Between the fever, body aches and exhaustion, the flu can turn an ordinary week into missed work, canceled plans and nights spent wondering if you’ll ever feel like yourself again.
But avoiding influenza isn’t always straightforward. Does last year’s flu shot still count? Can vaccination actually prevent infection? And with federal vaccine guidance sometimes feeling confusing, when should you actually roll up your sleeve?
Here’s what to know about common flu symptoms, why the illness can become dangerous and when it’s usually best to get vaccinated.
What Is the Flu? What Are Common Flu Symptoms?
Influenza is a contagious respiratory infection that comes “with an abrupt onset of symptoms,” says Dr. Sandra Nelson, the inpatient clinical director of the Division of Infectious Diseases at Massachusetts General Hospital. She says that such symptoms often include fever or chills, cough, sore throat, a runny or stuffy nose, headaches, muscle aches and fatigue.
Some people also experience vomiting or diarrhea, though these symptoms are more common in children than adults.
While an estimated 1 in 3 people get the flu every year, notes Dr. David Brett-Major, an epidemiologist at the University of Nebraska Medical Center’s College of Public Health, not everyone who gets it is adequately diagnosed. One reason for that is that sometimes flu symptoms overlap with symptoms associated with COVID-19 and other respiratory infections, so testing is often needed to distinguish them.
Why Can the Flu Be Dangerous?
Most people recover from the flu within several days to less than two weeks, though cough and fatigue symptoms can linger.
Calling influenza “just the flu” overlooks what the infection can do beyond making someone uncomfortable, notes Nelson. Flu-related complications that often require hospitalization include pneumonia, inflammation of the heart or brain and sepsis, a life-threatening response to infection. Flu can also severely worsen many common chronic conditions a person already has, she adds.
While most people who get the flu don’t usually develop serious complications, “there are many groups of people at risk for poor outcomes,” cautions Brett-Major.
These include adults 65 and older, pregnant women, people with severe obesity and those with weakened immune systems or chronic conditions such as asthma, chronic obstructive pulmonary disease (COPD), diabetes or kidney disease, notes the Centers for Disease Control and Prevention (CDC). “Health care workers, including those who work in long-term care facilities and nursing homes, are also at increased risk,” adds Brett Major.
Children under 5 are especially vulnerable to serious and sometimes fatal flu complications. CDC statistical modeling estimates that some 790 children in the United States died from influenza during the 2024–2025 season alone.
How Effective Is the Flu Vaccine, and When Should You Get It?
Among these pediatric deaths, the agency adds, “about 90% occurred in children who were not fully vaccinated against flu.” Many other infected at-risk populations – and everyone else – experience significantly improved symptoms if vaccinated. Vaccines can also “shorten the amount of time that those who are infected feel sick,” says Brett-Major.
That’s because vaccines prompt the body to develop protective antibodies, helping the immune system respond to influenza viruses. They can also prevent illness altogether – though protection varies with factors such as age, health and how closely current vaccines match the circulating viruses. “In some years, effectiveness against infection may be as low as 30%, and in other years as high as 80%,” says Nelson. But the more important measure, she adds, “is that the vaccine prevents severe outcomes like hospitalization or death.”
As for timing when to get vaccinated, the CDC recommends annual flu vaccination for everyone 6 months and older, with rare exceptions.
For most people, September or October is the preferred window, ideally by October’s end. That’s because “it can take a few weeks following vaccination for immunity to develop, and we want to be fully protected before the onset of flu season,” says Nelson. This year, experts have been recommending people get vaccinated sooner (i.e., now) given an early start to the season.
But vaccinating too early, such as in July or August, can leave protection waning later in the season. “Most flu seasons reach late March and into April,” says Brett-Major. He also stresses that last year’s shot doesn’t replace this season’s because protection declines over time, and vaccine formulations are updated annually to account for changing flu virus strains.
At the same time, if you’re pregnant, concerned about a vaccine reaction or have other questions, Nelson says your doctor can help you better understand your options and choose the best vaccine and timing for you.
For most people, she adds, “vaccination remains extremely safe and is the best tool that we have to keep ourselves and our loved ones healthy.”
Reporting by Daryl Austin, USA TODAY
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