“We just had no idea he could get this sick. No one ever told us about this. Of course we would have vaccinated him!”
This is what the parents of a healthy school-aged child in the intensive care unit with an influenza infection told one of us as their child laid in the bed struggling to breathe. The child had received all his childhood vaccines except for the influenza vaccine. While some families recognize the risk of disease and believe it “just won’t happen to them,” many are not even aware of the spectrum of illness caused by influenza.
The 2024-2025 influenza season caused almost 290 children to die from their infections — the most since pediatric influenza mortality started being recorded in 2004. Further, the 2025-2026 influenza season is off to a concerning start, with year-over-year numbers matching or surpassing those of last season. For these reasons, this month our “Fast Facts” series returns with a focus on influenza. We hope you’ll take a few minutes to review the information not only to remember the spectrum of disease caused by influenza but also to prepare yourself to effectively discuss this infection with patients, families, and community members.
Recognizing influenza
Each year, two different types of influenza virus, known as type A and type B, circulate. Further, multiple subtypes can circulate. In recent years in the U.S., we have seen infections caused by two type A influenza viruses and one type B virus. While two type B viruses circulated before the COVID-19 pandemic, only one type B virus has been circulating since the pandemic — leading to a change in the composition of influenza vaccines from quadrivalent to trivalent.
Because influenza viruses change as they replicate, people often have multiple influenza infections over their lifetime. Symptoms come on suddenly and typically include fever, chills, headache, muscle aches and decreased energy and appetite. People also experience respiratory symptoms, including cough, sore throat and runny nose. Children may experience additional disease presentations, including ear infections, croup (i.e., irritation of their upper airways, leading to a barking cough), and gastrointestinal symptoms, including nausea, vomiting and diarrhea. Symptoms start about one to four days after exposure to the virus.
The virus spreads between people most often when an infected person coughs or sneezes, but people can also be exposed through touching an infected surface and then touching their eyes, nose or mouth.
While many cases resolve on their own, younger children, older adults, pregnant women, and those with chronic medical conditions are at risk for more severe infections or unusual presentations that can include viral pneumonia; secondary infections, such as bacterial pneumonia; and worsening of medical conditions, including chronic obstructive pulmonary disease (COPD), asthma and heart disease. Rare but serious complications can include reduced brain function, such as confusion, behavior change or decreased alertness (encephalopathy); inflammation of the brain (encephalitis); irritation of muscles (myositis); inflammation of the heart (myocarditis); Guillain-Barré syndrome (GBS); or overall functional decline in the elderly. Importantly, and often not realized by the public, serious complications can occur in otherwise healthy individuals.
Most years, influenza causes tens of thousands of adult deaths and about 150 child deaths. However, as demonstrated during the 2024-25 season when we witnessed almost 290 pediatric deaths, the number of influenza-related deaths can vary.
Making an influenza diagnosis
During influenza season, typically December to April or May in most of the U.S., many clinicians will make a clinical diagnosis, meaning they will diagnose influenza based on a person’s symptoms. At times, there is a role for ensuring that the infection is more definitively caused by influenza, and in these cases, viral testing can be done.
Three diagnostic testing options are available:
- Rapid influenza diagnostic tests: These are the typical “flu swabs” available in many clinics, urgent care centers, and now even in homes. While these tests are helpful when positive, they can produce false negatives, meaning someone has an influenza infection, but the test results are negative.
- Rapid molecular assays: These tests use molecular techniques to identify the genetic material of the virus, so they can reliably detect the virus as well as its type (type A or type B). These tests are used more often in emergency departments and in some outpatient settings because they are fast and accurate.
- Reverse transcription polymerase chain reaction (RT-PCR): This test requires sending the sample to a lab, so results may not be available for hours or up to one day. These tests can identify both the type and subtype of influenza, so they tend to be more commonly used in hospitals and for public health surveillance. This type of test can be a stand-alone test, meaning for influenza only, or it can be part of a panel test for multiple viruses.
Testing is most accurate when done early during illness (within the first three to four days). Tests should be used when:
- The result will help with clinical care of the patient, such as for deciding to start antiviral treatment
- Needed to guide isolation practices, such as in hospitals
- Confirming the diagnosis is important for informing whether high-risk individuals may need post-exposure prophylaxis due to their proximity to the infected patient
For an otherwise healthy individual who will only require symptomatic treatment at home, testing is generally not necessary.
