How Effective Is the Flu Vaccine?

Medically Reviewed by Carol DerSarkissian, MD on August 26, 2021

According to the CDC, the flu vaccine reduces the odds of getting the flu by about 70%. But that number varies from year to year and among different groups of people.

You may wonder why there's such a wide range. And in fact, it's even wider than it seems: that statistic only applies to healthy adults. It turns out that the effectiveness of the flu vaccine depends on a number of different factors. Here's a rundown of what they are.

Your Age

The flu vaccine doesn't work equally well in all people. It’s most effective in healthy adults. In young children under 24 months, the flu vaccine is a little less effective at preventing the flu. It’s more effective as children get older.

After middle age, immunity naturally becomes weaker. The flu vaccine won’t work as well as it once did. But because the flu virus is much more dangerous for older people, it’s crucial that they get the vaccine. Even in cases where it doesn’t prevent the flu, it can still reduce the risk of serious side effects. Studies show that in older people who do not live in a care facility, the flu vaccine can cut the risk of hospitalization (for flu and pneumonia) by 30% to 70%. In people who do live in a nursing home or care facility, the flu vaccine is 50% to 60% effective in preventing hospitalization and 80% effective in preventing death from a flu complication.

There may also be slight differences depending on which vaccine you get.

This season, the American Academy of Pediatricians recommends that pediatricians offer flu shot to all children ages 6 months and older. The shot has provided the most consistent protection against the flu virus in recent years. Along with the CDC, the AAP supports the use of the nasal spray vaccine for children ages 2 and older for the 2021-22 season, but the spray' has not been shown to be effective against the A/H1N1 strain of virus.

A high-dose vaccine called Fluzone is recommended for adults age 65 and older when available. The high-dose flu shot contains four times as much active ingredient as a regular flu shot tp provide better immunity.

Your General Health

Vaccines work by spurring the immune system into action. In a sense, a vaccine "teaches" your body how to identify a virus and how to defend against it. Then, when you come into contact with the actual virus, your immune system quickly recognizes it and fights it off.

So the effectiveness of a vaccine depends on how vigorously the immune system responds to it. If you have a weak immune system to begin with, a vaccine may just not work as well. Many chronic illnesses can weaken a body’s defenses. The CDC estimates that the flu vaccine reduces the risk of hospitalization (for flu and pneumonia) by 30% to 70% in people with chronic illnesses.

When You Get the Flu Vaccine

While the flu vaccine was once only be available between October and the end of November, experts stress that you can now get it into December and January. Keep in mind that the flu season often doesn’t peak until February or later.

But the sooner you get it, the better. Why? Simple: the further you get into the flu season, the higher your risk of getting flu. Here’s something else to keep in mind: it can take two weeks for the flu vaccine to take effect. So if you’re exposed to the flu within that two week period, you might still get sick.

Once a flu season is over, the old vaccine is not as effective, so an annual flu shot is needed for optimal protection.

How Well the Vaccine Is Matched With the Dominant Flu Strains

Unlike other vaccines, the flu vaccine is often updated each season to protect against what researchers believe will be the dominant strains of the flu that year. Predictions are based on world-wide monitoring of viruses. While predictions are generally accurate, they aren't foolproof. The effectiveness of the flu vaccine in a given year depends on their accuracy.

Unfortunately, getting the flu vaccine isn’t a guarantee that you won’t get the flu, but it is thought to provide at least partial immunity. If you catch the flu despite getting the vaccine, your symptoms may be milder.

So, don't skip the vaccine -- especially if you're at high risk for flu complications. Even though the flu vaccine may not work quite as well in young children, older adults, and the ill, these very same people are the most likely to have severe and even life-threatening complications from the flu. It’s crucial that they get vaccinated. While it may not be perfect, the flu vaccine is the best defense we have.

One more thing to keep in mind: the flu vaccine does not protect against cold viruses. Some people believe that the flu shot doesn’t work because they get sick despite being vaccinated. But in most of these cases, experts say, the flu vaccine did work -- it’s just that these people came down with an unrelated cold virus.

Show Sources

Curtis Allen, spokesman, CDC, Atlanta. 
CDC web site: "Questions and Answers: How Well Does the Seasonal Flu Vaccine Work?"
Christine Hay, MD, assistant professor, University of Rochester Medical Center, Rochester, N.Y. 
Hayden, F. "Infectious Disease: Respiratory Viral Infections," ACP Medicine, 2006. 
Trish M. Perl, assistant professor of medicine, Johns Hopkins Medical School, Baltimore. 
William Schaffner, MD, professor of medicine and preventive medicine, chairman of the department of preventive medicine, Vanderbilt University School of Medicine, Nashville. 
CDC: "Nasal Spray Flu Vaccine in Children 2 through 8 Years Old." 

American Academy of Pediatricians: "AAP Issues Flu Vaccine Recommendations for 2018-2019."

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