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The truth about the flu shot: It may not always prevent illness, but it will reduce risk of death

Why get a flu vaccination that doesn’t always prevent the flu? A lot of reasons, as it turns out.
Image: Flu vaccine vial
Vials of flu vaccine are displayed in Philadelphia, Pennsylvania.Matt Rourke / AP

We had only been back from our family trip to Vietnam for a week when my son began running a low-grade fever. We had stuck to touristy areas and taken anti-malarial medication, but it was best to get him checked out just in case. Indeed, when his rapid flu test results came back, his physician joked that it was just an ordinary old influenza B, nothing tropical or exotic.

I had actually expected a flu diagnosis because we’d returned in the midst of the worst flu season the U.S. has seen in years. A whopping 7.4 percent of the country saw their doctor for influenza-like illnesses at the virus’s peak in early February, according to the Centers for Disease Control and Prevention (CDC). Even now, the CDC reports, flu activity remains strong at 5 percent — more than twice what’s usually seen this time of year.

Plus, my son’s symptoms were mild, not the high fever I’d expect from malaria, dengue or typhoid — or influenza in someone unvaccinated. As a science journalist who has reported on vaccines for years, I hadn’t vaccinated my children expecting to prevent them from getting flu. I know the vaccine is often lousy at that. I vaccinated them to make it far less likely they would end up in the hospital — or dead, like 114 other children so far this winter.

For the average person reading about the flu epidemic on the news, I understand the frustration. What is the point of a vaccination if it doesn’t actually prevent the flu?

But for the average person reading about the flu epidemic on the news, I understand the frustration. What is the point of a vaccination if it doesn’t actually prevent the flu? Is the shot ineffective? Does it simply not work sometimes? As it turns out, that depends on what you mean by “work.”

Too many people expect too much of the vaccine while not appreciating what it does do. It’s true that the flu vaccine isn’t very good at preventing illness in the way that most of our best vaccines do — measles, HPV, polio, diphtheria and hepatitis B, for example. Frankly, the virus is so well adapted to infecting humans that our best scientists haven’t been able to create a vaccine that prevents flu more than about 40 to 60 percent of the time (though that still cuts the risk in half).

“It’s what we call a leaky vaccine,” Brendan Flannery, PhD, an epidemiologist in the CDC’s Influenza Division, told me. “It provides partial protection for vaccinated people. It reduces their risk.”

The flu vaccine’s arguably more important purpose is preventing flu hospitalizations and deaths. This year, despite worry that the vaccine would only be about 10 percent effective, the CDC’s preliminary surveillance shows it actually is preventing 36 percent of all lab-confirmed flu, 59 percent of flu in children and 25 percent of infections from H3N2, the most virulent strain this year.

It’s also reducing how severe infections are. “It’s hard to measure effectiveness against more severe illness,” Flannery said. “Severe disease is less common, but usually we see estimates [of hospitalizations and deaths] that are very similar or a little bit better.”

In other words, getting the flu vaccine doesn’t guarantee someone won’t be hospitalized or die from the flu, but it makes it substantially less likely. As in past years, most children who died from flu this year weren’t vaccinated. Flannery led the Pediatrics study last year that found, for the first time, that flu vaccines cut the risk of dying from flu by almost two thirds (65 percent) for healthy children. For children with an underlying high-risk medical condition, the vaccine cut their risk of death in half.

In fact, a person hospitalized for the flu is two to five times more likely to die if they didn’t get the vaccine. In 2015-2016, the most recent year for which data exist, the vaccine prevented about 5.1 million infections, 71,000 hospitalizations and flu/pneumonia 3,000 deaths. And that number is almost certainly underestimated by two to four times because of how deaths are estimated: Total adult flu cases are not tracked, and many people with flu are never tested or seek medical care too late to detect the virus. Further, deaths often occur two to three weeks after initial infection, so it is usually flu complications, such as a secondary infection or an underlying condition worsened by the flu, listed as cause of death without mention of influenza on the death certificate.

A person hospitalized for the flu is two to five times more likely to die if they didn’t get the vaccine. In 2015-2016, the vaccine prevented 71,000 hospitalizations and flu/pneumonia 3,000 deaths.

All of which is to say I wasn’t mad that my vaccinated son got the flu anyway. That shot was probably why he spent his days home from school playing Minecraft and racing Hot Wheels around the house instead of moaning in bed from the aches and fever.

Yet fewer than half of Americans of any age got the vaccine this year. The most common reasons people give for skipping the vaccine is the idea that it isn’t necessary or, also incorrect, that it will give them the flu itself. First off, the idea that you don’t need the vaccine because you “never get the flu” makes about as much sense as never wearing a seatbelt because you’ve never been in a car accident. Never having the flu in the past doesn't mean zero risk of it in the future. And this year’s numbers should remind people how bad the flu can be, even in healthy people — and especially in healthy people who have never been infected with flu virus before.

Skipping the vaccine because you think it will give you flu is also faulty reasoning. Some people develop fever or aches as their immune system does its job responding to the vaccine, but flu shots are made with inactivated flu viruses ground into tiny pieces. The shot’s about as likely to give someone the flu as a chopped-up zombie returning to life. (Even when the nasal vaccine, made with a weakened live virus, returns next year, studies show it doesn’t give people flu, Flannery said.)

It’s ultimately up to each person to decide — every year — whether to get the flu vaccine. But it’s easier to make that decision more confidently with an understanding of what the vaccine actually does — and what it can’t do.

Tara Haelle is a freelance science journalist, Forbes contributor, author of "Vaccination Investigation: The History and Science of Vaccines" and co-author of "The Informed Parent: A Science-Based Resource for Your Child's First Four Years." Follow her @tarahaelle.