Influenza (Flu) in Adults

Reviewed on 5/27/2022

Things to Know About Flu in Adults

Flu treatment includes resting in bed, staying hydrated, and taking over-the-counter fever-reducing drugs.
Flu treatment includes resting in bed, staying hydrated, and taking over-the-counter fever-reducing drugs.
  • Influenza (flu) is an acute viral infection of the nose, throat, and lungs. It is a common cause of acute respiratory illness and can affect people of all ages.
  • It occurs every year beginning in late fall, with the peak season for the flu in the northern hemisphere extending from November through March.
  • It infects in a widespread fashion, affecting people of different ages at the same time.
  • Although influenza is one of many viruses circulating during the season, it is an important cause of illness and hospitalization. Up to 80% of people hospitalized due to the flu have conditions that put them at risk of severe infection or even death.
  • The flu is also associated with an increased occurrence of heart attacks and strokes.
  • While most people are aware that older or chronically ill people are at risk for serious complications of flu, groups such as pregnant women, young children, and obese people are also at high risk.
  • Common symptoms of flu and COVID-19 include fever, cough, and shortness of breath, or difficulty breathing.
  • Both flu and COVID-19 are associated with an increased risk of bacterial infections like pneumonia.

Some also confuse influenza with the term stomach flu. However, the latter is an illness associated with nausea, vomiting, and diarrhea; bacteria and viruses can cause stomach flu but not the influenza viruses. It is a stomach illness, whereas influenza virus infection causes the flu, an illness of the respiratory system.

What Causes Flu in Adults?

There are four types of influenza viruses. Types A and B cause epidemics of severe respiratory illnesses known as "the flu," and type C causes a mild illness not associated with epidemics. Type D does not cause human disease. Type A has two different subtypes or strains, based on the chemical structure of the virus. The H1N1 swine flu virus is a type A influenza virus. Type B is not divided into subtypes. Both type A and type B are responsible for the seasonal influenza outbreaks.

  • Outbreaks occur more frequently in the winter months. Many factors may play a role in this seasonal pattern:
    • The virus survives for longer periods indoors in winter because the relative humidity of indoor air is very low in comparison to the outside air.
    • The virus is in droplets that are coughed or sneezed; it infects others via inhalation or by landing on sensitive body areas such as the eyes, nose, or mouth. These droplets generally travel no further than 6 feet.
    • In the winter, humans tend to be indoors more and thus have closer contact with each other, which makes it easier for the virus to spread.
  • Health officials may classify flu outbreaks as epidemics (occurring in a set geographical area) or pandemics (a worldwide occurrence). A flu pandemic can occur when a new influenza A virus emerges against which there is very little immunity already in the human population. Because there is little immunity, the new virus can spread from person to person very easily and can sicken more people. In 2009, a pandemic influenza strain began circulating called "novel" H1N1 influenza or swine flu (also referred to as "A(H1N1)pdm09" or "2009H1N1").
  • Influenza is a contagious disease. The virus is spread when you either inhale infected droplets in the air (spread when an infected person coughs or sneezes) or when you come in direct contact with an infected person's secretions (for example, by kissing, sharing of handkerchiefs and other items, and through use of objects such as spoons and forks). Flu virus survives on surfaces for up to 48 hours. Touching surfaces, such as doorknobs, elevator buttons, keyboards, and phones, are other ways to transfer the virus to your hands, which may then contact the nose, mouth, or eyes, where the virus gets absorbed.
  • A sudden increase in the number of school-aged children sick at home with flu-like illness may indicate the arrival of flu season. Similar infections in other age groups, especially among adults, soon follow this outbreak.

What Are the Flu Symptoms in Adults?

Flu symptoms and signs usually come on suddenly. Flu symptoms include the following:

  • Fever (usually high)
  • Severe aches and pains in the joints and muscles (especially in the back) and around the eyes
  • Generalized weakness
  • Ill appearance with warm, flushed skin and red, watery eyes
  • Headache
  • Dry cough
  • Sore throat and watery discharge from the nose or nasal congestion
  • Vomiting or diarrhea in some cases, particularly in children

What Is the Incubation Period for Flu in Adults?

The incubation period is the time between exposure to the virus and symptoms of illness. The incubation period or illness onset time (time between exposure and symptoms) for the flu is from 2-4 days.

Flu vs. Cold

Many people commonly and incorrectly confuse influenza infection (the flu) with the common cold. The common cold is a mild infection frequently caused by many viruses other than the influenza virus.

Differentiating a cold from the flu by symptoms alone can sometimes be difficult or impossible, but in general, flu symptoms are more severe than cold symptoms. People with the flu get sick more suddenly, look much sicker, and feel much weaker than if the ailment were a common cold.

