OVERVIEW

What is influenza A?

Influenza A is a type of flu caused by the influenza virus.

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

Currently, the main strains infecting humans are the H1N1 and H3N2 subtypes of influenza A virus, as well as the Victoria and Yamagata lineages of influenza B virus, commonly referred to as the pathogens of "flu A and flu B."

What is influenza A (H1N1)?

Influenza A (H1N1), also known as human swine flu, is caused by a swine flu virus that has undergone genetic reassortment, incorporating segments of human and avian influenza virus genes.

Its main clinical symptoms are similar to seasonal flu, including fever, cough, headache, muscle/joint pain, runny nose, and fatigue, sometimes accompanied by vomiting and diarrhea. Symptoms range from mild to severe. Mild cases may resolve without treatment, while severe cases can progress to lung infections, respiratory failure, multi-organ failure, or even death.

The primary treatments are antiviral medications and supportive care. Most patients recover fully without lasting complications.

Influenza A (H1N1) is highly contagious, so isolation and personal hygiene are crucial. Good preventive measures not only speed up recovery but also help prevent transmission to others.

What is influenza A (H3N2)?

Influenza A viruses are classified into subtypes based on two surface proteins: hemagglutinin (H) and neuraminidase (N).

There are 18 H subtypes (H1-H18) and 11 N subtypes (N1-N11).

H3N2 is one such subtype, representing a specific combination of hemagglutinin and neuraminidase.

Are there many cases of influenza A?

Influenza A (H1N1) caused a pandemic in 2009. As a novel virus (the H1N1 strain contained genetic segments from swine, avian, and human flu viruses, enabling human-to-human transmission), most people lacked immunity, leading to widespread infections.

Influenza A (H3N2) typically does not cause large outbreaks but is more common, especially in winter. The severity of H3N2 infection is similar to seasonal flu.

Seasonal flu, however, occurs annually. Due to partial immunity in the population, the total number of cases is usually not exceptionally high.

Is influenza A the same as seasonal flu?

No, they are not the same.

Seasonal flu includes multiple types, with influenza A being one of them, primarily occurring in winter.

Which department should I visit for influenza A?

Infectious diseases, respiratory, or infection department.

SYMPTOMS

What are the common symptoms of H1N1 influenza?

The common symptoms of H1N1 influenza are similar to seasonal flu, including fever (may exceed 38°C), cough, headache, muscle and joint pain, sore throat, runny nose, extreme fatigue, and sometimes vomiting and diarrhea. Some patients may experience sudden and rapid deterioration.

Symptoms of H1N1 influenza patients range from mild to severe. Mild cases only involve minor upper respiratory symptoms and can recover without treatment, while severe cases may develop into lung infections, respiratory and multi-organ failure, and even death.

How does H1N1 influenza develop?

H1N1 influenza occurs due to antigenic shift (a major genetic change in the virus) in the new influenza virus, enabling efficient human-to-human transmission. Since humans have little to no immunity against this newly emerged virus, it spreads more rapidly and widely, leading to a "pandemic."

What other diseases can H1N1 influenza cause?

Severe cases may develop lung infections, sepsis, shock, respiratory and multi-organ failure, and even death.

Most patients experience mild symptoms, but those with underlying conditions (e.g., heart disease, asthma, emphysema) or special conditions (e.g., pregnant women) are more likely to progress to severe illness.

CAUSES

Who are the high-risk groups for H1N1 influenza?

The most susceptible groups are young people, pregnant women, healthcare workers, and individuals with underlying medical conditions (such as lung disease, heart disease, asthma, etc.).

In contrast, as age increases, the infection rate of H1N1 influenza gradually decreases.

Are H1N1 influenza patients highly contagious? How long does it last?

H1N1 influenza patients are the source of infection and are highly contagious. Generally, patients can transmit the virus to others from one day before symptoms appear until seven days after onset. If symptoms persist beyond seven days, the contagious period may extend until symptoms disappear.

Patients should only return to school or work after being fever-free for at least 24 hours and must cover their mouth and nose when coughing or sneezing.

How is H1N1 influenza transmitted?

The virus spreads easily from person to person:

• Droplet transmission: When an H1N1 influenza patient coughs or sneezes, virus-containing droplets can land on the mucous membranes of a healthy person's mouth, nose, or eyes, causing infection.
• Indirect contact transmission: Hands that have touched respiratory secretions, bodily fluids of an infected person, or virus-contaminated objects (studies show the flu virus can survive on hard surfaces like doorknobs and tables for 2–8 hours) can spread the virus if they then touch mucous membranes in the mouth, nose, or eyes.

DIAGNOSIS

How is influenza A diagnosed?

Generally, doctors first infer whether it is influenza A based on the patient's clinical symptoms, then quickly conduct a routine RT-PCR test for confirmation. The RT-PCR test has high sensitivity and specificity for detecting influenza A subtypes H1N1 and H3N2.

