Influenza (Flu)

Picture of the influenza virus

Influenza, commonly called "the flu," is an illness caused by RNA viruses (Orthomyxoviridae family) that infect the respiratory tract of many animals, birds, and humans. In most people, the infection results in the person getting a fever, cough, headache, and malaise (tired, no energy); some people also may develop a sore throat, nausea, vomiting, and diarrhea. The majority of individuals has flu symptoms for about 1-2 weeks and then recovers with no problems. However, compared with most other viral respiratory infections, such as the common cold, influenza (flu) infection can cause a more severe illness with a mortality rate (death rate) of about 0.1% of people infected with the virus.

The above is the usual situation for the yearly occurring "conventional" or "seasonal" flu strains. However, there are situations in which some flu outbreaks are severe. These severe outbreaks occur when a portion of the human population is exposed to a flu strain against which the population has little or no immunity because the virus has become altered in a significant way. These outbreaks are usually termed epidemics. Unusually severe worldwide outbreaks (pandemics) have occurred several times in the last hundred years since influenza virus was identified in 1933. By an examination of preserved tissue, the worst influenza pandemic (also termed the Spanish flu or Spanish influenza) occurred in 1918 when the virus caused between 40-100 million deaths worldwide, with a mortality rate estimated to range from 2%-20%.

In April 2009, a new influenza strain against which the world population has little or no immunity was isolated from humans in Mexico. It quickly spread throughout the world so fast that the WHO declared this new flu strain (first termed novel H1N1 influenza A swine flu, often later shortened to H1N1 or swine flu) as the cause of a pandemic on June 11, 2009. This was the first declared flu pandemic in 41 years. Fortunately, there was a worldwide response that included vaccine production, good hygiene practices (especially hand washing), and the virus (H1N1) caused far less morbidity and mortality than was expected and predicted. The WHO declared the pandemic's end on Aug. 10, 2010, because it no longer fits into the WHO's criteria for a pandemic.

Researchers identified a new influenza-related viral strain, H3N2, in 2011, but this strain has caused only about 330 infections with one death in the U.S. Since 2003, researchers identified another strain, H5N1, a bird flu virus, that caused about 650 human infections. This virus has not been detected in the U.S. and easily spreads among people in contrast to other flu strains. Unfortunately, people infected with H5N1 have a high death rate (about 60% of infected people die). Currently, H5N1 does not readily transfer from person to person like other flu viruses.

The most recent data for the mortality (death rates) from the influenza rate (death rate) for the United States in 2016 indicates that mortality from influenza varies from year to year. Death rates estimated by the CDC range from about 12,000 during 2011-2012 to 56,000 during 2012-2013. In the 2017-2018 season, deaths reached a new high of about 79,000. The CDC estimates between 24,000-62,000 deaths occurred in the 2019-2020 flu season. Experts suggest that a large percentage of people went unvaccinated or refused to vaccinate family members, causing an increased number of deaths due to the flu.

Haemophilus influenzae is a bacterium incorrectly considered to cause the flu until the virus was demonstrated to be the correct cause in 1933. This bacterium can cause lung infections in infants and young children, and it occasionally causes ear, eye, sinus, joint, and a few other infections, but it does not cause the flu.

Another confusing term is stomach flu. This term refers to a gastrointestinal tract infection, not a respiratory infection like influenza (flu). Influenza viruses do not cause stomach flu (gastroenteritis). Another name problem is with the condition called swine flu. Swine flu is a flu-like illness that usually infects pigs, but the term swine flu was applied to a flu strain that also could infect humans (H1N1). In 2018-19, the pig version of the virus (not infecting humans to date) killed the majority of pigs in China, forcing that country to begin to utilize its emergency stockpile of pork. The viral strain has now been detected in South Korea.