Common colds tend to cause low-grade fever, if at all. High fever, body aches, extreme tiredness, and dry cough are more often symptoms of the flu. Milder respiratory symptoms like runny or stuffy nose are more often cold symptoms.

Is the Coronavirus (COVID-19) a Type of Flu?

Both the flu and the coronavirus are considered respiratory infections, but COVID-19 is not a type of flu.

COVID-19 (also called the Wuhan coronavirus, 2019 novel coronavirus, and 2019-nCoV) is a new coronavirus related to the SARS and MERS coronaviruses. The virus is easily spread from person to person and has caused a worldwide pandemic.

Can You Get COVID-19 and the Flu at the Same Time?

Yes, you can. Some studies from around the world showed that up to 60% of people hospitalized with COVID-19 also had the influenza virus.

The two viruses share similarities and differences. It is possible for both to spread from person to person even though the infected person is showing no symptoms or before they know they are sick with the virus. It is estimated that 25%-40% of infected people with COVID-19 do not have symptoms and may be more likely to spread it compared to flu. This may be a factor in the higher transmission rate of COVID-19 compared to flu.

Symptoms for both viruses may be very difficult to distinguish, though the timeline of infection may differ. The incubation period (the time from infection to symptoms) for flu is 1-4 days. COVID-19 has a longer incubation period of as few as 2 days to as many as 14 days.

When flu symptoms begin, they are usually more sudden and severe than COVID-19. COVID-19 may start with milder daily fevers over several days before a more severe illness with difficulty breathing begins. COVID-19 symptoms may also last for many weeks compared to flu.

Both viruses may cause severe disease resulting in hospitalization, and both may cause infection at the same time (co-infection). It's possible that co-infection with SARS-CoV2 and influenza might cause more severe illness than either one alone. Both flu and COVID-19 are associated with increased risk of bacterial infections like pneumonia on top of or after the initial viral syndrome.

Symptoms of COVID-19 include flu-like symptoms such as:

  • Fever
  • Cough
  • Shortness of breath, or difficulty breathing

Other COVID-19 symptoms can include:

  • Sore throat
  • Diarrhea
  • Aches and pains
  • Stuffy or runny nose

Will the Flu Shot Help Fight the Coronavirus?

Since the flu is a different type of virus than coronaviruses, the flu vaccine will not prevent COVID-19 or other human coronavirus illnesses. However, even people who are at low risk can develop serious complications or death from the flu. Getting a flu shot can keep you from missing work or even getting hospitalized with flu complications. A flu shot can also keep you from spreading the flu before you realize you have it, which helps protect vulnerable people around you.

You can also get both flu and COVID-19 at the same time. Experts are concerned that having both infections might cause more severe illness, hospitalizations, and deaths. Two serious viruses circulating during the winter when most people must be indoors may severely overwhelm the health care system. Getting a flu shot can help flatten the curve of both flu and COVID-19 hospitalizations.

Is It Safe to Have a COVID-19 Vaccine if You Already Had a Flu Shot?

It will probably be safe, but they may need to be separated so that there is no interference with each vaccine's effectiveness. Most vaccines can be given either together or separated by a short period of time. This is different for each vaccine. There are several COVID-19 vaccines being studied, and there is no answer to this question yet.

When Should Adults With the Flu Call a Doctor?

Most people with the flu take care of themselves at home and do not seek medical care.

Quite a few groups of people are at high risk for developing complications of the flu (of course, anyone can develop serious complications and may be unaware of being at high risk). Groups at high risk include the following:
  • Anyone with chronic medical conditions that affect a major organ system
  • Smokers
  • Pregnant women and women up to two weeks after birth
  • Infants and children younger than 5 years of age
  • Native American and Alaskan Native populations
  • People with extreme obesity (body mass index or BMI over 40)
  • People with respiratory problems such as asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis
  • People with endocrine problems, such as diabetes or thyroid disease
  • People with chronic kidney disease, especially on dialysis
  • People with chronic liver disease, such as hepatitis C or cirrhosis
  • People with chronic heart disease, such as coronary artery disease, heart failure, or birth defects
  • People with disorders of the brain, spinal cord, peripheral nerves, or muscles (examples include cerebral palsy, seizures, intellectual disability, stroke, and spinal cord injury)
  • People with weak immune systems due to disease or medication (such as people with HIV infection, have had organ or bone marrow transplants, or who are on chronic steroids or tumor necrosis alpha inhibitor drugs)
  • People with cancer, including cancer survivors
  • People with disorders of metabolism or mitochondria
  • Residents of nursing homes and other facilities
  • People older than 65 years of age
  • People younger than 19 years of age on long-term aspirin therapy
  • People who provide care to those at high risk for complications of flu, such as home caregivers, preschool workers, or health care workers

Unlike cold viruses, influenza virus kills the protective cells that help sweep germs out of the airways. This makes it much easier for pneumonia-causing bacteria to invade the lungs. Pneumococcus is one of the most common causes of bacterial pneumonia and is preventable by both influenza vaccine and pneumococcal vaccine.

