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2022-2023 Flu Season

No two flu seasons are the same, and it is impossible to predict when flu season will peak. However, public health officials in the U.S. often look to the Southern Hemisphere’s summer flu season for what to expect during the next U.S. flu season. From mid-April to July 2022, the number of confirmed flu cases in Australia exceeded the numbers for every year since 2017. Experts expect a similarly bad 2022-2023 flu season in the U.S. 

Influenza vaccines are a key defense against the flu. Each year, the Advisory Committee on Immunization Practices (ACIP) of the U.S. Centers for Disease Control and Prevention (CDC) updates its recommendations for seasonal influenza vaccination. Because the vaccine changes every year to match circulating influenza virus strains, everyone six months of age or older needs a flu vaccine every year

For the 2022-2023 season, all flu vaccines are quadrivalent, which means they protect against four different influenza virus strains, two influenza A and two influenza B strains. Inactivated, recombinant, and live attenuated versions of the vaccine are available for 2022-2023. 

The inactivated flu vaccine is available as:

  • The standard quadrivalent flu shot produced in eggs (IIV4) for those ages six months and older;
  • A cell-cultured version (ccIIV4) for those ages six months and older who cannot use egg-based vaccines;
  • A high-dose version only for seniors 65 years or older (HD-IIV4); and
  • A version containing an adjuvant (a substance that increases the general immune response) only for seniors 65 years or older (aIIV4);.

The recombinant flu vaccine is available as a quadrivalent flu shot for ages 18 and up (RIV4). 

The live attenuated flu vaccine is available as a quadrivalent nasal spray for ages 2-49 (LAIV4). Note that those with PI and their household members should receive inactivated or recombinant versions of the vaccine, not the live attenuated version.

New this year, ACIP recommends that people ages 65 or older receive HD-IIV4, aIIV4, or RIV4 over the other types of flu vaccine because there is evidence that these formulations provide greater protection in older individuals. ACIP also recommends that children eight years old or younger who have either never been vaccinated for the flu or have only received one previous flu vaccine dose be vaccinated with two doses of flu vaccine spaced four weeks apart.

Heading into flu season, COVID-19 is still circulating at medium and high levels in many areas of the U.S. Because symptoms of COVID-19 and the flu are very similar, testing for one or both may be necessary to determine the appropriate treatment. Note that it is also possible to get both COVID-19 and the flu at the same time. 

Related article: Colds, flu, and COVID, oh my…

Use this table of contents to navigate the information below:

Influenza basics

Flu symptoms and complications

Influenza, or “the flu,” typically comes on suddenly and includes fever and/or chills, aching muscles, sore throat, and a dry cough. Additional symptoms may include a runny or stuffy nose, fatigue, headache, a burning sensation in the chest, eye pain, and sensitivity to light. Although many people think of the flu as harmless, it is a specific and serious respiratory disease that can result in hospitalization and death. During the 2019-2020 flu season, more than 50,000 people in the U.S. died of flu-related complications.

Influenza symptoms are not the same in every infected person. For example, people with PI may not spike a fever. Someone who has been previously exposed to similar virus strains (through natural infection or vaccination) is less likely to develop serious symptoms. 

Flu versus cold symptoms.
Centers for Disease Control and Prevention,
Cold Versus Flu:

Flu and the common cold are both respiratory illnesses, but they are caused by different viruses. Cold symptoms are usually milder than symptoms of the flu. People with colds are more likely to have a runny or stuffy nose. Colds generally do not result in serious health problems such as pneumonia, bacterial infections, or hospitalizations, whereas the flu can have very serious complications.

The flu also shares many symptoms with COVID-19. COVID-19 is more contagious than the flu. If COVID-19 and the flu are both circulating in your area, you may need to be tested to determine which illness you have. Testing is important because the treatments for COVID-19 and the flu are different and must be started as soon as possible after symptoms begin.

The most frequent complication of the flu is bacterial pneumonia. Viral pneumonia is a less common complication, but it has a high fatality rate. Other complications include inflammation of the heart (e.g., myocarditis) and worsening of lung diseases (e.g., bronchitis). Except for those with interferon pathway deficiencies, people with PI are not necessarily at high risk of developing complications from the flu unless they meet other high-risk criteria, such as: 

  • Five years old or younger (especially younger than two); 
  • 65 years or older;
  • Have asthma, diabetes, or chronic lung, heart, kidney, liver, metabolic, or blood conditions;
  • Have had a stroke;
  • Have a secondary immunodeficiency caused by disease (e.g., HIV or blood cancer) or medication (e.g., immunosuppressive therapy);
  • Are pregnant.