Although initially, symptoms of influenza may mimic those of a cold, influenza is more debilitating with symptoms of fatigue, fever, and respiratory congestion. Colds can be caused by over 100 different virus types, but only influenza viruses (and subtypes) A, B, and C cause the flu. In addition, colds do not lead to life-threatening illnesses like pneumonia, but severe infections with influenza viruses can lead to pneumonia or even death.

Compared with most other viral respiratory infections, such as the common cold, influenza (flu) infection usually causes a more severe illness with a mortality rate (death rate) of about 0.1% of people infected with the virus. Cold symptoms (for example, sore throat, runny nose, cough (with possible phlegm production), congestion, and slight fever) are similar to flu symptoms, but the flu symptoms are more severe, last longer, and may include vomiting, diarrhea, and cough that is often a dry cough.

The following table from the CDC helps to distinguish between a cold and influenza:

Although some of the symptoms of influenza may mimic those of food poisoning, others do not. Most symptoms of food poisoning include nausea, vomiting, watery diarrhea, abdominal pain, cramps, and fever. Note that the majority of food poisoning symptoms are related to the gastrointestinal tract, except for fever. The common flu signs and symptoms include fever but also include symptoms that are not typical for food poisoning, because the flu is a respiratory disease. Consequently, respiratory symptoms of nasal congestion, dry cough, and some breathing problems help distinguish the flu from food poisoning.

Influenza virus information

Influenza viruses cause the flu and are divided into three types, designated A, B, and C. Influenza A and influenza B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates of hospitalization and death. Influenza type C differs from types A and B in some important ways. Type C infection usually causes either a very mild respiratory illness or no symptoms at all. It does not cause epidemics and does not have the severe public health impact of influenza types A and B. Efforts to control the impact of influenza are aimed at types A and B, and the remainder of this discussion will be devoted only to these two types.

Influenza viruses continually change over time, usually by mutation (change in the viral RNA). This constant changing often enables the virus to evade the immune system of the host (humans, birds, and other animals) so that the host is susceptible to changing influenza virus infections throughout life. This process works as follows: A host infected with influenza virus develops antibodies against that virus; as the virus changes, the "first" antibody no longer recognizes the "newer" virus and infection can occur because the host does not recognize the new flu virus as a problem until the infection is well under way. The first antibody developed may provide partial protection against infection with a new influenza virus. In 2009, almost all individuals had no antibodies that could recognize the novel H1N1 virus immediately.

Type A viruses are divided into subtypes or strains based on differences in two viral surface proteins called the hemagglutinin (H) and the neuraminidase (N). There are at least 16 known H subtypes and nine known N subtypes. These surface proteins can occur in many combinations. When spread by droplets or direct contact, the virus, if not killed by the host's immune system, replicates in the respiratory tract and damages host cells. In people who are immune compromised (for example, pregnant women, infants, cancer patients, asthma patients, people with pulmonary disease, and many others), the virus can cause viral pneumonia or stress the individual's system to make them more susceptible to bacterial infections, especially bacterial pneumonia. Both pneumonia types, viral and bacterial, can cause severe disease and sometimes death.

Antigenic shift and drift

Figure 2. An example of influenza antigenic shift and drift

Influenza type A viruses undergo two major kinds of changes. One is a series of mutations that occurs over time and causes a gradual evolution of the virus. This is called antigenic "drift." The other kind of change is an abrupt change in the hemagglutinin and/or the neuraminidase proteins. This is called antigenic "shift." In this case, a new subtype of the virus suddenly emerges. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift and therefore do not cause pandemics.

The 2009 pandemic-causing H1N1 virus was a classic example of antigenic shift. Research showed that novel H1N1 swine flu has an RNA genome that contains five RNA strands derived from various swine flu strains, two RNA strands from bird flu (also termed avian flu) strains, and only one RNA strand from human flu strains. According to the CDC, mainly antigenic shifts over about 20 years led to the development of novel H1N1 flu virus. A diagram that illustrates both antigenic shift and drift (see Figure 2) and features influenza A types H1N1 and bird flu (H5N1), but almost every influenza A viral strain can go through these processes that changes the viral RNA. A recent flu epidemic in India was partially blamed on antigenic drift/shift.

