OVERVIEW

What is influenza?

Influenza is an acute respiratory infection caused by the influenza virus^[1].

Patients with influenza typically experience obvious fever (armpit temperature: 38.1°C–40.0°C), fatigue, and body aches, while symptoms like cough, nasal congestion, and runny nose are relatively mild^[1].

Influenza is highly contagious, primarily spreading through airborne droplets and contact transmission. It often occurs in northern regions during winter and spring, while southern regions experience outbreaks year-round^[1].

Influenza has different types, including seasonal influenza (prevalent annually in winter, peaking from November to March), the 2009–2010 H1N1 influenza (also called swine flu), and avian influenza^[1].

Although influenza is a self-limiting disease, infants, the elderly, and those with weakened immune systems may develop severe cases, so early antiviral treatment is essential. Symptomatic treatment should follow, and confirmed or suspected cases should be isolated^[1].

Most seasonal influenza patients recover fully. However, some may develop complications like pneumonia, leading to lung congestion, edema, or even respiratory failure in severe cases. A small number of critical patients may die.

What’s the difference between influenza and the common cold?

The differences include:

• Different pathogens: Pathogens are microorganisms that cause illness^[1]. Influenza is caused by the influenza virus, such as H5N1, H7N9, H1N1, or avian influenza viruses. The common cold is caused by other viruses, such as rhinovirus or coronavirus.
• Different symptoms: The common cold mainly affects the nose, causing nasal congestion, runny nose, and sneezing, with milder symptoms like fever, fatigue, and body aches. It usually resolves within a week. Influenza, however, has milder respiratory symptoms (nasal congestion, runny nose, sore throat, cough) but more pronounced fever, fatigue, headache, and body aches. Some patients (e.g., children) may also experience abdominal pain, bloating, or vomiting. Influenza has an incubation period of 1–5 days (average 2 days) with no noticeable symptoms^[1].
• Different contagiousness: Influenza is more contagious. Once diagnosed, patients should rest at home, avoid crowded places, refrain from work or school, and wear masks when going out.

Is influenza a legally notifiable infectious disease?

Yes, influenza is a Category C notifiable infectious disease in China, known for its strong contagiousness and rapid spread^[2].

China’s notifiable infectious diseases are classified into Categories A, B, and C, totaling 39 types^[2].

Which department should I visit for influenza?

Depending on symptoms and hospital setup, patients may go to the fever clinic, emergency department, infectious disease clinic, or respiratory clinic.

SYMPTOMS

What are the symptoms of the flu?

Most people infected with the flu virus experience an incubation period of 1–3 days, during which no discomfort or symptoms occur^[1].

This is followed by a sudden onset of fever (armpit temperature between 38.1°C and 40.0°C), accompanied by chills, headache, body aches, fatigue, loss of appetite, indigestion, and other symptoms. Additional symptoms may include sore throat, dry cough, nasal congestion, and runny nose. In severe cases, respiratory failure, circulatory failure, shock, or even death may occur^[1].

If there are no complications, the fever usually subsides within 3–4 days, but complete recovery typically takes 1–2 weeks^[2]. Infants, the elderly, and those with weakened immune systems are more prone to complications, which can be life-threatening.

What diseases or complications can the flu cause?

• Pneumonia: When the flu virus invades the lower respiratory tract, it can cause pneumonia, manifesting as cough, chest pain, phlegm, fever, difficulty breathing, and, in severe cases, respiratory failure^[1].
• Otitis media: The flu virus can infect the inner side of the eardrum, leading to otitis media, characterized by ear pain, fever, and hearing abnormalities.
• Heart damage: Myocarditis, pericarditis, or even heart failure may occur, presenting as palpitations, chest tightness, fever, shortness of breath, and swelling^[1].
• Neurological damage: Conditions such as encephalitis, meningitis, or myelitis may develop, with symptoms including headache, nausea, fever, neck stiffness, confusion, abnormal limb sensations, or paralysis^[1].
• Myositis: More common in children, symptoms include severe muscle pain (most often in the legs), vomiting, and confusion^[1].

The presence of complications indicates severe flu and requires prompt medical attention and treatment.

What symptoms suggest that the flu may be severe?

• Persistent high fever for more than 3 days, accompanied by severe cough, purulent or bloody sputum, or chest pain;
• Rapid breathing, difficulty breathing, or bluish lips;
• Altered mental state, such as sluggishness, drowsiness, agitation, or seizures;
• Severe vomiting or diarrhea leading to dehydration^[2].

Is the flu highly dangerous?

