Wondering if you should get a flu shot? Here’s what science says

By Brenda Goodman, CNN

(CNN) — Flu is making people miserable from coast to coast, thanks in large part to a new strain called subclade K.

That’s why infectious disease experts are telling people to run, not walk, to get this season’s flu shot if they haven’t yet.

But you may be wondering whether getting a flu vaccine will still help or even if getting the vaccine might be riskier than getting sick. It can help, and the flu is far riskier — but misinformation about vaccines is almost as rampant as the germs they protect against, and it can be hard to know what to believe.

Here are some of the most common myths about flu vaccines and what the science shows.

Myth: The flu shot doesn’t work.

The reality is that people who get a flu shot are less likely to become severely ill, be hospitalized or die.

In a social media post Tuesday, Jim O’Neill, acting director of the US Centers for Disease Control and Prevention, said that a review by the US Department of Health and Human Services “found no randomized controlled trials demonstrating reduction of community transmission, hospitalizations, or mortality in children from the pediatric [flu] vaccine.”

He’s right, says Dr. Mark Loeb, a flu researcher at McMaster University in Ontario – but that’s because randomized controlled trials aren’t the right tool for measuring severe outcomes of the flu.

“For randomized controlled trials, most of them are underpowered to detect outcomes like even hospitalizations or deaths,” Loeb said, meaning they don’t have enough people in them to accurately detect differences in these outcomes, which are less common than infections.

Loeb published a meta-analysis, or study of studies, in October in the journal Clinical Microbiology and Infection that included 165 observational studies with “test-negative designs,” which look at all the people who are treated for the same set of symptoms in an emergency room or clinic and then compare the vaccination status of those who tested positive for the flu against those who tested negative.

If a vaccine is effective, there will be fewer vaccinated people in the group that tests positive for the flu compared with those with the same symptoms who tested negative.

Test-negative studies are powerful because they eliminate an important type of bias in observational studies: that people who go to the doctor may be generally more concerned about their health and more likely to get vaccinated than those who don’t go to the doctor.

Together, the studies included more than 600,000 participants. They found that vaccines provide protection against severe flu complications at any age and regardless of how well the shots match the circulating flu strains.

The analysis found that, on average, flu vaccines cut the risk of hospitalization or intensive care admission for children by about half and the risk of pneumonia by 70%. For adults up to 65, vaccination reduced the odds of hospitalization or pneumonia by 40%. Those over 65 saw a roughly 30% reduction in the risk of hospitalization, a 45% reduction in the risk of pneumonia and a 53% drop in the odds of needing intensive care.

“I think it is particularly important year for people to get a flu shot, given the increased severity of the [virus] that’s circulating, and our paper provides good proof for this,” Loeb said.

Myth: There’s no point in getting a flu vaccine since it doesn’t match the new strain, subclade K.

The reality is that the current vaccine is still offering important protection.

It’s true that there’s a new flu virus going around and that it’s not included in this year’s shots. But that doesn’t seem to be affecting how well the vaccines work as much as scientists first feared it might.

Subclade K is an A-type flu strain, specifically an H3N2 virus.
H3N2 strains change quickly and often bedevil the best efforts of scientists to choose strains to include in each year’s vaccines.

That happened this year. After the three strains for this year’s shots were chosen, H3N2 began circulating in the Southern Hemisphere, driving a record flu season in Australia.

One big question has been how well this season’s flu shots protect against this new strain. Years when an H3N2 virus is the main driver of infections tend to have more severe disease, making answers about vaccine effectiveness even more urgent.

There’s been mixed evidence on this point. World Health Organization scientists reported in September that the blood of ferrets vaccinated with the current season’s shots didn’t neutralize subclade K viruses very effectively.

In November, though, the United Kingdom’s Health Security Agency analyzed nearly 8,000 emergency room visits and hospitalizations and determined that this year’s flu shots were still surprisingly effective, despite subclade K mismatch. The vaccines cut the risk of kids needing to go to the emergency department or be hospitalized by about 75%. They reduced the same risks for adults by between 30% and 40%.