People in a high-risk group should receive the flu vaccine and pneumococcal vaccines before the start of flu season. Pneumococcus is one of the most common causes of bacterial pneumonia that is preventable by vaccine. They should be especially aware of when to see a doctor or go to the hospital.

People at high risk may benefit from early treatment with antiviral drugs that combat the influenza virus. You should contact your health care provider or seek care in a hospital's emergency department for the following symptoms, which may be a sign of complications:

  • Dehydration (feeling lightheaded when standing up) and unable to drink fluids
  • Bloody or brown sputum (blood mixed with mucus and coughed up)
  • Difficulty breathing
  • Turning blue (a sign of poor oxygenation)
  • Worsening fever
  • Return of fever, cough, and other symptoms in the second week after the onset of the flu or worsening after symptoms have begun to improve
  • Confusion or difficulty waking up

These respiratory symptoms and signs may signify a more severe and complicated attack of flu (most importantly, the development of pneumonia). Pneumonia is an infection of the lungs; the flu virus or a bacterial infection that may occur when the person is weakened during a flu attack can cause pneumonia.

How Long Does the Flu Last?

The illness from uncomplicated flu lasts from 3-7 days in most adults. Cough and malaise (feeling tired or weak) can last up to 2 weeks following illness. Complications such as pneumonia can occur in some cases, which causes a more prolonged illness.

When Is the Flu Contagious?

  • Medical professionals can detect flu virus in respiratory secretions and is transmissible up to 24 hours before the onset of symptoms. A person with flu is thus contagious from 24 hours before they feel sick and up to 7 days after the illness onset. They are most contagious during the first 4 days of symptoms.
  • Some people can have the flu and may not develop symptoms but still shed flu virus to others.
  • In young children and people with weakened immunity, the virus can spread in body secretions into the second week of illness.
  • To avoid spreading the illness, the U.S. Centers for Disease Control and Prevention (CDC) recommends that people stay home until 24 hours after the fever is gone without using fever reducers, except to obtain necessities or to seek medical care.

How Is the Flu Diagnosed in Adults?

  • In most cases, a person's symptoms determine the diagnosis of the flu, especially when these occur during the peak flu season. Sometimes, medical professionals may need to perform special tests to be sure the influenza virus is responsible for the symptoms and not one of the many other viruses that circulate at the same time. If an influenza virus test is positive, antiviral medication may be helpful.
  • A health care professional takes a sample from the back of the throat or nose. To take a sample, a medical professional rubs a long sterile swab against the back of the throat or inside the nose. The health care provider then seals the swab in a packet for transporting specimens and sends it to a lab.
  • Some offices may use a rapid test that doctors can perform in the office with the result available in 30 minutes. Some rapid tests detect only influenza A virus, while others can detect both influenza A and influenza B. Rapid tests may miss some cases of the flu, thus a clinical diagnosis of flu (based on symptoms and signs) may be made even if the test is negative.

What Foods Should You Eat When You Have the Flu?

  • Although no particular foods decrease the length of the illness, it is recommended to drink plenty of fluids, including water and diluted juices to avoid dehydration, and eat food even though the appetite may be decreased.
  • Eat foods with protein, such as meats, beans, and nuts and those that may soothe, such as warm soup or cold juice pops.

What Is the Treatment for Flu in Adults?

  • Treatments for flu (influenza) include home remedies such as resting in bed, avoiding physical exertion, and avoiding alcohol and tobacco use.
  • Hydration is important, and over-the-counter (OTC) fever-reducing medication such as acetaminophen (Tylenol), ibuprofen (Advil), and naproxen (Aleve) can relieve minor aches and pains.
  • Avoid spreading germs and the flu virus by washing hands often or using alcohol-based hand sanitizers.
  • Stay away from others until you are fever free for 24 hours.
  • Cover sneezes or coughs with the inside of your elbow, or use a tissue and throw it away promptly.
  • Medical treatment for the flu may include medications (antiviral drugs) prescribed to decrease the severity and duration of the infection, as well as reduce shedding of virus. Physicians may recommend these for those at highest risk for flu-related complications or their caregivers. In special cases (see "Flu Chemoprophylaxis"), antivirals may be prescribed in addition to or instead of influenza vaccine as a preventive treatment (prophylaxis) during flu season. These drugs include the class known as neuraminidase inhibitors and the newer class, polymerase acidic (PA) endonuclease inhibitors.
  • Antibiotics do not treat viruses like colds or influenza, and they do not prevent bacterial infections that may occur because of the flu. To avoid resistance to antibiotics, medical professionals only prescribe antibiotics when there is evidence of a bacterial infection, such as pneumonia.