When does flu season begin and end?

Flu season officially begins in October of each year and extends to May of the following year. According to the CDC, people can follow the development of flu across the United States by following CDC's weekly update of the locations where flu is developing in the U.S. (see the flu map).

As mentioned previously, has hemagglutinin on the viral surface. The viral hemagglutinins have at least 18 types, but these types are broken into two main influenza A virus categories. For example, one of the two main categories include human H1, H2, and avian H5 viruses while the other major category includes human H3 and avian H7 viruses. Researchers in 2016 at UCLA and the University of Arizona discovered that if you were exposed to one of these groups as a child, you had a much better chance of being protected against other viruses in that same group or category later in life. For example, if you are exposed to H2 as a child and then later in life to H2 or H5 viruses, you may have as high as a 75% chance of protection against those H2 and/or H5 strains. But if you are exposed to the other major category that included H3 or H7, you would be much more susceptible to these viral types. The reverse situation would be true if you were exposed as a child to H3 or H7 viruses. The researchers concluded that the immunological imprinting early in life helps determine the response (immune response) to these viral types or categories. Consequently, the first strain of flu that a person is exposed to in childhood likely determines that person's risk in the future for severity of the flu depending upon the exact category of the first viral strain that infects the child. The researchers hope to exploit these new findings in the development of new and more effective flu vaccines.

Typical clinical symptoms of the flu may include

• fever (usually 100 F-103 F in adults and often even higher in children, sometimes with facial flushing and/or sweating),
• chills,
• respiratory symptoms such as
  • cough (more often in adults),
  • sore throat (more often in adults),
  • runny or stuffy nose (congestion, especially in children),
  • sneezing,
• headache,
• muscle aches (body aches), and
• fatigue, sometimes extreme.

Although appetite loss, nausea, vomiting, and diarrhea can sometimes accompany influenza infection, especially in children, gastrointestinal symptoms are rarely prominent. The term "stomach flu" is a misnomer that some people use to describe gastrointestinal illnesses caused by other microorganisms. H1N1 infections, however, caused more nausea, vomiting, and diarrhea than the conventional (seasonal) flu viruses. Depending upon the severity of the infection, some patients can develop swollen lymph nodes, muscle pain, shortness of breath, severe headaches, chest pain or chest discomfort, dehydration, and even death.

Most individuals who contract influenza recover in a week or two, however, others develop potentially life-threatening complications like pneumonia. In an average year, influenza is associated with about 36,000 deaths nationwide and many more hospitalizations. Flu-related complications can occur at any age; however, the elderly and people with chronic health problems are much more likely to develop serious complications after the conventional influenza infections than are younger, healthier people. When people ignore or refuse flu vaccination, the death rate increases as shown by the recent higher death rates.

How can you get influenza?

Flu easily spreads from person to person both directly and indirectly. Human-to-human flu transmission occurs via droplets contaminated with the virus. Produced by coughing, sneezing, or even talking, these droplets land near or in the mouth or the nose of uninfected people, and the disease may spread to them. The disease can spread indirectly to others if contaminated droplets land on utensils, dishes, clothing, or almost any surface that uninfected people then touch. If the infected person touches their nose or mouth, for example, they transfer or spread the disease to themselves or others.

Incubation period for the flu, which means the time from exposure to the flu virus until initial symptoms develop, typically is 1-4 days with an average incubation period of 2 days.

The flu is typically contagious about 24-48 hours before symptoms appear (from about the last day of the incubation period) and in normal healthy adults is contagious for another 5-7 days. Children are usually contagious for a little while longer (about 7-10 days). Individuals with severe infections may be contagious as long as symptoms last (about 7-14 days).