Now, scientists at the University of Pennsylvania’s Perelman School of Medicine have weighed in. For a preprint study, published ahead of peer review, the research team took blood samples from 76 adults about a month after they’d gotten this year’s flu shot. They tested the serum, or the clear part of the blood, against several flu strains to see whether the antibodies in it could effectively recognize and bind to the viruses.

“We were surprised to find that many individuals that were vaccinated with this year’s vaccine produced substantial amounts of antibodies that could effectively bind and neutralize subclade K viruses,” study author Dr. Scott Hensley said. His lab is part of the Centers of Excellence for Influenza Research and Response, a network of flu researchers supported by the National Institutes of Health, and they do rapid work to get answers to urgent question about the flu. Hensley said his team worked through their Thanksgiving break to complete the experiments.

Before vaccination, only eight of the 76 study participants, or 11%, had neutralizing antibodies against subclade K viruses. After vaccination, 30 out of 76 people, or 39%, did.

“The message is very clear here,” Hensley said. “In years when there are antigenic mismatches, as is the case this year, the vaccine can still provide protection.”

Whether or not a person made it into the group that developed antibodies against subclade K probably depends on what viruses they were exposed to in the past, Hensley said. So there’s no way of knowing whether you’d be in that group.

“This isn’t perfect, but if you can decrease your chance of getting flu this year by fourfold, man, that’s a vaccine that I want,” Hensley said.

Myth: The flu shot can make you sick.

The reality is that people may feel poorly shortly after the shot, but it’s not severe, and it’s not flu.

Plenty of people – about 1 in 3, according to some studies — feel bad for a day or two after they get a flu shot. The most common symptoms include headaches, muscle aches, fatigue and fever, all of which can feel an awful lot like the flu itself. But it’s not.

“It’s biologically impossible,” said Dr. William Schaffner, an infectious disease expert at Vanderbilt University. Because flu shots contain only pieces of the influenza virus, there’s no way these can cause an infection.

“Yes, we can get a sore arm that’s a little swollen, and some people may get a degree of fever and have a headache after receiving influenza vaccine, but that’s not the flu. That’s just your immune system responding to the vaccine that’s in the immediate period right after getting the vaccine.”

Vaccines may use slightly different technology, but they all work according to the same basic principle: They show the immune system a piece of a virus or bacteria so it will recognize and respond to the real thing more quickly. Think of your adorable preschooler sneezing in your face as you tuck them into bed or the stranger in the subway who coughs a cloud of germs in your direction. A vaccine primes your immune system to spring into action quickly after you’re exposed.

In the process of building that immunity, vaccines can trigger inflammation. One small study that tracked symptoms in 56 women in the days after their flu shots found that most reported a sore arm. This was generally mild and resolved within a day or two.

Ten women reported bodywide symptoms like fatigue, headache or sore throat. Six women said their symptoms were gone after a day. Two had symptoms on the second day after vaccination, and one still had symptoms on the third day. Most described their symptoms as mild. Women who reported symptoms after their flu shot also had an increase in certain proteins in their blood that are markers for inflammation, leading the researchers to conclude that inflammation might be driving their symptoms.

The good news is that everyone in the study developed protective antibodies against the flu, whether they experienced symptoms or not.

Myth: I got the vaccine, so I won’t get the flu.

The reality is that you can still get the flu after vaccination, but the vaccine can protect from severe illness or death.

Studies show that flu vaccines do prevent some people from getting the flu, but in general, vaccines aren’t great at preventing respiratory infections, and it’s not really their main benefit. What they do best is prevent the worst consequences of flu infections: hospitalizations, pneumonia, heart attacks and death.

Here’s why: Vaccines create immunity all over your body in the form of Y-shaped scavengers called neutralizing antibodies. These proteins have two outstretched arms that recognize specific places on flu viruses and latch on to block them from infecting cells. These antibodies live in your blood, the clear fluid that surrounds cells called lymph fluid, and in body tissues.

Flu viruses usually enter the body through the moist tissues – the mucosa – that line our nose and throat. That’s where the infection begins. To stop an infection completely, vaccines would need to create more of a different type of antibody, called IgA antibodies, that lives in these mucosal tissues and could block the viruses from getting a foothold there.

