How the Flu Can Affect Your Health Even After You Feel Better

There are plenty of reasons to get your flu shot every year to protect yourself against the contagious and occasionally severe respiratory illness known as influenza, as defined by the Centers for Disease Control and Prevention (CDC). For instance, the CDC estimates that 12,000–52,000 people die from the flu every year. And while the shot doesn't offer 100% protection against the virus, it's certainly better than nothing. Even if you do get sick, being vaccinated reduces your risk of getting a severe case of the flu and winding up in the hospital, according to the CDC. It also makes it less likely that you'll pass the flu on to others.

But, there's another, lesser-known, argument for getting the flu shot and for taking other precautions against influenza.

The flu isn't just a health risk for the seven days or so that you're physically sick with the virus—it can also have some lasting effects that could affect your health for weeks, months, or even permanently, according to the CDC.

Heart Complications

Respiratory illnesses can cause a heart attack, according to a 2018 study in the New England Journal of Medicine (NEJM). In this study, researchers found that the risk of having a heart attack was six times higher during the week after being diagnosed with the flu compared to the year before or after a flu infection.

Influenza has been linked to an increased risk of heart complications. Roughly half of the adults that are hospitalized due to the flu also have heart disease, according to the CDC. And another study from 2020 in the Annals of Internal Medicine found that heart complications occurred in 12% of the patients that were hospitalized with the flu.

"Most people who have studied this agree that two to four weeks, and maybe even into that second month, there is an increased risk of heart attack and stroke," said William Schaffner, MD, an infectious disease specialist at Vanderbilt University. But the message hasn't yet reached the general public, Dr. Schaffner added, or even a lot of healthcare providers. "When I mention this to doctors during continuing education classes, they sit up in their chairs; they've never heard this before."

Those with heart disease or who have had a stroke are at higher risk for complications associated with the flu, according to the CDC. It is even more important for these people to get their flu shots to prevent these complications.

Secondary Infections

The flu can also do a number on the immune system, which can leave people vulnerable to other illnesses and infections—like pneumonia, for example, according to the American Lung Association.

And while pneumonia is often referred to as a complication of the flu, it's also not unusual for a person to come down with it once their initial flu symptoms have passed. "It happens quite often," said Sharon Nachman, MD, chief of the division of Pediatric Infectious Diseases at Stony Brook University School of Medicine. "People feel poorly, then they start to feel better, and all of a sudden they feel poorly again, and they wonder why they're not getting better. And actually, it's because you don't still have the flu; you have a new, secondary infection."

Those infections are sometimes bacterial, said Dr. Nachman, which means that antibiotics are likely needed to treat them. It can be normal to feel like you're not at 100% following a bad case of the flu, said Dr. Schaffner—but if you don't feel better after a few weeks, talk to your healthcare provider to rule out something more serious.

Abnormal Test Results

The body has to work hard to fight the flu virus, and it can take time to recover to its pre-flu state. So it shouldn't be surprising that a recent case of influenza can affect the results of blood tests and medical screenings, said Dr. Nachman.

"After most viral illnesses, your white blood cell count is going to be low," Dr. Nachman said. If you're scheduled for any routine testing after a bout with the flu, be sure to mention it to your healthcare provider so they know that it could be a factor in your results.

Physical Decline in Older Adults

For older adults, getting the flu could be the first step in a continual downward spiral when it comes to their health and their ability to take care of themselves. Not only do older adults face a higher risk of serious complications and death while they have the flu, but they're also at greater risk of reduced quality of life afterward, said Dr. Schaffner.

"When an infection like the flu puts you in a bed, it's remarkable how much muscle tone you lose every day," Dr. Schaffner added. "And if you're already on the edge of frailty, it can send you on the downhill slide, and it's very difficult to get your strength and your confidence back completely."

For this reason, Dr. Schaffner said, healthcare providers and loved ones should pay close attention to older adults after a flu diagnosis. "Make sure they're getting the help they need to get back to their normal routine," Dr. Schaffner said. "In some cases, they may even need some physical therapy to help them do that."

Lost Strength and Endurance

That loss of muscle tone and strength is especially dangerous for older adults, but it's also likely to affect younger flu victims as well to a lesser extent, said Dr. Schaffner. Exercise after the flu can help you feel better, Dr. Schaffner added, but he recommended starting with a low-intensity activity like brisk walking. "When I've gone back to the gym after having influenza, I've reduced my weights and my reps for some time before I work my way back up to normal," Dr. Schaffner said.

Dr. Nachman agreed. "Think about it like a trauma: After they take the cast off from your broken leg, you can't run a mile right away," Dr. Nachman said. "And after you've had a really bad viral infection, your body needs time to recuperate—and overdoing it right away can make you feel sicker." Listen to your body, Dr. Nachman said, and if you feel short of breath or overly fatigued, scale back for a few days.

A Quick Review

These post-flu risks may not be as well known or as well publicized as the more obvious symptoms and immediate complications of influenza itself. "It makes flu an even nastier virus than we thought," said Dr. Schaffner, "and we thought it was plenty nasty already, even without these lasting effects." But they should serve as even more reason to get vaccinated and hopefully prevent getting infected in the first place.