What Are Home Remedies for Flu in Adults?

  • Rest in bed. Avoid physical exertion.
  • Avoid using alcohol and tobacco.
  • Drink plenty of fluids such as water, diluted fruit juices (such as apple juice mixed with equal parts of water), and clear soups (like chicken broth).
    • Water should never be the sole or main liquid consumed because it does not contain adequate electrolytes (sodium and potassium, for example) that the body requires.
    • Commercially available products such as Gatorade and other similar sports drinks can be useful in this regard.
    • Commercial pediatric ORS (oral rehydration solution) in powder, packets, or bottles is another good way to replenish the body's fluids in the elderly. A health care provider may recommend an ORS of salt, sugar, and water that patients can make at home. It is critical to follow the recommended proportions very carefully with homemade ORS, especially for the elderly, to avoid serious nervous system or brain complications from too much or too little sodium intake.
  • Treat fever and aches with over-the-counter medications such as acetaminophen (Tylenol), ibuprofen (Advil or Motrin), and naproxen (Aleve or Naprosyn).
  • Use cough suppressants and expectorants to treat the cough.
  • Cough and sneeze into a soft tissue or handkerchief. Carefully dispose of tissues after using them.
  • Stay away from who are well until you have not had fever for 24 hours.
  • Doctors should closely monitor the very old and immunosuppressed (including pregnant or recently delivered mothers). Caregivers should consult with a health care provider, in case of the need to hospitalize, if the ill person is not improving or appears to be worsening.

What Is the Follow-up for Flu in Adults?

Generally, patients do not need follow-up for most flu cases unless fever or cough returns along with other new symptoms, which could signal a complication.

What Is the Best Medicine for the Flu in Adults?

Antibiotics are not antivirals and may cause unnecessary complications if given to treat a virus like the flu. They do not prevent influenza-related bacterial pneumonia. If a person who has received unnecessary antibiotics develops pneumonia, the bacteria may be resistant, and there is a higher risk of hospitalization and treatment failure.

Antiviral drugs are not a substitute for the flu vaccine. Getting the annual seasonal flu vaccine is still the best way to prevent the flu.

Antiviral drugs, however, are useful to decrease the severity and duration of a flu attack. They are most effective when given within 48 hours of the onset of illness. They can decrease the duration of the disease by 1 day if used within this early period. They may be able to reduce the incidence of complications from the flu as well as decrease or shorten the amount of flu virus shed by the sick person.

Oseltamivir (Tamiflu), zanamivir (Relenza), and peramivir (Rapivab) are neuraminidase inhibitor drugs recommended to treat seasonal flu. They are active against both influenza A and B, although resistance to these drugs may develop in some strains of influenza A. Side effects may include nervousness, poor concentration, nausea, and vomiting. Discuss side effects with a pharmacist or health care provider. The neuraminidase inhibitor class includes oral, inhaled, or injected drugs. Those approved by the U.S. FDA include:

  • Oseltamivir (available as generic, or under the trade name, Tamiflu)
  • Zanamivir (Relenza)
  • Peramivir (Rapivab)

Oseltamivir is given by mouth as a solution or pill and is FDA-approved for use in people age 14 days and older. Zanamivir is inhaled and is not recommended in those with lung problems like asthma or COPD. It is approved for use in people ages 7 years and above. People usually take both for a period of about 5-7 days. Oseltamivir is safe and recommended for use by pregnant women.

Physicians administer peramivir as a single dose intravenously and is currently U.S. FDA approved for adults 18 years and older. The polymerase acidic (PA) endonuclease inhibitor class includes FDA-approved Xofluza (baloxavir marboxil), which is taken orally in one dose.

For nasal congestion, a health care professional may suggest the use of over-the-counter decongestants. Anyone with high blood pressure, heart disease, diabetes, thyroid disease, enlarged prostate, glaucoma (high pressure inside the eye), or pregnancy should not use decongestants without a health care provider's advice.

