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The Flu Shot is Less Effective in Certain Populations

Research finds specific groups are less protected by the flu shot than the general population.

Credit: Brian Snyder | Reuters/Landov

Adults over 65 are at higher risk of developing serious complications from the flu — you’ve heard that before. Seniors’ immune systems are weaker, and the antibodies that come with the vaccine dissipate more quickly than in the rest of the population.

It’s an unfortunate symptom of aging that researchers have also observed in overweight and obese patients, including younger ones. And providers need to keep this in mind as flu season ramps up. At the Centers for Disease Control and Prevention’s last count, ten states are experiencing widespread influenza activity.

Obesity and the Flu Shot: What the Science Says

The CDC considers obese people at high risk for severe flu complications, and numerous studies indicate that obese people experience worse flu symptoms, NPR reports. One, in particular, found that obese participants exhaled more flu virus than those with lower body weight.

“The virus is able to grow faster … and spread more in overweight/obese animal models, and there are studies now that suggest that’s also true in people,” Stacey Schultz-Cherry, Ph.D., an infectious disease specialist at St. Jude Children’s Research Hospital, stated.

After observing this, researchers began investigating how the flu shot affected obese people. According to Dr. Schultz-Cherry, one of the most salient findings Is that the amount of antibodies present immediately after vaccination is the same in obese and lower-weight people — but the obese population “loses their antibody response much faster,” she says.

“The lack of vaccine efficacy in overweight/obese people looks very similar to what we see in elderly adults,” Dr. Schultz-Cherry adds. “We’re trying to understand the underlying mechanisms and reasons for this.”

What Other Populations Aren’t As Protected by the Flu Shot?

Many populations are at risk for severe flu complications, from pregnant women to people with respiratory and cardiac illnesses. But falling into this group doesn’t necessarily mean the flu shot is less effective for these individuals.

For example, for pregnant women, the cause is “more anatomical,” says William Schaffner, MD, medical director of the National Foundation for Infectious Disease and professor of infectious diseases at Vanderbilt University School of Medicine.

“Women who are pregnant who get influenza have complications similar to the rates of older people,” Dr. Schaffner tells Florence Health. “[They] hold on to body fluids more, and a wet lung is more apt to get complications of pneumonia. Also, a baby in the uterus pushing up against the diaphragm can squish the lung, which also predisposes to pneumonia.”

That said, pregnant women could fall into the category of “less protected by the current flu shot” — alongside obese and elderly people. But there needs to be more research first, Dr. Schultz-Cherry says. She also wants more studies on how new babies and people with underlying conditions (especially respiratory and cardiac) respond to the flu shot.

“The question is: How well do the vaccines work in these populations?” Dr. Schultz-Cherry explains. “That’s becoming more important as we see news reports each year about the vaccine’s efficacy decreasing … [The way we make the vaccine] hasn’t changed since it came out in the ’70s. What’s changed is our population.”

What Can Clinicians Do?

Dr. Schultz-Cherry and Dr. Schaffner recommend that clinicians discuss the flu shot with high-risk patients and emphasize preventive measures. 

Talk to Patients About How the Flu Shot Works

Above all, clinicians should advise patients to get the flu shot even if it’s less effective for them. Acknowledge their concerns — “There’s nothing wrong with admitting it needs to be better,” Dr. Schultz-Cherry says — but stress that it’s still the best prevention strategy.

If they remain hesitant, it can help to explain that the vaccine protects against four different strains of the virus, which Dr. Schultz-Cherry says many patients don’t realize. In addition, they may not understand that if they get sick, the vaccine will minimize symptoms, complications (including heart attack and stroke), and further spread of the disease.

“It can prevent you from dying, which is even more important in our high-risk population,” she adds.

For patients still on the fence, add that even if there’s “a mismatch,” where the shot doesn’t combat the annual strain as well as anticipated, inoculated patients still will experience less severe illness. “Many doctors don’t know that,” Dr. Schaffner says.

Double-Check You’re Using the Correct Needle

Dr. Schaffner notes that it’s not uncommon for people administering the flu shot to use the incorrect needle when vaccinating an obese person. (This problem doesn’t appear to contribute to the challenges previously described by Dr. Schultz-Cherry.)

“[Without] the appropriate length needle, the vaccine may be inoculated into fatty tissue, where the immune cells might not have the same physical access to the vaccine and therefore the response isn’t what we’d expect from a leaner population,” he says.

The CDC has guidelines about needle length for the flu shot based on weight.

When Possible, Use Population-Specific Vaccines

Multiple flu vaccines are available that research indicates are more effective in people 65 and older, but it’s unclear if they offer the same benefit to younger, obese adults. 

Traditionally, facilities need to request these vaccines a year in advance. You can talk to an administrator to make sure your workplace is prepared.

If an older adult requests such a vaccine but you don’t have one, encourage them to get vaccinated with what you have, per CDC recommendations. “A vaccination deferred is a vaccination never received,” Dr. Schaffner says.

Stress the Standard Precautions

For patients with a high risk for complications — and for whom the vaccine may be less effective — the following preventive behaviors are especially important:

  • Wash your hands often. This is more effective than hand sanitizer.
  • Avoid contact with sick people.
  • If you start to experience symptoms, especially around sick people, call your provider and ask for a prophylactic antiviral, such as Tamiflu. These only work if you use them within the first 72 hours of exposure.

Unfortunately, a universal flu vaccine is still several years away. Until then, ensure you provide patients with the best care by considering their circumstances

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