Influenza Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses. Symptoms include fever, cough, sore throat, muscle aches, fatigue, and more. In severe cases, complications such as pneumonia can arise. Influenza can have more serious consequences for children, the elderly, pregnant women, and individuals with underlying health conditions.

An acute respiratory infectious disease caused by the influenza virus

High fever, headache, fatigue, cough, and body aches

Highly infectious through droplets and contact

Usually self-resolving within 3 to 14 days, but severe cases can be life-threatening

Influenza, commonly known as the flu, is an acute respiratory disease caused by the influenza virus and is classified as a Class C infectious disease.

Influenza is more common in China during the winter and spring seasons. Clinical manifestations mainly include high fever, fatigue, headache, cough, and systemic muscle aches as the main symptoms of systemic intoxication, with relatively mild respiratory symptoms.

The influenza virus is prone to mutations, highly infectious, and the population is generally susceptible, resulting in a high incidence rate. Historically, it has caused several outbreaks worldwide, making it an important global public health issue.

The influenza virus belongs to the Orthomyxoviridae family and is a single-stranded, negative-sense, segmented RNA virus. Based on differences in nucleoprotein and matrix protein, it is divided into four types: A, B, C, and D.

Currently, the main types of influenza virus infecting humans are the H1N1 and H3N2 subtypes of influenza A virus, as well as the Victoria and Yamagata lineages of influenza B virus.

The influenza virus is sensitive to common disinfectants such as ethanol, iodine tincture, and povidone-iodine, and it is sensitive to ultraviolet light and heat. It can be inactivated at 56°C for 30 minutes.

Epidemic Characteristics

Influenza spreads rapidly and can cause seasonal outbreaks every year, particularly in places where people gather such as schools, daycare centers, and nursing homes.

A global study showed an infection rate of 10.7% and an illness rate of 4.4% among adults, with those aged 65 and above at 7.2%.

In China, the annual cycle of influenza varies with latitude and exhibits diverse spatial patterns and seasonal features:

Northern provinces above 33 degrees north latitude experience a winter epidemic pattern with a single annual peak in January to February.

The southernmost provinces below 27 degrees north latitude experience a single annual peak from April to June.

The intermediate latitude regions between these two areas experience dual peaks in January to February and June to August each year.

According to the World Health Organization (WHO), seasonal influenza globally leads to 3 to 5 million severe cases and 290,000 to 650,000 deaths annually.

All children under 5 years old are considered at high risk for severe influenza, with those under 2 years old at the highest risk, and infants under 6 months having the highest hospitalization and mortality rates.

High-risk groups such as pregnant women, infants, the elderly, and those with chronic underlying diseases have a higher risk of severe illness and death after contracting influenza.

Vaccination against influenza is the most effective measure to prevent infection with the influenza virus and its severe complications.

Sources of Infection

Influenza patients and those with hidden infections are the main sources of transmission.

Patients are infectious 1 to 7 days after the onset of illness, with peak infectivity within the first 2 to 3 days.

Modes of Transmission

Influenza primarily spreads through respiratory droplets from sneezing and coughing. The influenza virus can survive in the air for about half an hour. Infection can occur through direct or indirect contact with the mucous membranes of the mouth, nose, and eyes, as well as through contact with virus-contaminated items.

In crowded and poorly ventilated places, influenza may also spread in aerosol form.

Susceptible Population

The general population is generally susceptible to influenza.

Some individuals are at higher risk of developing severe cases after virus infection, including:

Children under 5 years old (those under 2 years old are more prone to severe complications);

Individuals aged 65 and above;

Those with the following diseases or conditions: chronic respiratory diseases, cardiovascular diseases (excluding hypertension), kidney diseases, liver diseases, blood system diseases, neurological and neuromuscular diseases, metabolic and endocrine system diseases, malignant tumors, immunosuppression, etc.;

Obese individuals (BMI > 30);

Pregnant and postpartum women.

Vaccination against influenza can effectively prevent infection with the corresponding subtypes of the influenza virus.

Influenza can be classified into two types based on the virus: type A influenza and type B influenza.

Based on the severity of the illness, it can be categorized as mild influenza, severe influenza, and critical influenza.