Reye’s syndrome is a complication that occurs almost exclusively in children—patients suffer from severe vomiting and confusion, which may progress to coma because of swelling of the brain. To decrease the chance of developing Reye’s syndrome, infants, children, and teenagers should not be given aspirin for fever reduction or pain relief.


What causes the flu and how it spreads

The flu is caused by influenza viruses. The two main types that infect people, influenza A and influenza B, are genetically different. Influenza A can cause moderate to severe illness in all age groups and infects many species, including pigs, birds, and horses. Influenza B usually causes milder disease and primarily affects children. Both influenza A and influenza B strains cause seasonal outbreaks, typically during the winter months in the U.S.

Influenza A strains can also cause flu pandemics, like the 1918 flu and the swine flu in 2009. Subtypes of influenza A are named for two proteins, hemagglutinin (H) and neuraminidase (N), on the surface of the virus. These antigens (proteins that cause an immune response) change in the virus population over time. When a major change in the antigens occurs, the new influenza A subtype can cause a flu pandemic since the human immune system has not ‘seen’ the subtype before, and the population has no immunity to the new strain. Pandemic strains typically go on to become seasonal strains once the human population develops some immunity to them.

Like COVID-19, the flu is transmitted from person to person by tiny airborne droplets formed during coughing, sneezing, or talking. These droplets are inhaled or land on mucus membranes inside the nose, mouth, or eyes. Influenza virus also can be transmitted orally, most commonly by kissing, or by touching a surface or object that has flu virus on it and then touching the mouth, nose, or eyes. Individuals with PI have the same risk of getting the flu as the rest of the population. 

A person is most likely to pass on the influenza virus beginning one to two days before symptoms and ending four to five days after symptoms begin. The incubation period for influenza, which is the time from when someone is infected to when they start to show symptoms, ranges from one to four days. For most people, the flu lasts only a few days, but some people get much, much sicker. 

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Preventing the flu

Get annual flu vaccines

The most effective way to avoid the flu is to get an influenza vaccine (‘flu shot’) annually. Influenza virus strains change from year to year, so a new vaccine is needed every year to protect against current strains. For households with a person who has PI, all members of the household and other close contacts should get the inactivated (killed) or recombinant flu vaccine

Why should everyone be immunized? First, some people with PI may benefit from the vaccine and develop at least some antibodies or a T cell response, which is important for any viral infection. Immunoglobulin (Ig) replacement therapy products (IVIG or SCIG) typically do not have protective levels of antibodies against current flu strains because manufacturing takes six months or more from the initial plasma donation. Even if the person with PI does not respond to immunization, there is little downside to receiving the vaccine. Flu vaccines are safe, and, contrary to a common misconception, they do not cause the flu itself.

Importantly, household members who do respond to the vaccine create a “protective cocoon” around the person with PI. The person with PI benefits from having close contacts protected from infection and is less likely to be exposed to the virus if those around them are vaccinated.

Flu vaccines usually become available in late summer. Because protection from the flu vaccine decreases over time, particularly for individuals age 65 and older, the CDC recommends getting a flu vaccine in September or October to extend protection throughout the entire flu session. However, studies have shown that immunization can still be effective when given well into February or March in some years, so even late vaccination is worthwhile. 

Note that those on Ig therapy do not need to coordinate flu vaccine timing with their treatment schedule. Also, the flu vaccine can be given at the same time as a COVID-19 vaccine booster dose.

In general, there are three types of flu vaccines available (see the season-specific section at the top for any yearly exceptions):

  • Inactivated Influenza Vaccine (IIV): IIV contains purified H antigen from influenza virus particles that have been produced in eggs or cell culture and then killed. 
  • Recombinant Influenza Vaccine (RIV): RIV contains purified H antigen produced in insect cells. Because it is not produced using eggs, RIV is safe for those who have egg allergies.
  • Live Attenuated Influenza Vaccine (LAIV): LAIV is made from live but weakened whole influenza virus. It is only for healthy people aged 2-49 years who are not pregnant. Although it is unlikely to cause problems for any but the most severely immunocompromised, people with PI should not receive LAIV.