Individuals with mild flu symptoms may not require the care of a physician unless they are a member of a high-risk group as described above. For many individuals, treatment is provided by their primary care physician or provider (including internists or family medicine specialists and physician assistants and other primary caregivers) or pediatrician. Complicated or severe flu infections may require consultation with an emergency-medicine physician, critical care specialist, infectious-disease specialist, and/or a lung specialist (pulmonologist).

Medical professionals clinically diagnose the flu by evaluating the patient's history of association with people known to have the disease and their symptoms listed above. Usually, a health care professional performs a quick test (for example, nasopharyngeal swab sample) to see if the patient has an influenza A or B viral infection. Most of the tests can distinguish between A and B types. The test can be negative (no flu infection) or positive for types A or B. If it is positive for type A, the person could have a conventional flu strain or a potentially more aggressive strain such as H1N1. Most of the rapid tests are based on PCR technology that identifies the genetic material of the virus. Some rapid influenza diagnostic tests (RIDTs) can screen for influenza in about 10-30 minutes.

Swine flu (H1N1) and other influenza strains like bird flu or H3N2 are definitively diagnosed by identifying the particular surface proteins or genetic material associated with the virus strain. In general, this testing is done in a specialized laboratory. However, doctors' offices are able to send specimens to specialized laboratories if necessary.

First, individuals should be sure they are not members of a high-risk group that is more susceptible to getting severe flu symptoms. Check with a physician if you are unsure if you are a higher-risk person. The CDC recommends home care if a person is healthy with no underlying diseases or conditions (for example, asthma, lung disease, pregnant, or immunosuppressed).

Increasing liquid intake, warm showers, and warm compresses, especially in the nasal area, can reduce the body aches and reduce nasal congestion or head congestion. Nasal strips and humidifiers may help reduce congestion, especially while trying to sleep. Some physicians recommend nasal irrigation with saline to further reduce congestion; some recommend nonprescription decongestants like pseudoephedrine (Sudafed). Over-the-counter fever-reducing medications like acetaminophen (Tylenol) or ibuprofen (Advil, Motrin and others) can treat a fever. Read labels for safe dosage. Cough drops, over-the-counter cough syrup, or cough medicine that may contain dextromethorphan (Delsym) and/or guaifenesin (Mucinex) can suppress a cough. Notify a doctor if an individual's symptoms at home get worse.

The CDC published the following guidance concerning antiviral drugs:

Antiviral medications with activity against influenza viruses (anti-influenza drugs) are an important adjunct to influenza vaccine in the control of influenza.

• Influenza antiviral prescription drugs treat influenza or to prevent influenza.
• Oseltamivir, zanamivir, and peramivir are chemically related antiviral medications known as neuraminidase inhibitors that have activity against both influenza A and B viruses.
• In October 2018 (10/24/2018), the FDA approved a new antiviral drug (baloxavir marboxil [Xofluza]) for flu treatment that prevents viral replication.

The CDC recommended the following antiviral medications for the treatment of influenza (flu) for the 2020-2021 season: oral oseltamivir (Tamiflu), inhaled zanamivir (Relenza), intravenous peramivir (Rapivab), and oral Baloxavir.

Over-the-counter medications that may help reduce symptoms of congestion (decongestants), coughing (cough medicine), and dehydration include diphenhydramine (Benadryl), acetaminophen (Tylenol), NSAIDs (Advil, Motrin, Aleve), guaifenesin (Mucinex), dextromethorphan (Delsym), pseudoephedrine (Sudafed), and oral fluids. Aspirin may be used in adults but not in children.

Antibiotics treat bacterial infections, not viral illnesses like the flu.

Individuals with the flu may also benefit from some additional bed rest, throat lozenges, and possibly nasal irrigation; drinking fluids may help prevent symptoms of dehydration (for example, dry mucus membranes and decreased urination).