When the influenza virus enters the respiratory tract, it binds to specific receptors on the surface of the ciliated columnar epithelial cells of the respiratory tract, enters the cells, replicates within the cells, and continuously releases new viral particles, leading to systemic symptoms such as fever, headache, and muscle pain. It can also cause viral pneumonia, encephalopathy, and other complications.

Environmental Factors

Research from the Chinese Center for Disease Control and Prevention has found that climate variables and latitude are the strongest factors associated with the seasonal characteristics of influenza. Low temperatures predict the occurrence and annual periodic intensity of winter influenza in northern regions, while influenza activity in southern regions during spring is related to rainfall.

Individual Factors

Having underlying chronic diseases, smoking, and immunosuppression due to fatigue and stress can all make individuals more susceptible to influenza.

High-Risk Groups for Severe Influenza

Children under 5 years old (those under 2 years old are more prone to severe complications);

Individuals aged 65 and above;

Those with the following diseases or conditions: chronic respiratory diseases, cardiovascular diseases (excluding hypertension), kidney diseases, liver diseases, blood system diseases, neurological and neuromuscular diseases, metabolic and endocrine system diseases, immunosuppression (including the use of immunosuppressive agents or HIV infection leading to immunosuppression);

Obese individuals (Body Mass Index (BMI) greater than 30, BMI = weight (kg)/height (m)2);

Pregnant and postpartum women.

In general, influenza presents as an acute onset of symptoms. The prodromal phase is characterized by fatigue, followed quickly by high fever (up to 39°C-40°C), chills, shivering, headache, and systemic symptoms such as muscle and joint pain, which are indicative of systemic intoxication. Local symptoms may include nasal congestion, runny nose, sore throat, dry cough, discomfort behind the sternum, facial flushing, and conjunctival congestion, which may or may not be present.

The course of influenza typically lasts 4-7 days, and in a few patients, coughing may persist for several weeks. In children, fever is usually higher than in adults, and symptoms such as nausea, vomiting, and diarrhea are more common when infected with type B influenza.

Mild Influenza

In mild cases, fever is mild to moderate, and both systemic and respiratory symptoms are mild. Self-recovery or complete recovery can occur within 2-3 days.

Influenza Viral Pneumonia

The onset of pneumonia in influenza is similar to typical influenza symptoms initially, but 1-3 days later, the condition rapidly worsens, with high fever, cough, chest pain, and in severe cases, respiratory failure and multi-organ failure, rendering antibiotic treatment ineffective. This type of influenza is more common in the elderly, infants and young children, patients with chronic diseases, and those with compromised immunity. It can lead to respiratory circulatory failure within 5-10 days, posing a life-threatening risk with a high mortality rate.

Meningoencephalitic Type

Patients may experience altered consciousness, headache, vomiting, and signs of meningeal irritation such as neck stiffness.

Myocarditis and Pericarditis Type

The virus may invade the heart muscle or pericardium, leading to symptoms such as chest tightness and chest pain. Laboratory tests may show abnormal cardiac enzymes, abnormal findings on electrocardiogram, and in severe cases, heart failure may occur.

Myositis Type

This occurs only in pediatric patients and is characterized by muscle pain, tenderness, weakness, dark or red urine, elevated levels of serum creatine kinase, and myoglobin, indicating rhabdomyolysis.

Severe or Critical Influenza

In the process of identifying and diagnosing influenza symptoms, it is crucial to diagnose and treat severe or critical influenza cases. A case is considered severe if it meets one of the following criteria:

- Persistent high fever (>3 days) with severe cough, sputum, or chest pain

- Rapid respiratory rate, dyspnea, cyanosis

- Altered consciousness: delayed response, drowsiness, agitation, convulsions, etc.

- Severe vomiting, diarrhea, signs of dehydration

- Complicated by pneumonia

- Significant worsening of pre-existing comorbidities

A case is considered critical if it meets one of the following criteria:

- Respiratory failure

- Acute necrotizing encephalopathy

- Septic shock

- Multi-organ dysfunction

- Presence of other severe clinical conditions requiring intensive care

Doctors typically diagnose influenza based on epidemiological history, patient symptoms, and pathogen test results.