For healthy adults, the flu is generally not very harmful, typically causing fever, body aches, fatigue, and mild respiratory symptoms. Most cases resolve on their own without treatment.

However, for infants, the elderly, and those with weakened immune systems, the flu poses a greater risk, potentially leading to severe complications like pneumonia or even death.

For example, according to World Health Organization data as of December 2020, the H7N9 avian influenza virus (a type of flu virus) has caused 1,568 confirmed human cases globally, with 616 deaths—a mortality rate of nearly 40%^[3].

When is flu season?

Autumn and winter are peak seasons for influenza, with most cases occurring between November and March. This seasonal pattern is why it is called "seasonal flu"^[1].

CAUSES

What causes the flu?

The cause of the flu is infection with the influenza virus^[1].

The influenza virus is highly contagious and spreads easily among people. Whether it's healthy young and middle-aged adults or elderly individuals and children with weaker immune systems, anyone can get infected (medically termed as "universal susceptibility"). However, those who have received the flu vaccine may have a chance to avoid infection or experience milder symptoms if infected.

What is the influenza virus?

The influenza virus, also known as the "flu virus," can be classified into four types: A, B, C, and D.

Currently, the main strains infecting humans are H1N1 and H3N2 subtypes of influenza A virus, as well as the Victoria and Yamagata lineages of influenza B virus^[1].

How is the influenza virus transmitted?

• Droplet transmission: When flu patients or asymptomatic carriers sneeze or cough, they release droplets containing the influenza virus into the air. Contact with these droplets through the eyes, nose, or mouth can lead to infection^[1].
• Indirect contact transmission: Droplets containing the virus can adhere to surfaces (such as doorknobs, railings, tables, or chairs). Touching these surfaces and then touching the mouth, nose, or eyes may also result in infection^[1].
• Aerosol transmission: In specific settings (such as crowded and poorly ventilated rooms), the influenza virus may also spread via aerosols^[1].

If you suspect you have the flu, it's best to self-isolate and wear a mask to avoid infecting others.

How long is a flu patient contagious?

The virus can be shed in respiratory secretions for 3–7 days, while children, immunocompromised individuals, and critically ill patients may shed the virus for more than a week^[2].

Generally, contagiousness lasts from the end of the incubation period through the most severe 3–5 days of symptoms^[2].

When fever subsides without antipyretics for more than 24 hours, it suggests reduced contagiousness^[2].

How can the influenza virus be deactivated?

Heating at 100°C for 1 minute or 56°C for 30 minutes^[1].

Using common disinfectants such as 1% formaldehyde, peracetic acid, or bleach (sodium hypochlorite) for wiping or soaking.

Exposure to ultraviolet light or direct sunlight.

Who is more likely to develop severe flu symptoms?

The following groups are at higher risk of severe illness^[2]:

• Children under 5 years old (especially under 2);
• Adults aged 65 or older;
• Individuals with chronic respiratory diseases, cardiovascular diseases (excluding hypertension), kidney disease, liver disease, blood disorders, neurological or neuromuscular diseases, metabolic or endocrine disorders (e.g., diabetes), cancer, or immunosuppression;
• Obese individuals (BMI > 30 kg/m², where BMI = weight (kg) / height² (m));
• Pregnant women or those within 2 weeks postpartum or after a miscarriage.

DIAGNOSIS

How is influenza generally diagnosed?

Doctors make a diagnosis based on the current influenza epidemic situation, the patient's symptoms, physical examination, and the results of etiological tests.

What are the etiological tests for influenza?

Etiological tests aim to find evidence of the pathogen—the influenza virus.

Test samples typically include respiratory secretions (such as nasal swabs, throat swabs, nasopharyngeal or tracheal aspirates, sputum, etc.) and blood. Common testing methods include:

• Viral antigen detection: The advantage is fast results, aiding doctors in early diagnosis. The downside is low sensitivity, which may lead to missed diagnoses. A positive result supports the diagnosis, but a negative result does not rule out influenza^[2].
• Viral nucleic acid testing: Highly accurate with a low missed diagnosis rate, it can also distinguish virus types and subtypes, making it an ideal method for early confirmation of influenza^[2].
• Viral culture and isolation: Culturing and isolating the influenza virus from respiratory secretions is the gold standard for diagnosis. However, viral culture takes a long time, and waiting for results may delay treatment until the condition has already worsened. Therefore, doctors rarely use this method for rapid diagnosis.
• Serological testing: Detects influenza virus antibodies in the blood. IgM antibodies have low sensitivity and limited diagnostic value. A fourfold or greater increase in IgG antibodies during the recovery phase compared to the acute phase can confirm influenza^[2]. Thus, this test is also unsuitable for rapid early diagnosis.