Treatment and management of influenza
Most often, clinical management for influenza is primarily supportive and focuses on rest, ensuring the patient is well hydrated, and relieving symptoms, such as supporting any respiratory needs or reducing nausea or diarrhea. Most patients recover fully within days to weeks. Hospitalization is required for severe cases.
In some cases, use of antiviral therapies may be recommended. Multiple options are available for influenza, though oseltamivir is used most frequently.
Among children, treatment is indicated for:
- Those younger than 12 months of age.
- Those with chronic medical conditions.
- Healthy children between 12 and 24 months of age, less than 48 hours into their symptoms or who are presenting with severe disease.
- All hospitalized children should generally be given oseltamivir as well, unless they are late in their illness and the hospitalization is the result of a secondary infection.
For adults, treatment will likely benefit those who are hospitalized, severely ill or worsening, or at higher risk for complications, such as:
- Adults 65 years of age and older
- Pregnant women and those who delivered within the last six weeks
- People with chronic medical conditions
- Immunocompromised patients
- Adults with severe obesity
- Residents of nursing homes or long-term care facilities
Otherwise-healthy adults may have shorter symptom duration with the use of antivirals early in infection (within 48 hours of symptom onset).
Additional antivirals include:
- Baloxavir, an oral, weight-based medication given as a one-time dose.
- Peramivir, an intravenously administered antiviral, also given as a one-time dose.
- Zanamivir, an inhaled antiviral given to patients who do not have underlying respiratory disease, such as asthma or chronic lung disease. Treatment is for five days.
As each antiviral has specific age restrictions and dosing regimens, clinicians should consult a drug formulary or package insert for specific guidance.
Infection control: Reducing the spread of influenza
Influenza spreads through respiratory droplets, meaning tiny droplets produced by coughing, sneezing, or even talking. These virus-laden droplets then land in the nose or mouth of nearby people spreading the infection to them. The virus can also live on surfaces, causing infections when people touch the surface and then touch their nose, mouth or eyes, though this type of spread is less common than person-to-person transmission through respiratory droplets. People can spread the virus about three days before symptoms appear until about seven days after symptoms develop. However, transmission during this period will depend on a variety of factors, including their inoculum, immune response and activities.
The best protection against influenza is vaccination. Unfortunately, many opt out of using this tool each year, often because of misconceptions about the disease (“It’s not that bad.”) or the vaccine (“It gave me the flu.”). However, two points about the vaccine are important:
- The goal of the influenza vaccine is not to prevent infection, but rather to limit the severity of disease, thereby decreasing hospitalizations and deaths. Specifically, protection following vaccination leverages immunologic memory, which takes a few days to activate. After a person is exposed, the virus begins replicating in the respiratory tract. This means the person is infected and can spread the virus during this period. But once memory immunity is activated, it quickly alters the course of infection because memory immunity affects the infection more quickly than primary immunity.
- While the vaccine is not always well matched to the circulating strains of influenza, due to antigenic drift as the virus replicates during its travel from the southern to the northern hemisphere, the specific immunity it generates is still more effective than no immunity or non-specific immunity afforded by other forms of protection, such as masking and handwashing.
With this said, people can still decrease spread of the virus by masking and good hand hygiene practices. Influenza virus is susceptible to both alcohol-based hand sanitizers and soap and water. Likewise, people who are sick should be encouraged to stay home and minimize interactions with others.
Post-exposure prophylaxis is recommended for people who have a known exposure to influenza in the past 48 hours and are at high risk for complications, particularly if they are unvaccinated. When recommended, prophylaxis typically involves prescribing a course of antivirals that can reduce the likelihood of developing an infection, with the length of treatment depending on which antiviral is used. If someone is already showing symptoms, they should get treatment.
Influenza: Key clinical takeaways
At the peak of influenza season, providers are likely to see many different presentations of disease. Keep these fast facts at your fingertips to help with diagnosing, testing, treating, and guiding your patients through influenza season.
Resources for families
- Influenza: What You Should Know: English | Spanish | Japanese (Q&A)
- Vaccines & Diseases: Influenza (webpage)
- VEC Vaccine Notes: video | audio
- Who should get the influenza vaccine and influenza vaccine types (infographic)
- Be a Flu Fighter! (webpage)
- Can a flu vaccine give me the flu? (video)
- Why doesn’t the influenza vaccine work for more than one year? (video)
- HILLEMAN: A Perilous Quest to Save the World’s Children, influenza segment (video)
Resources for providers
Contributed by: Lori Handy, MD, MSCE , Charlotte A. Moser, MS, Paul A. Offit, MD
“We just had no idea he could get this sick. No one ever told us about this. Of course we would have vaccinated him!”