Clinical Diagnosis

A clinical diagnosis is made when there is an epidemiological history (close contact with suspected or confirmed influenza patients without effective personal protection within 7 days prior to onset, or being one of the clustered cases of influenza-like illness, or clear evidence of transmission to others) and typical influenza symptoms, while other diseases with similar presentations have been ruled out.

Confirmed Diagnosis

A confirmed diagnosis of influenza is made when there are clinical manifestations of influenza, and any one or more of the following test results are positive:

- Positive influenza virus nucleic acid test

- Positive influenza antigen test

- Positive influenza virus isolation and culture

- A fourfold or greater increase in specific IgG antibody levels against influenza virus in acute and convalescent phase serum samples.

- Complete Blood Count (CBC): A decrease in total white blood cell count can be observed in the early stages of the illness. Severe cases of influenza virus infection may lead to a decrease in lymphocyte count. In cases of bacterial infection, there may be an increase in total white blood cell count and neutrophils.

- Blood Chemistry Tests: Elevated levels of aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase, and creatinine may be present. Some patients may experience electrolyte imbalances such as hypokalemia. In a few cases, creatine kinase levels may be elevated. In cases of shock, blood lactate levels may be increased.

- Influenza Virology-Related Tests: Laboratory diagnosis of influenza relies on various detection methods including viral nucleic acid testing, viral culture isolation, antigen detection, and serological testing. The first two methods are based on respiratory specimens (nasal, throat swabs, sputum, nasopharyngeal or tracheal aspirates) for virus detection, while the latter two involve immunological examinations to indirectly "capture" traces of the virus.

- Imaging Tests: Routine influenza cases do not require imaging tests and often show no specific manifestations. In cases of concurrent pneumonia, imaging tests may reveal patchy, ground-glass opacities, and multi-segmental infiltrates in the lungs. In pediatric influenza patients with pneumonia, patchy shadows in the lungs may appear early, with a rapid progression and potential complications such as pneumothorax and mediastinal emphysema.

- Common cold: It has weaker infectivity and milder symptoms, primarily characterized by sneezing and runny nose. Virological testing for the influenza pathogen is negative, or evidence of corresponding infectious pathogens may be found.

- Other upper respiratory infections: Including acute pharyngitis, tonsillitis, rhinitis, and sinusitis. Infections and symptoms are mainly limited to the respective sites, and virological testing for the influenza pathogen is negative.

- Other lower respiratory infections: When influenza presents with cough or concurrent tracheobronchitis, it needs to be differentiated from acute tracheobronchitis. In cases of concurrent pneumonia, differentiation from other types of pneumonia including bacterial, chlamydial, mycoplasmal, viral, fungal, and tuberculosis pneumonia is necessary. Preliminary judgments can be made based on clinical features, and pathogen testing can confirm the diagnosis.

- COVID-19: Mild and moderate COVID-19 can manifest with symptoms similar to influenza, including fever, dry cough, and sore throat. Severe and critical cases of COVID-19 exhibit symptoms similar to severe and critical influenza. Epidemiological history and pathogen testing are necessary for differentiation.

Isolation: Patients with clinical diagnoses and confirmed cases should be isolated and treated as early as possible. Patients with mild symptoms can self-isolate at home and avoid close contact with others.

Rest and diet: Maintain good room ventilation, get plenty of rest, drink plenty of water, consume easily digestible and nutritious foods, and pay attention to nasal, throat, and oral hygiene.

Temperature reduction: Physically reduce high fever if present.

Close monitoring of the development of severe influenza: Seek medical attention promptly if severe symptoms such as persistent high fever, severe cough, difficulty breathing, confusion, severe vomiting, and diarrhea occur. Pregnant women, children, the elderly, and those with chronic illnesses are more likely to develop severe influenza and should seek medical attention early.

Due to significant individual differences, there is no absolute "best," "fastest," or "most effective" medication. Apart from common over-the-counter drugs, medication should be chosen under the guidance of a physician based on individual circumstances.

Symptomatic treatment:

Patients with high fever can use antipyretic drugs. It should be noted that children should not use aspirin or other salicylic acid preparations.