What additional tests might be needed for severe influenza cases?

Severe cases may require further tests such as complete blood count, blood biochemistry, C-reactive protein, arterial blood gas analysis, myocardial enzymes, electrocardiogram (ECG), cerebrospinal fluid routine and biochemistry, chest X-ray or CT, and brain CT or MRI to assess disease severity and potential complications.

TREATMENT

How to treat influenza?

• Antiviral treatment: The human body has the ability to clear the influenza virus on its own, so even without treatment, influenza can resolve spontaneously. However, for populations at higher risk of developing severe cases (as mentioned in the "Causes" section), antiviral treatment should be initiated as early as possible (within 48 hours of symptom onset) to alleviate symptoms, shorten the course of illness, reduce complications, and lower the risk of death.
• Symptomatic treatment: When symptoms such as fever, headache, body aches, sore throat, nasal congestion, or cough are prominent, symptomatic treatment can be used to relieve discomfort. Under medical guidance, targeted medications such as antipyretics/pain relievers (e.g., acetaminophen, ibuprofen), antihistamines (e.g., chlorpheniramine, diphenhydramine), and decongestants (e.g., pseudoephedrine, ephedrine nasal drops) may be used. Aspirin or aspirin-containing drugs and other salicylate preparations are contraindicated in children^[2].
• Complication treatment: For patients with complications, doctors will provide corresponding treatments, such as respiratory support, antibiotics, sputum expulsion assistance, cardiac load reduction, intracranial pressure reduction, sedation, etc.

What antiviral drugs are available for adults with influenza?

Currently, drugs sensitive to influenza virus approved in China include three types: neuraminidase inhibitors, RNA polymerase inhibitors, and hemagglutinin inhibitors^[5].

• Neuraminidase inhibitors: Common drugs include oseltamivir, zanamivir, and peramivir. Some patients may experience adverse reactions such as nausea, vomiting, and abdominal pain^[2].
• RNA polymerase inhibitors: Such as baloxavir marboxil and favipiravir^[5].
• Hemagglutinin inhibitors: Common drugs like arbidol may cause nausea, diarrhea, dizziness, etc., in some patients^[2].

What antiviral drugs are available for children with influenza?

Currently, antiviral drugs suitable for children include oseltamivir, zanamivir, baloxavir marboxil, and peramivir. For details, refer to pediatric influenza.

Do influenza patients need hospitalization?

Generally not. However, hospitalization is required if any of the following conditions occur^[2]:

• Significant worsening of underlying conditions (e.g., chronic obstructive pulmonary disease, diabetes, chronic heart failure, chronic kidney disease, cirrhosis).
• Persistent high fever > 3 days, accompanied by severe cough, purulent or bloody sputum, or chest pain.
• Rapid breathing, difficulty breathing, or cyanosis.
• Altered mental status (e.g., lethargy, agitation, seizures).
• Severe vomiting or diarrhea with signs of dehydration.
• Pneumonia.
• Respiratory failure.
• Acute necrotizing encephalopathy.
• Septic shock.
• Multiple organ dysfunction.
• Other severe clinical conditions requiring intensive care.

How is influenza treated in pregnant patients?

While large-scale studies on the safety of neuraminidase inhibitors during pregnancy are lacking, the benefits outweigh potential risks. Pregnant women suspected of having influenza should start antiviral treatment early^[4].

Oseltamivir is the first-line treatment for 5 days. If symptoms remain severe, the course may be extended^[4].

For nasal congestion, warmth and humidifiers may help. Severe fever, headache, or body aches require prompt medical attention.

Do breastfeeding mothers need to stop nursing if taking antivirals?

Large-scale safety data are lacking, but oseltamivir appears harmless to infants as it minimally transfers to breast milk^[4]. Breastfeeding mothers requiring antivirals may continue nursing while taking oseltamivir^[4].

Are antibiotics needed for influenza?

No. Since influenza is viral, antibiotics are ineffective^[2]. Unnecessary use may cause side effects or antibiotic resistance. However, bacterial co-infections (e.g., persistent fever, yellow sputum, pus discharge) require antibiotics under medical guidance.

Is antiviral treatment still useful for children after 48 hours of symptoms?

Yes for severe/high-risk cases; limited benefit for mild cases after 48 hours.

Children with severe symptoms, worsening conditions, or complications benefit even after 48 hours. The AAP recommends antivirals for high-risk children (e.g., those under 5) regardless of timing. For mild cases over 5, antivirals beyond 48 hours offer little benefit.