This is what the parents of a healthy school-aged child in the intensive care unit with an influenza infection told one of us as their child laid in the bed struggling to breathe. The child had received all his childhood vaccines except for the influenza vaccine. While some families recognize the risk of disease and believe it “just won’t happen to them,” many are not even aware of the spectrum of illness caused by influenza.
The 2024-2025 influenza season caused almost 290 children to die from their infections — the most since pediatric influenza mortality started being recorded in 2004. Further, the 2025-2026 influenza season is off to a concerning start, with year-over-year numbers matching or surpassing those of last season. For these reasons, this month our “Fast Facts” series returns with a focus on influenza. We hope you’ll take a few minutes to review the information not only to remember the spectrum of disease caused by influenza but also to prepare yourself to effectively discuss this infection with patients, families, and community members.
Recognizing influenza
Each year, two different types of influenza virus, known as type A and type B, circulate. Further, multiple subtypes can circulate. In recent years in the U.S., we have seen infections caused by two type A influenza viruses and one type B virus. While two type B viruses circulated before the COVID-19 pandemic, only one type B virus has been circulating since the pandemic — leading to a change in the composition of influenza vaccines from quadrivalent to trivalent.
Because influenza viruses change as they replicate, people often have multiple influenza infections over their lifetime. Symptoms come on suddenly and typically include fever, chills, headache, muscle aches and decreased energy and appetite. People also experience respiratory symptoms, including cough, sore throat and runny nose. Children may experience additional disease presentations, including ear infections, croup (i.e., irritation of their upper airways, leading to a barking cough), and gastrointestinal symptoms, including nausea, vomiting and diarrhea. Symptoms start about one to four days after exposure to the virus.
The virus spreads between people most often when an infected person coughs or sneezes, but people can also be exposed through touching an infected surface and then touching their eyes, nose or mouth.
While many cases resolve on their own, younger children, older adults, pregnant women, and those with chronic medical conditions are at risk for more severe infections or unusual presentations that can include viral pneumonia; secondary infections, such as bacterial pneumonia; and worsening of medical conditions, including chronic obstructive pulmonary disease (COPD), asthma and heart disease. Rare but serious complications can include reduced brain function, such as confusion, behavior change or decreased alertness (encephalopathy); inflammation of the brain (encephalitis); irritation of muscles (myositis); inflammation of the heart (myocarditis); Guillain-Barré syndrome (GBS); or overall functional decline in the elderly. Importantly, and often not realized by the public, serious complications can occur in otherwise healthy individuals.
Most years, influenza causes tens of thousands of adult deaths and about 150 child deaths. However, as demonstrated during the 2024-25 season when we witnessed almost 290 pediatric deaths, the number of influenza-related deaths can vary.
Making an influenza diagnosis
During influenza season, typically December to April or May in most of the U.S., many clinicians will make a clinical diagnosis, meaning they will diagnose influenza based on a person’s symptoms. At times, there is a role for ensuring that the infection is more definitively caused by influenza, and in these cases, viral testing can be done.
Three diagnostic testing options are available:
- Rapid influenza diagnostic tests: These are the typical “flu swabs” available in many clinics, urgent care centers, and now even in homes. While these tests are helpful when positive, they can produce false negatives, meaning someone has an influenza infection, but the test results are negative.
- Rapid molecular assays: These tests use molecular techniques to identify the genetic material of the virus, so they can reliably detect the virus as well as its type (type A or type B). These tests are used more often in emergency departments and in some outpatient settings because they are fast and accurate.
- Reverse transcription polymerase chain reaction (RT-PCR): This test requires sending the sample to a lab, so results may not be available for hours or up to one day. These tests can identify both the type and subtype of influenza, so they tend to be more commonly used in hospitals and for public health surveillance. This type of test can be a stand-alone test, meaning for influenza only, or it can be part of a panel test for multiple viruses.
Testing is most accurate when done early during illness (within the first three to four days). Tests should be used when:
- The result will help with clinical care of the patient, such as for deciding to start antiviral treatment
- Needed to guide isolation practices, such as in hospitals
- Confirming the diagnosis is important for informing whether high-risk individuals may need post-exposure prophylaxis due to their proximity to the infected patient
For an otherwise healthy individual who will only require symptomatic treatment at home, testing is generally not necessary.