Vaccination is the primary method for control of influenza; however, antiviral agents have a role in the prevention and treatment of mainly influenza type A infection. Regardless, antiviral agents should not be considered as a substitute or alternative for vaccination. Most effectiveness of these drugs is reported to occur if the antivirals are given within the first 48 hours after infection; some researchers maintain there is little or no solid evidence these drugs can protect people from getting the flu so some controversies exist regarding these agents.

Vaccine efficacy also varies from one person to another. Past studies of healthy young adults have shown influenza vaccine to be 70%-90% effective in preventing illness. In the elderly and those with certain chronic medical conditions such as HIV, the vaccine is often less effective in preventing illness. Studies show the vaccine reduces hospitalization by about 70% and death by about 85% among the elderly who are not in nursing homes. Among nursing home residents, vaccine can reduce the risk of hospitalization by about 50%, the risk of pneumonia by about 60%, and the risk of death by 75%-80%. However, these figures did not apply to the 2014-2015 flu vaccine because the quadrivalent (four antigenic types) vaccine did not match well with 2014-2015 circulating strains of the flu (vaccine effectiveness was estimated to be 23%). This occurs because the vaccine needs to be produced months before the flu season begins, so the vaccine is designed by projecting and choosing the most likely viral strains to include in the vaccine. If drift results in changing the circulating virus from the strains used in the vaccine, efficacy may be reduced. However, the vaccine is still likely to lessen the severity of the illness and to prevent complications and death, according to the CDC.

Although only a few different influenza virus strains circulate at any given time, people may continue to become ill with the flu throughout their lives. The reason for this continuing susceptibility is that influenza viruses are continually mutating, through the mechanisms of antigenic shift and drift described above. Each year, researchers update the vaccine to include the most current influenza virus strains that are infecting people worldwide. The fact that influenza viral genes continually change is one of the reasons people must get the vaccine every year. Another reason is that antibody produced by the host in response to the vaccine declines over time, and antibody levels are often low one year after vaccination so even if the same vaccine is used, it can act as a booster shot to raise immunity.

Many people still refuse to get flu shots because of misunderstandings, fear, "because I never get any shots," or simply a belief that if they get the flu, they will do well. These are only some of the reasons -- there are many more. The U.S. and other countries' populations need to be better educated about vaccines; at least they should realize that safe vaccines have been around for many years (measles, mumps, chickenpox, and even a vaccine for cholera), and as adults they often have to get a vaccine-like shot to test for tuberculosis exposure or to protect themselves from tetanus. The flu vaccines are as safe as these vaccines and shots that are widely accepted by the public. Consequently, better efforts need to be made to make yearly flu vaccines as widely acceptable as other vaccines. Susceptible people need to understand that the vaccines afford them a significant chance to reduce or prevent this potentially debilitating disease, hospitalization and, in a few, a lethal flu-caused disease.

In the United States, the flu season usually occurs from about November until April. Officials have decided each new flu season will start each year on Oct. 4. Typically, activity is very low until December, and peak activity most often occurs between January and March. Ideally, the conventional flu vaccine should be administered between September and mid-November. Flu season typically occurs between October and May. It takes about 1-2 weeks after vaccination for antibodies against influenza to develop and provide protection. The CDC has published a summary list of their current recommendations of who should get the current vaccine.

Summary of CDC influenza vaccination recommendations for 2022-2023

Updated CDC information and guidance in this report includes the following taken directly from the CDC:

Routine annual influenza vaccination is recommended for all persons aged ≥6 months who do not have contraindications.

A licensed vaccine appropriate for age and health status should be used. Consult package information for age indications.