Hand washing and other non-pharmaceutical interventions

Here are some additional steps that prevent the spread of respiratory illnesses like the flu:

  • Fully cover (nose and mouth) coughs and sneezes with a sleeve, elbow, or tissue—throw the tissue away after use.
  • Wash your hands often with soap and water for a full 20 seconds, especially after coughing or sneezing. Note that some studies have found that alcohol-based hand sanitizer does not kill influenza virus contained in wet/moist mucus. Washing with water, with or without soap, is more effective at killing influenza.
  • Stay at least six feet away from people who are sick.
  • Use a well-fitting mask worn over the nose and mouth to reduce inhalation of the droplets that spread respiratory viruses like influenza and COVID-19. See the CDC’s guidance on mask types. During the 2020-2021 and 2021-2022 flu seasons, not many people got the flu because they were wearing masks for COVID-19 prevention.
  • Try not to touch your eyes, nose, or mouth. Germs often spread this way.
  • Stay home from work or school for at least 3-4 days after symptoms start when sick with the flu. 

During flu season, people with PI may also want to stay away from crowded public places, such as shopping malls, if exposure is a concern.

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Flu testing and treatment

Make a plan with your healthcare provider

During influenza season, people with PI should discuss plans for dealing with the flu with their healthcare providers before it hits. How and where will you get tested for influenza or other, similar respiratory viruses like COVID-19? What medication does your healthcare provider recommend and how will you get it? What symptoms should you pay particular attention to?

If you become ill, contact your doctor immediately about getting tested for influenza and beginning antiviral treatment. Speed is important since antiviral medications are most effective if they are started within 48 hours of the beginning of symptoms. To facilitate early treatment, healthcare providers may provide an antiviral prescription based on symptoms, before testing or an office visit.

Although there are no at-home rapid tests, influenza can be diagnosed rapidly by a test done in physician's offices or at a pharmacy. If the test is positive, patients should immediately begin antiviral treatment. 


Antiviral medication

Antiviral medications for influenza work by blocking the action of critical viral proteins, thereby preventing the virus from replicating and/or spreading. Studies show that antiviral medications are most effective when they are taken right after symptoms begin, ideally within 48 hours. The same antiviral medicines that are effective for the general population are also effective for people with PI. 

There are four antiviral drugs approved by the U.S. Food and Drug Administration (FDA) and recommended by the CDC to treat the flu. All four work on both influenza A and influenza B viruses. Three antivirals are approved for the prevention of influenza (i.e., prophylactic use) as well. However, for people with severe immunodeficiencies or others who might not respond to influenza vaccination, the CDC recommends prophylactic use of influenza antivirals only after exposure to a person with influenza, not as routine prevention.

Three of the antivirals inhibit influenza’s neuraminidase (N) protein:

  • Oseltamivir phosphate (available as a generic or under the trade name Tamiflu®) is available as a pill or liquid suspension and is FDA approved for treatment of flu within 48 hours of symptoms in people 14 days and older. The CDC also recommends prophylactic use in those ages three months and older.
  • Zanamivir (trade name Relenza®) is a powder that is administered using an inhaler device and is approved for early treatment of flu in people seven years and older. It is also approved for prophylactic use in people five years and older. Note that it is not recommended for people with breathing problems like asthma or COPD or those with milk protein or lactose allergies.
  • Peramivir (trade name Rapivab®) is given intravenously by a healthcare provider and is approved for early treatment of flu in people two years and older.

The fourth antiviral inhibits influenza’s cap-dependent endonuclease protein:

  • Baloxavir marboxil (trade name Xofluza®) is a single-dose pill and is approved for the treatment of flu within 48 hours of symptoms and as post-exposure prophylaxis (i.e., after known exposure to the flu, but before symptoms begin) in people five years and older. 

Antiviral treatment should be initiated in symptomatic individuals as soon as possible when there is known influenza activity in the community. Treatment is also indicated when flu is suspected or confirmed for people at high risk of developing complications. A history of current season influenza vaccination does not exclude a diagnosis of influenza in an ill child or adult. Likewise, a positive COVID-19 test does not exclude influenza as there are documented cases of co-infection.

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Updated September 2022