Patients with severe cough and phlegm should be given antitussive and expectorant medications.

Appropriate oxygen therapy should be administered according to the degree of hypoxia.

Etiological treatment:

Etiological treatment for influenza involves antiviral therapy. It should be noted that influenza patients should avoid the blind use of antibacterial drugs (antibiotics) and should use them strictly under the guidance of a physician.

Patients with severe symptoms or high-risk factors for severe influenza should be given antiviral treatment as early as possible, without waiting for virus test results:

The best effect of antiviral drugs is achieved when used within 48 hours of onset.

Early virus treatment can reduce complications, lower mortality rates, and shorten hospital stays.

Severely ill patients who have been ill for more than 48 hours can still benefit from antiviral treatment.

Patients with non-severe symptoms and no high-risk factors for severe influenza should, within 48 hours of onset, consider antiviral treatment based on a risk-benefit assessment by a physician.

Neuraminidase inhibitors such as oseltamivir, zanamivir, and peramivir are effective treatments for type A and type B influenza. Early use, especially within 48 hours of onset, significantly reduces the occurrence of severe influenza and death.

It should be noted that antiviral drugs should be used under the guidance of a physician.

Oseltamivir (capsules/granules):

Adult dosage is 75mg per dose, twice daily.

For children aged 1 year and above, the dosage should be adjusted based on body weight:

For those weighing less than 15kg, 30mg per dose, twice daily;

For those weighing 15-23kg, 45mg per dose, twice daily;

For those weighing 23-40kg, 60mg per dose, twice daily;

For those weighing over 40kg, 75mg per dose, twice daily.

The course of treatment is 5 days, and it may be extended for severe cases. The dosage should be adjusted based on renal function for those with renal insufficiency.

Zanamivir (inhalation spray):

Suitable for adults and adolescents aged 7 and above.

Dosage: 10mg per dose, twice daily, with a 12-hour interval, for a course of 5 days.

Inhalants are not recommended for patients with pre-existing asthma or other chronic respiratory diseases.

Peramivir (intravenous administration):

The adult dosage is 300-600mg. For infants under 30 days old, the dosage is 6mg/kg; for infants aged 31-90 days, the dosage is 8mg/kg; for children aged 91 days to 17 years, the dosage is 10mg/kg. It is administered intravenously once daily for 1-5 days, and the course of treatment may be extended for severe cases.

Other:

Coagulation inhibitors such as abidol can be used for the treatment of type A and type B influenza in adults, but due to limited usage data, the efficacy and adverse reactions should be closely monitored.

M2 ion channel blockers such as amantadine and rimantadine are resistant to influenza virus according to current monitoring data, and their use is not recommended.

It is recommended that patients seek TCM treatment in regular medical institutions under the guidance of a physician and receive TCM treatment based on differentiation of symptoms.

The main TCM treatment plans for mild cases are as follows:

Wind-Heat Attacking the Defensive Qi type:

Main symptoms: In the early stages of the disease, fever or no fever, discomfort in the throat, mild cough with little phlegm, and no sweating.

Common TCM drugs: Wind-dispelling and heat-clearing drugs, such as Jinhua Qinggan granules, Lianhua Qingwen capsules (granules), Qingkailing granules (capsules, soft capsules, tablets), Shufeng Jiedu capsules, and Yinqiao Jiedu capsules, etc.

Children can use children's antiviral granules, Xia'er Chiqingre granules, etc.

Wind-Cold Constricting the Surface type:

Main symptoms: In the early stages of the disease, aversion to cold, fever or no fever, no sweating, body aches, headache, and clear nasal discharge.

Common TCM drugs: Jiawei Qianghuo pills (granules), Zhengchaihu decoction granules, and Antigripal granules (capsules), etc.

Surface Cold and Internal Heat type:

Main symptoms: Aversion to cold, high fever, headache, body aches, sore throat, nasal congestion, runny nose, and thirst.

Common TCM drugs: Lianhua Qingwen capsules, Jinhua Qinggan granules, etc.

Heat Toxins Attacking the Lungs type:

Main symptoms: High fever, coughing, sticky phlegm, yellow phlegm, unsatisfactory expectoration, thirst, sore throat, and red eyes.