Phenylephrine (Neo-Synephrine) and oxymetazoline hydrochloride (Neo-Synephrine 12 Hour, Afrin) are available as nasal sprays or drops. Use two to three sprays in each nostril as indicated on the label. Only use these nasal sprays or drops for up to 3 days. If they are used for more than that, the medication can actually worsen the congestion. These medications are not recommended for children.

Pseudoephedrine (Sudafed) comes in tablet form and may also help congestion. It may interact with certain foods and quite a few medications, elevate blood pressure and heart rate, and cause other serious side effects. People with chronic health conditions or on medications should consult a pharmacist or their health care professional before using pseudoephedrine.

How to Prevent the Flu

  • Annual influenza vaccination is the best way to prevent getting the flu, but for those who are at high risk and are unvaccinated, taking an antiviral drug after exposure can also help prevent disease.
  • Due to concerns that the flu virus may develop resistance to these drugs, medical professionals do not recommend widespread use of antiviral medication to prevent the flu.
  • In certain instances, for example, people with severe immunodeficiency who cannot receive flu vaccine or in whom it may not work, high-risk individuals exposed to flu, or residents of a health care facility or nursing home in which there is an influenza outbreak, may take oseltamivir or zanamivir for 7 days to prevent the flu.
  • As with COVID-19, wearing a face mask can also help prevent flu. Countries in the southern hemisphere that experienced the COVID-19 pandemic during their March to August flu season found extremely low rates of influenza. The only differences between their 2020 flu season and prior years was mandatory universal masking and increased flu vaccination.
  • Face masks can also reduce the infectious dose of respiratory droplets, which may result in less severe illness.

Does the Flu Shot Have Side Effects? What if I Always Get the Flu After I Get the Flu Vaccine?

Flu vaccines cannot give you an influenza infection. Vaccine manufacturers make the vaccines with inactivated flu viruses or with vaccines that only contain particles of flu virus. The live virus vaccine is only able to infect nasal tissue, enough to cause antibody production, but it is not able to infect other tissues to cause influenza.

As is expected, all of the vaccines do stimulate your immune system into thinking you have the flu, so that your body produces protective antibody within 2-3 weeks. Feelings of sore arm or flu-like illness within two weeks are often a sign of a good immune response. These symptoms may last 1-2 days.

Secondly, flu-like symptoms within 2 weeks of vaccine may indicate infection with influenza before antibodies have developed or infection by one of the hundreds of other viruses circulating at the same time.

No medical intervention is 100% perfect, and each strain in a vaccine may produce different levels of immunity. Therefore, it is certainly possible to get flu after antibodies have developed, usually from a different strain or a strain that has mutated from the original strain the vaccine was made for. This has happened occasionally because influenza viruses may change slightly even within the same flu season.

In either case, the influenza illness is likely to be less severe and less likely to be complicated compared to not getting the vaccine. Any protection is better than no protection, especially if you are in a high-risk group.

Who Should Get the Flu Vaccine?

Medical professionals recommend vaccination for the flu each year for all people over 6 months of age and anyone interested in reducing the risk of flu. Those at higher risk of complications should be especially sure to get the flu vaccine. High-priority individuals include the following:

  • Children aged 6 months up to 4 years of age
  • Anyone aged 50 years and above
  • Women who will be pregnant during the flu season and up to two weeks after delivery (Breastfeeding women can also get the flu shot without worry about harm to the baby.)
  • A person of any age with chronic diseases of the heart, lungs, liver, blood (such as sickle cell disease), nervous system, kidneys, or metabolism (diabetics)
  • People with a BMI of 40 or greater
  • A person of any age who is HIV positive or who has AIDS
  • A person of any age taking immune-suppressing therapies or drugs, such as corticosteroids, chemotherapy or tumor necrosis factor inhibitors
  • Residents of nursing homes and other long-term care facilities
  • Alaskan Native and Native American populations
  • Children 6 months to 18 years of age who are on long-term aspirin or salicylate therapy and therefore could develop Reye's syndrome after the flu
  • Health care professionals
  • Household members (including children) and caregivers of people in high-risk groups
  • Students or others living in institutional settings (for example, those who reside in dormitories or camps where close contact is likely)

The intranasal or live vaccine (FluMist and others) is an alternative to the flu shot in people who are healthy, 2-49 years of age, and not pregnant. Exceptions are health care professionals who care for severely immunosuppressed patients or people caring for children younger than 6 months of age, children age 2 through 4 with asthma, and children age 2-17 on long-term aspirin treatment.

People who are allergic to eggs or who have had Guillain-Barré syndrome (paralysis) within six weeks of a prior vaccination should check with their doctor before getting an influenza vaccine.

People who have had severe allergic reaction to flu vaccine itself should not receive flu vaccine even if they have risk factors for severe influenza. These individuals should ask their doctor if preventive antiviral therapy might be of benefit to them.