Treatment and management of influenza
Most often, clinical management for influenza is primarily supportive and focuses on rest, ensuring the patient is well hydrated, and relieving symptoms, such as supporting any respiratory needs or reducing nausea or diarrhea. Most patients recover fully within days to weeks. Hospitalization is required for severe cases.
In some cases, use of antiviral therapies may be recommended. Multiple options are available for influenza, though oseltamivir is used most frequently.
Among children, treatment is indicated for:
- Those younger than 12 months of age.
- Those with chronic medical conditions.
- Healthy children between 12 and 24 months of age, less than 48 hours into their symptoms or who are presenting with severe disease.
- All hospitalized children should generally be given oseltamivir as well, unless they are late in their illness and the hospitalization is the result of a secondary infection.
For adults, treatment will likely benefit those who are hospitalized, severely ill or worsening, or at higher risk for complications, such as:
- Adults 65 years of age and older
- Pregnant women and those who delivered within the last six weeks
- People with chronic medical conditions
- Immunocompromised patients
- Adults with severe obesity
- Residents of nursing homes or long-term care facilities
Otherwise-healthy adults may have shorter symptom duration with the use of antivirals early in infection (within 48 hours of symptom onset).
Additional antivirals include:
- Baloxavir, an oral, weight-based medication given as a one-time dose.
- Peramivir, an intravenously administered antiviral, also given as a one-time dose.
- Zanamivir, an inhaled antiviral given to patients who do not have underlying respiratory disease, such as asthma or chronic lung disease. Treatment is for five days.
As each antiviral has specific age restrictions and dosing regimens, clinicians should consult a drug formulary or package insert for specific guidance.
Infection control: Reducing the spread of influenza
Influenza spreads through respiratory droplets, meaning tiny droplets produced by coughing, sneezing, or even talking. These virus-laden droplets then land in the nose or mouth of nearby people spreading the infection to them. The virus can also live on surfaces, causing infections when people touch the surface and then touch their nose, mouth or eyes, though this type of spread is less common than person-to-person transmission through respiratory droplets. People can spread the virus about three days before symptoms appear until about seven days after symptoms develop. However, transmission during this period will depend on a variety of factors, including their inoculum, immune response and activities.
The best protection against influenza is vaccination. Unfortunately, many opt out of using this tool each year, often because of misconceptions about the disease (“It’s not that bad.”) or the vaccine (“It gave me the flu.”). However, two points about the vaccine are important:
- The goal of the influenza vaccine is not to prevent infection, but rather to limit the severity of disease, thereby decreasing hospitalizations and deaths. Specifically, protection following vaccination leverages immunologic memory, which takes a few days to activate. After a person is exposed, the virus begins replicating in the respiratory tract. This means the person is infected and can spread the virus during this period. But once memory immunity is activated, it quickly alters the course of infection because memory immunity affects the infection more quickly than primary immunity.
- While the vaccine is not always well matched to the circulating strains of influenza, due to antigenic drift as the virus replicates during its travel from the southern to the northern hemisphere, the specific immunity it generates is still more effective than no immunity or non-specific immunity afforded by other forms of protection, such as masking and handwashing.
With this said, people can still decrease spread of the virus by masking and good hand hygiene practices. Influenza virus is susceptible to both alcohol-based hand sanitizers and soap and water. Likewise, people who are sick should be encouraged to stay home and minimize interactions with others.
Post-exposure prophylaxis is recommended for people who have a known exposure to influenza in the past 48 hours and are at high risk for complications, particularly if they are unvaccinated. When recommended, prophylaxis typically involves prescribing a course of antivirals that can reduce the likelihood of developing an infection, with the length of treatment depending on which antiviral is used. If someone is already showing symptoms, they should get treatment.
Influenza: Key clinical takeaways
At the peak of influenza season, providers are likely to see many different presentations of disease. Keep these fast facts at your fingertips to help with diagnosing, testing, treating, and guiding your patients through influenza season.
Resources for families
- Influenza: What You Should Know: English | Spanish | Japanese (Q&A)
- Vaccines & Diseases: Influenza (webpage)
- VEC Vaccine Notes: video | audio
- Who should get the influenza vaccine and influenza vaccine types (infographic)
- Be a Flu Fighter! (webpage)
- Can a flu vaccine give me the flu? (video)
- Why doesn’t the influenza vaccine work for more than one year? (video)
- HILLEMAN: A Perilous Quest to Save the World’s Children, influenza segment (video)
Resources for providers
Contributed by: Lori Handy, MD, MSCE , Charlotte A. Moser, MS, Paul A. Offit, MD