Common TCM drugs: Heat-clearing and detoxifying drugs, lung-clearing and cough-suppressing drugs, such as Lianhua Qingwen capsules (granules), Jinhua Qinggan granules, Shufeng Jiedu capsules, and Yinhuang preparations, etc.

Children can use Xia'er Feire Kechuan granules (oral liquid), etc.

Other considerations:

For severe influenza patients and those in the recovery period, TCM treatment plans can also be provided under the guidance of a physician.

During the use of TCM drugs, pregnant women should refer to the adult treatment plan, but should avoid the use of contraindicated drugs during pregnancy, treat the disease while protecting the fetus to prevent miscarriage, and pay attention to the dosage.

Children's medication can refer to the adult treatment plan and adjust the dosage according to pediatric regulations. TCM drugs without pediatric indications should not be used.

If there is hypoxemia or respiratory failure, appropriate treatment measures should be given in a timely manner, including oxygen therapy or mechanical ventilation;

Appropriate shock treatment should be given when combined with shock;

When there is damage to other organ functions, appropriate supportive treatment should be given;

If there is secondary infection, appropriate anti-infective treatment should be given.

Pneumonia

If the influenza virus invades the lower respiratory tract, it can cause primary viral pneumonia.

Some critically ill patients may also have infections with bacteria, fungi, and other pathogens, and severe cases may develop acute respiratory distress syndrome (ARDS).

Nervous system damage

Including meningitis, encephalitis, myelitis, Guillain-Barré syndrome, etc.

Cardiac injury

Including myocarditis, pericarditis, and severe cases may develop heart failure.

In addition, infection with the influenza virus increases the risk of hospitalization and death related to myocardial infarction and ischemic cardiomyopathy.

Other

Myositis, rhabdomyolysis: may present as muscle pain, weakness, etc., and severe cases may lead to acute kidney injury.

Shock.

Children with influenza may develop complications such as pharyngitis, otitis media, and bronchitis, which are more common than in adults.

Maintaining good personal hygiene habits is an important measure for preventing respiratory infectious diseases such as influenza.

Enhancing physical fitness and immunity;

Frequent handwashing;

Maintaining a clean and ventilated environment;

Minimizing activities in crowded places and avoiding contact with respiratory infection patients;

When coughing or sneezing, cover the mouth and nose with the upper arm or a tissue, and wash hands after coughing or sneezing, while avoiding touching the eyes, nose, or mouth as much as possible;

If experiencing respiratory infection symptoms, stay home and seek medical attention early.

Vaccination

Annual influenza vaccination is the most effective way to prevent influenza, significantly reducing the risk of influenza and severe complications for the vaccinated.

It is recommended that priority for influenza vaccination be given to individuals aged 60 and above, children aged 6 months to 5 years, pregnant women, family members and caregivers of children under 6 months, individuals with chronic illnesses, and healthcare workers.

Influenza vaccines come in two types: trivalent and quadrivalent. The trivalent vaccine contains one lineage each of influenza A(H3N2), A(H1N1), and influenza B viruses, while the quadrivalent vaccine contains two influenza B lineages, Victoria and Yamagata, in addition to the A(H3N2) and A(H1N1) viruses.

In China, the approved influenza vaccines include trivalent inactivated influenza vaccine and quadrivalent inactivated influenza vaccine. It is important to note that there are currently no approved inactivated influenza vaccines for use in infants under 6 months of age.

Antiviral Prophylaxis

Antiviral prophylaxis cannot replace vaccination.

For severely ill individuals at high risk for severe influenza who have not been vaccinated or have not yet acquired immunity after vaccination, antiviral prophylaxis with drugs such as oseltamivir and zanamivir may be considered as an emergency temporary preventive measure.

General Preventive Measures

Maintain good personal hygiene habits: frequent handwashing, keeping living and working environments clean and well-ventilated, and reducing visits to crowded places during the influenza season;

Maintain good respiratory hygiene habits: when sneezing or coughing, cover the mouth and nose with the upper arm or a tissue, and wash hands afterward;

When experiencing influenza-like symptoms, voluntarily self-isolate and wear a mask when going to public places.