Updates on the seasonal flu vaccine are available on the CDC web site, influenza (flu) (

How Effective Is the Flu Shot?

  • Vaccination remains the most effective way to prevent the flu.
  • The effectiveness varies according to the age and health of the person receiving it and the closeness that it matches the strain that is circulating in any given year.
  • Numerous studies have shown that the flu shot can reduce hospitalizations from flu from 52% to 92% in various populations, such as adults with chronic health problems like diabetes, chronic lung disease, and heart disease as well as in older adults, infants, and pregnant women.
  • People who do get the flu despite receiving the flu shot may have a milder and shorter illness.

What Is Avian Flu, and Why Is It Important?

  • Influenza virus is one of the few viruses that infects humans and other species like birds and pigs. Waterfowl in the wild, such as geese and gulls, are natural species for influenza A strains.
  • Some strains are highly pathogenic (very likely to cause disease) and highly contagious in birds and produce almost 100% death rates within days; these strains cause severe epidemics on poultry farms. Not only do they cause economic losses to farming, but they pose a potential for spreading to humans if the virus changes to a variant for which human infection becomes possible. This could lead to the ability to spread from human to human and could spark a global human pandemic with a highly lethal flu, so health researchers closely track these events.
  • H5 avian flu strains have occasionally spread to humans, with 50% mortality (death rate) and severe pneumonia, but human-to-human spread has been highly limited.

What Is Swine Flu, and Why Is It Important?

  • Similar to avian flu viruses, there are influenza A strains that typically infect pigs. On occasion, variants of swine flu have developed the ability to infect humans, such as those who raise and work with pigs. Rarely, like avian flu, a swine flu strain has spread from pigs to humans at petting zoos or a pig farm, and some human-to-human spread has occurred. These are variant strains. An example of a swine flu variant that has spread to humans is H3N2 variant or H3N2v.
  • Epidemics are usually limited to a continent and occur yearly due to small changes in flu strains (antigenic drift). Pandemics spread globally and occur due to major changes in a strain (antigenic shift). Before the flu vaccine became widely used, pandemics occurred about every 20 years. This is now much less frequent but remains a risk.
  • Pigs may play a unique role because human, pig, and bird flu strains can infect them at the same time. Infection in a pig creates an opportunity for a bird strain to share genes with a flu strain that spreads easily from person to person. This can create a new human strain that sparks a global pandemic.
  • This occurred in 2009 when an H1N1 strain caused the first pandemic since the deadly Spanish Flu pandemic of 1918. The Spanish Flu killed people within days. Between 20-40 million people died from flu worldwide, far more than those who died because of World War I during those years, and more than those who died during the Black Plague in the Middle Ages.
  • Fortunately, the 2009 H1N1 strain was not as severe as Spanish Flu, but it did cause unexpectedly serious complications in younger individuals, obese people, and pregnant women, and it continues to circulate yearly, including during the summer.
  • The most important preventive tools against pandemics are vaccination against circulating strains every year and ongoing public health surveillance.

Is It Possible to Prevent Flu in Adults?

Personal Hygiene

  • Limit touching high-touch surfaces where flu viruses may remain alive; examples include handrails, doorknobs, faucets, keyboards, and elevator buttons.
  • Wash your hands often, especially after touching high-touch surfaces, being in public places, or at work.
  • Avoid touching your eyes, nose, or mouth before washing one's hands.
  • Avoid close contact with people who appear ill.
  • Do not share clothes or other personal items with another person during a flu outbreak.
  • Those infected with influenza should stay at home for 24 hours after fevers have resolved.

Flu Vaccination

The best means of preventing the flu is getting an influenza vaccination. The CDC recommends an annual flu vaccine for everyone 6 months of age and older.

Two general types of vaccines are available:

  • One is the injectable vaccine (known as the flu shot) made from inactivated virus. The flu shot contains only killed influenza viruses A and B. The other is a live attenuated influenza virus, or weakened, vaccine (LAIV) that health care providers squirt into the nose. This is the intranasal vaccine or nasal spray vaccine.
  • The intranasal form is indicated for certain people who may prefer it to a shot, and it is approved for people from 2 through 49 years of age. Doctors do not recommend it for people who are immuno-suppressed or have other conditions (see below for a list).

There were concerns about poorer effectiveness against H1N1 flu virus than injected vaccines, and the Advisory Committee on Immunization Practices (ACIP) recommended against its use during flu seasons in 2016-17 and 2017-18. Since 2018-19, the LAIV (FluMist) includes an H1N1 component, and the ACIP and the CDC recommend it along with all of the other flu vaccines.