Emphasis should be placed on the vaccination of high-risk groups and their contacts/caregivers. When vaccine supply is limited, vaccination efforts should focus on delivering vaccination to (no hierarchy is implied by order of listing):

• Children aged 6 through 59 months
• Adults aged ≥50 years
• Persons with chronic pulmonary (including asthma), cardiovascular (excluding isolated hypertension), renal, hepatic, neurologic, hematologic, or metabolic disorders (including diabetes mellitus)
• Persons who are immunocompromised due to any cause, including (but not limited to) medications or HIV infection
• Women who are or will be pregnant during the influenza season
• Children and adolescents (aged 6 months through 18 years) receiving aspirin- or salicylate-containing medications who might be at risk for Reye syndrome associated with influenza
• Residents of nursing homes and long-term care facilities
• American Indians/Alaska natives
• Persons who are extremely obese (BMI ≥40 for adults)
• Caregivers and contacts of those at risk:
  • Health care personnel, including all paid and unpaid persons working in health care settings who have the potential for exposure to patients and/or to infectious materials, whether or not directly involved in patient care;
  • Household contacts and caregivers of children aged ≤59 months (for example, <5 years), particularly contacts of children aged <6 months, and adults aged ≥50 years;
  • Household contacts and caregivers of persons with medical conditions associated with increased risk of severe complications from influenza.

For more information and details too extensive to include here (for example, vaccine types, scheduled doses, side effects), the following site is recommended: Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices -- United States, 2022-23 Influenza Season.

For additional information: MMWR Recomm Rep 2020;69(No. RR-8), at https://www.cdc.gov/vaccines/hcp/acip-recs/vacc-specific/flu.html.

Thimerosal is a preservative that contains mercury and is used in multidose vials of conventional flu vaccines to prevent contamination when the vial is repeatedly used to extract the vaccine. Although thimerosal is being phased out as a vaccine preservative, it is still used in flu vaccines in low levels. There is no data that indicates thimerosal in these vaccines has caused autism or other problems in individuals. However, flu vaccine that is produced for single use (not a multidose vial) contains no thimerosal; however, these vials are not as readily available to doctors and likely cost more to produce. Consequently, the FDA has published these two questions with clear answers that are quoted below:

"Is it safe for children to receive an influenza vaccine that contains thimerosal?"
"Yes. There is no convincing evidence of harm caused by the small doses of thimerosal preservative in influenza vaccines, except for minor effects like swelling and redness at the injection site."

"Is it safe for pregnant women to receive an influenza vaccine?"
"Yes. A study of influenza vaccination examining over 2,000 pregnant women demonstrated no adverse fetal effects associated with influenza vaccine. Case reports and limited studies indicate that pregnancy can increase the risk for serious medical complications of influenza. One study found that out of every 10,000 women in their third trimester of pregnancy during an average flu season, 25 will be hospitalized for flu-related complications."

However, as stated above, the FDA goes on to say that single-dose vial of conventional and other flu vaccines will not contain the preservative thimerosal, so that if a person wants to avoid the thimerosal, they can ask for vaccine that comes in a single-dose vial. The nasal spray vaccine contains no thimerosal, but it is not recommended for use in pregnant women. The CDC further states, that after numerous studies, there is no established link between flu shots with or without thimerosal and autism.

Although annual influenza (injectable) vaccination has long been recommended for people in the high-risk groups, many still do not receive the vaccine, often because of their concern about side effects. They mistakenly perceive influenza as merely a nuisance and believe that the vaccine causes unpleasant side effects or that it may even cause the flu. The truth is that influenza vaccine causes no side effects in most people. In the past, patients with egg allergy had restrictions on getting the vaccine. However, extensive research has indicated that there is not enough egg protein in the vaccine to trigger an immune response, and all the recommendations about allergies to eggs has been dropped for the 2018-2019 flu season by several organizations that regulate vaccines. The vaccine is not recommended while individuals have active infections or active diseases of the nervous system. Less than one-third of those who receive the vaccine have some soreness at the vaccination site, and about 5%-10% experience mild side effects, such as headache, low-grade fever, or muscle cramps, for about a day after vaccination; some may develop swollen lymph nodes. These side effects are most likely to occur in children who have not been exposed to the influenza virus in the past. The intradermal shots reportedly have similar side effects as the IM shot but are less intense and may not last as long as the IM shot.