Some vaccines — typically nasal sprays like FluMist — are designed specifically to create mucosal immunity. FluMist is one option for vaccination, and a good one for people who don’t like needles, but in general, injected vaccines have been shown to provide more consistent protection, especially for adults.

Flu infections are typically mild when they get started. These early symptoms may feel awful, but they’re not nearly as serious as what happens days to weeks later if the infection moves into the lungs and turns into pneumonia or causes widespread inflammation, which increases the risk for cardiovascular events like heart attacks and strokes. One study found the risk of a heart attack jumps sixfold in the week after a confirmed flu infection. Vaccination, on the other hand, cuts the risk of heart complications by 34% to 45%.

Doctors have also been seeing more of a rare but serious complication called acute necrotizing encephalopathy, which causes brain damage after the flu. These complications are far more common in people who were not vaccinated before they got sick.

An updated evidence review looking at the effectiveness of vaccines against Covid-19, RSV and influenza, published in October in the New England Journal of Medicine, found that for children, flu vaccination cuts the risk of needing to see a doctor by about 55% and reduces the risk of hospitalization by 67%.

“So vaccination basically eliminated about two-thirds of the hospitalizations that would have occurred without it,” said the study’s lead author, Dr. Jake Scott, an infectious disease expert at Stanford University.

For adults through age 64, flu vaccines cut the risk of needing to see a doctor or being hospitalized with the flu by 49% and 48%, respectively.

Seniors, 65 and older, had a 41% and 42% reduced risk of needing to see the doctor or being hospitalized with the flu, respectively.

“We can’t expect a shot that we get in our arm to put up this magic force shield that blocks those respiratory viruses from entry in our nose, but they do prevent complications,” Scott said.

Myth: Kids aren’t supposed to get the flu shot anymore.

The reality is that pediatricians still strongly recommend that most children get the flu vaccine every year.

This week, HHS pared back the number of vaccines routinely recommended for American children, saying the decision to get a flu vaccine should be made only after consultation with a health care provider. Vaccinations against rotavirus, Covid-19, hepatitis A and B and meningitis are also now in this “talk to your doctor” category. HHS says these shots will still be covered by insurance if families want to get them.

Pediatricians and infectious disease experts have responded, saying that putting the shots in this category only increases barriers to vaccination and may prevent kids from getting needed protection against dangerous germs. They say they’ve long had discussions with patients about the benefits and risks of vaccines before offering the shots, so this doesn’t really change what they do in their practice. Instead, they say, the change suggests that there are safety issues that aren’t being discussed before families vaccinate children. It casts doubt on the safety of vaccines, despite the fact that there’s no new evidence of increased risks that would suggest such enhanced discussion are warranted.

Some states and professional associations, such as the American Academy of Pediatrics, say they are following the science and continue to recommend annual flu shots for kids 6 months and older.

During the last flu season, 280 children died of complications of the flu, a record for a non-pandemic year, according to the CDC. Of the children whose vaccine status was known, almost 90% of those who died were not fully vaccinated. Almost half who died had no underlying medical conditions before they got sick.

Myth: Everyone needs the same flu shot.

The reality is that which flu shot you get depends on your age and your preferences.

Starting in 2022, the CDC’s Advisory Committee on Immunization Practices recommended that people 65 and older get stronger types of flu vaccines: those made with either a higher dose of active ingredients or those that have an added ingredient, called an adjuvant, that boosts immune response. Those are brand-named Fluad, Flublok or Fluzone high-dose.

“Those three work better for older adults,” Schaffner said. “All of them provide about a 25% enhanced protection for people who are age 65 and older.

“Just as we physically become not as robust as we get older, such as when we were 22 years old and could run the hundred-yard dash very, very quickly, most of us can’t do that when we get older, so our immune system does not function as vigorously, and these three vaccines give an extra punch, if you will, to our immune system,” Schaffner said.

For people who prefer to avoid needles, especially young kids and non-pregnant adults ages 18 to 49, the nasal spray called FluMist also offers good protection.

Because FluMist contains a live but weakened form of the virus, it is not recommended for children younger than 2, adults 50 and older, pregnant women or those with immunity that’s been severely compromised by disease or immunosuppressive medications.

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