There are different injected flu vaccines, such as the quadrivalent flu shot, which contains two type A viruses and two type Bs, rather than the standard trivalent that has two type As and one type B:

  • There is a high-dose shot formulation for people over 65 years of age and an intradermal (into the skin) version for people ages 18-65, and it uses a tiny needle.
  • There is also a flu vaccine licensed for people age 65 and over that contains an adjuvant (an added immunity booster). This vaccine (Fluad) may produce higher levels of flu antibodies and may offer greater protection. In August 2014, the FDA approved Afluria (as both a trivalent and a quadrivalent formula), which is injected into the muscle through a needle-free jet injector. Medical professionals approve Afluria for ages 18-64.

At this time, there is no hard evidence that one vaccine is better than any other. The ACIP and the CDC have not made any recommendations of specific flu vaccines. The most important thing is that people get whichever flu vaccine is available or is recommended by their health care professional. People can find a complete listing of currently available flu vaccines at

An important point is that no one vaccine is recommended over the others and one should not delay vaccination in order to wait for one of the others if there is a vaccine available.

It is also important to note a widely circulated study that reported a weak, inconclusive link of flu vaccination with miscarriage; in essence, there was a slightly higher number of miscarriages in a group of women who received flu vaccine compared to usual, but there was no evidence that the vaccine caused this. Because the risk of serious flu complications and death during pregnancy is clear and far higher, the American College of Obstetricians and Gynecologists (ACOG) recommends flu vaccination as an essential part of prenatal care.

Lastly, research has shown that flu vaccination is safe in most people who have all but the most severe egg allergy. People who can eat lightly cooked eggs, for example, are not likely to be allergic to eggs. People who have only experienced hives after exposure to eggs can receive any of the licensed flu vaccines that approved for their age and health. They may also receive the vaccine at any facility licensed to give it, such as a local pharmacy or health fair.

People who have had reactions more severe than hives after eating eggs may receive any licensed flu vaccine, but they should receive it in a facility that is staffed with health professionals and equipped to manage severe allergic reactions. This need not be a doctor's office or hospital, nor is there a prescribed 30-minute observation period after receiving the vaccine.

Health professionals giving vaccines to anyone should be able to recognize severe allergic reactions. Symptoms and signs of a severe allergic reaction may include:

  • Throat or tongue swelling
  • Lightheadedness
  • Repeated vomiting
  • Difficulty breathing, and may require the use of epinephrine or emergency medical treatment

Health care providers administer the influenza vaccine every year prior to flu season. Immunity to the flu virus develops after about 2 weeks. The CDC recommends that vaccine administration as soon as it becomes available each fall.

  • Because of substantial vaccine distribution delays during previous influenza seasons and the possibility of similar delays in the future, medical professionals recommended that anyone at high risk for complications from the flu get a flu shot as early as September. This same group of people can still get the vaccine throughout the flu season if they were unable to obtain vaccination earlier.
  • While there is some data suggesting that flu vaccine antibodies drop a small percentage in older individuals after a few months, there is no evidence that this lowers the effectiveness of the vaccine if given early before flu season. Even if antibody levels drop, immune memory cells can ramp up antibodies very quickly if you are exposed to flu.
  • Vaccination reduces the risk of flu illness by 40%-60% during seasons when the circulating flu viruses are well-matched to the flu vaccine. Flu vaccines tend to work better against influenza B and influenza A(H1N1) viruses and offer lower protection against influenza A(H3N2) viruses.
  • In some years, the vaccine is less effective because the circulating flu virus genetically changed from the ones that were anticipated and included in the vaccine. The H3N2 strain mutates more often and more quickly than other strains. The 2014-15 flu vaccine was only 23% effective due to changes in the circulating flu virus that season.
  • Not only does the flu vaccine decrease the risk of getting flu infection, but it can also decrease the number of visits to a doctor's office, lost work days, hospitalizations, and risk of death from the flu virus.

