada, told Business Insider that preliminary data from America's northern neighbor suggests the Canadian boomers are just as sick as their American counterparts.

Why are older adults getting so sick this year?

Flu experts believe that something called "imprinting" is key when it comes to fighting off viruses like the flu.

Here's how scientists think imprinting works: when we're young, our first "exposure" to a flu virus (be it from a shot, or be it from a full-blown illness) teaches the budding immune system to fight against that first strain of flu we encounter.

It's great preparation if you're exposed to another flu that's just like the initial imprinted strain, but not if you catch a different strain. Scientists think that's what's happening to older adults this year, who were born before the vile H3N2 strain, which is circulating wildly this year, even existed.

This years worst strain of the flu didnt show up until after boomers were born

The H3N2 flu (sometimes referred to as "Aussie flu") first showed up globally during the disastrous 1968 flu pandemic, which killed more than a million people around the world. By that point, adult boomers (the youngest of whom are in their early 50s) were already toddling around, and had likely picked up a different H2N2 virus as their imprint.

Most children today probably have their first exposure to flu not through a full-blown illness, but through the flu vaccines they can get once they reach six months old. Kids who got this year's shot as their first dose of flu were exposed to a few different strains of flu virus. The most common flu shot used this year had portions of three strains: H3N2, H1N1 (sometimes called "swine flu") and an influenza B.

There's some evidence to suggest that our "imprinting" isn't as strong when it's done this way through a vaccine, but it is broader coverage against future flus than the protection from a single strain after being sick.

Imprinting can be a handicap

Imprinting might not always be helpful, even when it comes to fighting similar flus. New research suggests it can actually hurt your chances of fighting off another flu if you're born in a pandemic flu year, where the virus is raging around the world.

Miller thinks it's possible that kind of dramatic first imprint might damage a patient's immune system, and that could be why it's harder for them to fight off other, similar strains when they come back into circulation.

This year's flu may feel particularly nasty, especially if you're ill or caring for a sick family member, but we haven't reached "pandemic" level of flu, and death rates in the US aren't shocking authorities at the CDC, who typically report tens of thousands of flu deaths every year.