Nevertheless, some older people remember earlier influenza vaccines that did, in fact, produce more unpleasant side effects. Vaccines produced from the 1940s to the mid-1960s were not as highly purified as modern influenza vaccines, and it was these impurities that caused most of the side effects. Since the side effects associated with these early vaccines, such as fever, headache, muscle aches, and/or fatigue and malaise, were similar to some of the symptoms of influenza, people believed that the vaccine had caused them to get the flu. However, injectable influenza vaccine produced in the United States has never been capable of causing influenza because it consists of killed virus.

Another type of influenza vaccine (nasal spray) is made with live attenuated (altered) influenza viruses (LAIV). This vaccine is made with live viruses that can stimulate the immune response enough to confer immunity but do not cause classic influenza symptoms (in most instances). The nasal spray vaccine (FluMist) was only previously approved for healthy individuals ages 2-49 years of age and was recommended preferentially for healthy children aged 2 through 8 who did not have contraindications to receiving the vaccine. However, this season, the CDC and others report there is no preference expressed for any vaccine over another. The American Academy of Pediatrics (AAP) recommends that all children 6 months and older receive a seasonal flu vaccine (some children under the age of 9 will need 2 doses). AAP and others recommend both inactivated influenza vaccines (IIV) and live attenuated influenza vaccine (LAIV) as vaccine options for the 2020-2021 season with no preference for any vaccine type. However, FluMist, a live attenuated vaccine, is recommended for ages 2-49 only. For additional information about vaccines, see https://www.aap.org/en-us/advocacy-and-policy/aap-health-initiatives/immunizations/Influenza-Implementation-Guidance/Pages/Annual-AAP-Influenza-Policy.aspx or your child's doctor. This nasal spray vaccine contains live attenuated virus (less able to cause flu symptoms due to a designed inability to replicate at normal body temperatures). Side effects of the nasal spray vaccine include nasal congestion, sore throat, and fever. Headaches, muscle aches, irritability, and malaise have also been noted. In most instances, if side effects occur, they only last a day or two. This nasal spray has been produced for conventional flu viruses and should not be given to pregnant women or anyone who has a medical condition that may compromise the immune system because in some instances the flu may be a side effect.

Some people do not receive influenza vaccine because they believe it is not very effective. There are several different reasons for this belief. People who have received influenza vaccine may subsequently have an illness that is mistaken for influenza, and they believe that the vaccine failed to protect them. In other cases, people who have received the vaccine may indeed have an influenza infection. Overall vaccine effectiveness varies from year to year, depending upon the degree of similarity between the influenza virus strains included in the vaccine and the strain or strains that circulate during the influenza season. Because the vaccine strains must be chosen 9-10 months before the influenza season, and because influenza viruses mutate over time, sometimes mutations occur in the circulating virus strains between the time the vaccine strains are chosen and the next influenza season ends. These mutations sometimes reduce the ability of the vaccine-induced antibody to inhibit the newly mutated virus, thereby reducing vaccine effectiveness. This commonly occurs with the conventional flu vaccines as the specific virus types chosen for vaccine inclusion are based on reasoned projections for the upcoming flu season. Occasionally, the vaccine does not match the actual predominating virus strain and is not very effective in generating a specific immune response to the predominant infecting flu strain.

The CDC urges people to seek emergency medical care for a sick child with any of these flu effects (symptoms or signs):

1. Fast breathing or trouble breathing (shortness of breath)
2. Bluish or gray skin color
3. Not drinking enough fluids
4. Severe or persistent vomiting
5. Not waking up or not interacting
6. Being so irritable that the child does not want to be held
7. Flu-like symptoms improve but then return with fever and cough

The following is the CDC's list of symptoms that should trigger emergency medical care for adults:

1. Difficulty breathing or shortness of breath
2. Pain or pressure in the chest or abdomen
3. Sudden dizziness
4. Confusion
5. Severe or persistent vomiting
6. Influenza-like symptoms improve but then return with fever and worse cough
7. Having a high fever for more than 3 days is another danger sign, according to the WHO, so the CDC has also included this as another serious symptom.