More Flu Prevention Tips

  • Travel: The risk of getting the flu during travel depends on the destination and time of the year. In countries in the northern hemisphere, such as the United States, influenza occurs between November and March. In the southern hemisphere, most influenza occurs between April and September. In the tropical regions, such as the Caribbean, the flu occurs year-round. For people who are traveling, the recommendations are as follows:
    • Because the availability of the vaccine in North America is limited in the summer season, those needing the vaccine for travel purposes should discuss the best option for flu prevention, vaccine versus carrying of antiviral medications, with their doctor.
    • Those who are at high risk of getting complications from the flu and anyone who wants to decrease the chances of getting the flu and is visiting the tropics or the southern hemisphere from April through September should take the vaccine at least two weeks before departure if they were not vaccinated in the previous winter or fall.
    • Anyone at high risk who received the previous season's vaccine should receive the current vaccine before travel in the fall or winter.
    • Those who have missed the flu shot can still get the vaccine during an outbreak. However, the best method of prevention is to get the shot prior to the flu season.
    • For more travel information, check the CDC National Center for Infectious Diseases Infectious Disease Information, Influenza (Flu, Influenza Virus Infection).
  • Side effects of the flu shot: The most frequent side effects of vaccination are soreness and redness at the vaccination site that may last for up to 2 days. These reactions are generally mild and rarely interfere with the ability to conduct usual daily activities. Fever, weakness, muscle aches, and other symptoms can occur, most often in children, following vaccination. These reactions begin six to 12 hours after vaccination and can last for 1-2 days.
    • Side effects from the intranasal vaccine are usually mild. The nasal-spray vaccine contains weakened viruses and will not cause severe symptoms often associated with influenza illness. In children, side effects can include runny nose, headache, vomiting, muscle aches, and fever. In adults, side effects can include runny nose, headache, sore throat, and cough.
    • Most licensed flu vaccine is prepared using hens' eggs, and it is possible for them to contain very tiny amounts of egg proteins. Life-threatening allergic reactions are very rare but can be unpredictable in people who are genuinely allergic to eggs. Thus, medical professionals have cautioned people with egg allergy about flu vaccination for several years. However, there no longer significant restrictions related to egg allergy and flu vaccines.
    • People who develop more than just hives after eating eggs should receive the flu vaccine in a medical facility that is equipped to treat severe allergic reactions. Because allergic reactions may begin up to a day after an exposure, a 30-minute observation period after vaccination is no longer required.
    • Anyone who can eat soft cooked eggs without hives can get a flu vaccine.
    • Of course, those who have had a serious allergic reaction to a previous dose of flu vaccine should not receive it again.
  • Myths about the flu vaccine: It's not true that you can get the flu from the shot. You cannot get the flu from the shot. The shot contains only an inactivated (killed) form of the virus and therefore cannot cause influenza. The intranasal vaccine (squirted into the nose) contains live virus, but it is weakened to the point where it cannot cause influenza in healthy people.
    • It is not true that you only need to have the vaccine once in your life. People must get the flu shot every year in order to keep from getting the flu. The fact that influenza viruses continually change their structure is one of the reasons people must get the vaccine every year. Antibodies formed by the body's immune system after the vaccination decline over time. Thus, one's own defenses may not be effective for the next flu season. Each year, medical researchers update the vaccine to include the most current influenza virus subtype.
    • Even if you have a flu shot, you may still get the flu when the flu season arrives because the vaccine is not 100% effective in preventing the flu. The virus can be of a different subtype, so you may not be protected against it. Medical researchers must make the vaccine several months ahead from the virus subtypes that are predicted to be circulating in the current season. Sometimes the new vaccine may not match the all the virus types that are causing flu the next year, as was the case for the 2014-2015 flu vaccine.
    • People at high risk for complications from the flu, and who have not had the shot in time to be protected, can be given one of the antiviral medications for prevention during an outbreak.

What Is the Prognosis of Flu in Adults?

Flu symptoms start to go away after 2-5 days. Fever may last for up to 5 days, while other symptoms, including weakness and fatigue, may persist for several weeks. The very young, the very old, and those in the high-risk groups are at risk for complications requiring hospitalization. Some people may die from the flu.

Where Can People Find More Information About the Flu?

  • Centers for Disease Control and Prevention, Influenza (Flu)
  •, Seasonal Influenza
  • National Institute of Allergy and Infectious Diseases, National Institutes of Health, Flu Fact Sheet
  • American Lung Association, Influenza (Flu)

Health Solutions From Our Sponsors

Flu Vaccine Side Effects

In general, all medications, including intramuscular, intradermal, and nasal-spray vaccines, have side effects and the potential for allergic reactions. For most medicines and vaccines, the side effects or reactions are infrequent and are minimal if they do occur. Consequently, the pros far outweigh the cons for vaccination.

Reviewed on 5/27/2022
United States. Centers for Disease Control and Prevention (CDC). "Coronavirus (COVID-19)." Mar. 19, 2020. <>.

United States. Centers for Disease Control and Prevention (CDC). "Influenza (Flu)." Mar. 13, 2020. <>.

United States. Gilead Sciences, Inc. "Gilead’s Investigational Antiviral Remdesivir Receives U.S. Food and Drug Administration Emergency Use Authorization for the Treatment of COVID-19." <>.