Should a child retake antivirals after vomiting?

No official guidelines exist. General advice:

• If vomiting occurs immediately after taking, redose fully.
• If vomiting within 30 minutes with visible drug residue, redose half/full amount.
• After 30 minutes, assume successful absorption.

Must oseltamivir be taken for 5 full days?

Yes. Complete the 5-day course (twice daily).

Stopping early may lead to relapse due to the drug’s short half-life.

Can baloxavir replace oseltamivir if a child vomits?

Yes, if oseltamivir is intolerable. Switch within 48 hours for optimal effect. For uncertain cases, consult a doctor.

Oseltamivir or baloxavir for pregnant/breastfeeding women?

Oseltamivir is preferred—safe during pregnancy/lactation. Baloxavir is not recommended due to insufficient safety data unless benefits outweigh risks.

DIET & LIFESTYLE

What should flu patients pay attention to in daily life?

For flu patients who do not require hospitalization, the following should be noted during the illness:

• Self-isolate at home, keep the room ventilated, wear a mask, and avoid spreading the flu to others. When going out, wear a mask and avoid crowded or poorly ventilated places.
• Get plenty of rest, drink plenty of water, and eat easily digestible and nutritious food to aid recovery.
• Maintain appropriate indoor temperature and humidity, which can help relieve symptoms such as sore throat, nasal congestion, and cough.
• Practice cough etiquette—cover your mouth and nose with a tissue when coughing or sneezing. If using your hands, wash them promptly.
• Wash hands frequently with soap or hand sanitizer.
• Monitor symptoms closely. Symptoms usually improve within 3–4 days and resolve within 1–2 weeks^[1]. Seek medical attention if symptoms worsen or show no improvement.

Can a mother with the flu continue breastfeeding?

Yes^[4]. However, during the contagious phase, the mother should isolate from the baby. She can use a breast pump to express milk, and a healthy family member can feed the baby with a bottle.

PREVENTION

How to prevent the flu?

The flu can be prevented^[1]. Prevention methods include:

• Get vaccinated against the flu annually.
• Wash hands frequently with soap or hand sanitizer; avoid touching your nose or rubbing your eyes with unwashed hands.
• Ventilate rooms regularly by opening windows.
• Avoid close contact with diagnosed flu patients. If a family member has the flu, the patient should wear a mask, and personal items like towels and cups should be stored separately and disinfected promptly.
• During flu outbreaks, avoid crowded public places and poorly ventilated areas.
• High-risk or exposed individuals may take oseltamivir phosphate for prevention. Additionally, baloxavir marboxil has been approved abroad for prevention in certain cases. (Note: Preventive medications are prescription-only and must be used under medical supervision).

Can the flu vaccine prevent all types of flu?

No.

The flu vaccine targets only specific subtypes of the influenza virus and can prevent only some strains of the flu for that year. It does not protect against the common cold and cannot guarantee 100% prevention of the flu.

However, vaccination is still recommended, as it can reduce the severity of symptoms if infection occurs^[2].

Can pregnant women get the flu vaccine?

Yes^[4], and it is recommended for pregnant women to provide better protection.

Refer to the "Precautions" section of the flu vaccine for details.

When is the best time to get the flu vaccine?

The optimal time for vaccination is 1–2 months before the peak flu season. For residents in China, September to November is the most suitable period^[2]. However, vaccination is still recommended even if the best timing is missed.

Due to regional and climatic differences, local CDC announcements via platforms like WeChat or Weibo may provide more tailored recommendations.

Who should get the flu vaccine?

All individuals aged ≥6 months without contraindications should receive the flu vaccine^[5].

Priority is given to the following high-risk groups^[5]:

• Healthcare workers, including clinical, public health, and quarantine personnel.
• People aged 60 and above.
• Individuals with chronic medical conditions.
• Vulnerable populations and staff in crowded settings like nursing homes and welfare institutions.
• Pregnant women.
• Children aged 6–59 months.
• Family members and caregivers of infants under 6 months.
• People in key settings such as kindergartens, schools, and detention facilities.

Can the flu be prevented with medication?

Yes, but medication cannot replace vaccination^[2]. For high-risk groups mentioned earlier who have not received the current flu vaccine or were vaccinated less than 2 weeks prior (before full immunity develops):

If exposed to a flu patient (from before symptom onset until 24 hours after fever resolves), preventive medication can be taken within 48 hours of exposure^[2]. Options include oseltamivir or zanamivir, with oseltamivir preferred for children. Medical guidance is required.