In adults, flu symptoms usually last about 5-7 days, but in children, the symptoms may last longer (about 7-10 days). However, some symptoms such as weakness and fatigue may gradually wane over several weeks.

In general, the majority (about 90%-95%) of people who get the flu feel terrible (see symptoms) but recover with no problems. People with suppressed immune systems historically have worse outcomes than uncompromised individuals; current data suggest that pregnant individuals, children under 2 years of age, young adults, and individuals with any immune compromise or debilitation are likely to have a worse prognosis. Complications of long-term problems from the flu may worsen medical conditions such as asthma, congestive heart failure, and diabetes. Other complications may include ear infections, sinus infections, dehydration, pneumonia, and even death. In most outbreaks, epidemics, and pandemics, the mortality rates are highest in the older population (usually above 50 years old). Complications of any flu virus infection, although relatively rare, may resemble severe viral pneumonia or the SARS (severe acute respiratory syndrome caused by a coronavirus strain) outbreak in 2002-2003, in which the disease spread to about 10 countries with over 7,000 cases, over 700 deaths, and had a 10% mortality rate. Guillain-Barré syndrome (GBS), a rare immune disorder that can result in weakness or paralysis, may occur after having the flu or very rarely, after vaccination against the flu (estimated by the CDC to be about one person per every million people vaccinated).

Yes. However, associated deaths per year depend upon the virulence of the particular strain of virus that is circulating. That means for any given year, the likelihood of dying from the flu varies according to the specific infecting viruses. For example, from 1976-2007 (the most reliable available data according to the CDC), deaths associated with the flu range from a low of about 3,000 per year to a high of about 49,000 per year. The CDC estimates about 36,000 deaths/year (average) in the U.S. in recent years, but these may increase if vaccination rates continue to fall. The 1918 influenza pandemic (1918-1919) was estimated to cause 20-50 million deaths worldwide.

Flu vaccine

Annual influenza vaccination can prevent most of the illness and death that influenza causes. The CDC's current Advisory Committee on Immunization Practices (ACIP) issued recommendations for everyone 6 months of age and older, who do not have any contraindications to vaccination, to receive a flu vaccine each year.

Flu vaccine (influenza vaccine made from inactivated and sometimes attenuated [noninfective] virus or virus components) is specifically recommended for those who are at high risk for developing serious complications from influenza infection.

Other simple hygiene methods can reduce or prevent some individuals from getting the flu. For example, avoiding kissing, handshakes, and sharing drinks or food with infected people and avoiding touching surfaces like sinks and other items handled by individuals with the flu are good preventive measures. Washing one's hands with soap and water or by using an alcohol-based hand sanitizer frequently during the day may help prevent the infection. Individuals with the flu should avoid coughing or sneezing on uninfected people; quick hugs are probably okay as long as there is no contact with mucosal surfaces and/or droplets that may contain the virus. Wearing a mask may help reduce your chances of getting the disease, and if you unknowingly or know you have the infection, help to reduce spreading it to others.

While a person has the flu, good nutrition can help the recovery process. Anyone with the flu needs to avoid dehydration, soothe sore throat and/or upset stomach, and have a good protein intake. Avoid dehydration by maintaining an adequate fluid intake. Sore throat and upset stomach may be relieved by broths or warm soups (chicken, vegetable, or beef) and plain crackers, toast, and ginger tea or noncarbonated ginger ale. Scrambled eggs, yogurt, and/or protein drinks are good protein sources. In addition, bananas, rice, and applesauce are food that are often recommended for those with an upset stomach. This list is not exhaustive but should provide a balanced approach to help speed recovery from the flu.

It is possible to get the flu and COVID-19 at the same time. There is little or no data available to determine how often such concurrent infections may happen.