What special tests are needed for influenza A?

If influenza A is suspected, an RT-PCR test may be required to confirm the type of influenza.

Which diseases can influenza A be easily confused with? How to differentiate?

Influenza A can be easily confused with seasonal flu caused by other pathogens. An RT-PCR test may be necessary to distinguish the type and viral subtype of influenza.

Will you definitely have a fever if you get influenza A? Are there cases without fever?

Not necessarily.

After contracting influenza A, there can also be asymptomatic carriers, so not having a fever is quite normal.

For those with symptoms, fever is the most common symptom, occurring in over 95% of cases. For adults and older children, symptoms usually include sore throat, headache, and muscle pain.

However, for children under 3 years old, some may primarily experience gastrointestinal symptoms, such as nausea, vomiting, or diarrhea.

The peak seasons for influenza and infectious diarrhea roughly overlap from November to March each year. During this period, children presenting with nausea, vomiting, or diarrhea are often initially diagnosed with digestive disorders. Even if they have a fever, it may be attributed to infections like norovirus or rotavirus. However, it’s reasonable to consider influenza as a possibility during this time.

If a child only has gastrointestinal symptoms, they can be closely monitored at home. If symptoms persist, worsen, or a fever develops, seek medical evaluation promptly.

Can influenza A cause acute brain necrosis?

Yes, but it is extremely rare.

Among children with influenza A, 5.7%–18.2% require hospitalization. Among hospitalized cases, approximately 1.7% are diagnosed with encephalopathy. This 1.7% includes all types of encephalopathy, so it is very uncommon.

Acute brain necrosis, medically termed acute necrotizing encephalopathy, is thought to result from an excessive immune response to viral infection, leading to increased vascular permeability in the brain, causing cerebral edema, hemorrhage, and cell necrosis. It mostly occurs in children aged 6–18 months. Early aggressive treatment may lead to full recovery.

Although the probability of severe cases is low, parents may find it difficult to assess at home. If any uncertain symptoms arise, it is advisable to seek medical attention promptly.

TREATMENT

Can H1N1 Influenza (Swine Flu) Resolve on Its Own?

H1N1 influenza is caused by a viral infection. Patients with mild symptoms may recover on their own.

However, pregnant women, children, and individuals with chronic underlying conditions may develop complications leading to severe illness. These individuals should seek medical attention promptly.

How Is H1N1 Influenza Treated?

Treatment for H1N1 influenza varies depending on the patient's condition.

For mild cases, no specific treatment may be necessary. Adequate rest and proper nutrition can lead to self-recovery.

For more severe cases, symptomatic treatment is required, such as:

• Fluid replacement to maintain water, electrolyte, and acid-base balance.
• Nutritional support therapy to replenish calories promptly.
• Fever reduction and pain relief using acetaminophen or ibuprofen.
• Antiemetic treatment for patients experiencing vomiting.
• Antiviral therapy. The primary antiviral drugs include oseltamivir (available as granules or capsules), zanamivir, peramivir, and baloxavir marboxil.

Is Hospitalization Required for H1N1 Influenza?

Mild cases can usually recover at home, but severe cases with complications require hospitalization for antiviral and supportive treatment.

What Are the Common Side Effects of H1N1 Influenza Medications?

Common medications like oseltamivir may cause side effects such as vomiting, nausea, insomnia, headache, and abdominal pain.

How Should H1N1 Influenza Patients Manage Their Condition at Home?

If symptoms are mild:

• Stay home as much as possible—avoid work, school, or crowded places. Rest and drink plenty of fluids.
• Cover your mouth and nose with a tissue when coughing or sneezing, and dispose of used tissues properly.
• Wash hands frequently with soap and running water, or use alcohol-based hand sanitizer.

If symptoms are severe, seek medical care:

• Inform the doctor about your symptoms, mention your suspicion of H1N1 influenza, and explain why (e.g., recent travel to areas with outbreaks).
• Wear a mask to cover your nose and mouth when visiting the hospital.

Can H1N1 Influenza Be Completely Cured?

Most cases of H1N1 influenza can be fully cured without lasting complications.

However, a small number of severe cases may lead to lung infections, sepsis, shock, respiratory or multi-organ failure, and even death.

DIET & LIFESTYLE

What should patients with influenza A pay attention to in their diet?

In terms of diet, eat foods that are easy to digest, rich in protein and fiber. If there is no appetite, do not force yourself to eat. Drink plenty of water and avoid fried or greasy foods.

What should patients with influenza A pay attention to in daily life?

In daily life, the following protective measures should be taken:

• Wash hands frequently with soap and water, or use alcohol-based hand sanitizer. Keep the home environment clean and hygienic.
• Cover your mouth and nose with a tissue when coughing or sneezing, and dispose of used tissues in the trash. If no tissue is available, use your elbow or sleeve.
• Stay home and rest when sick, and avoid going out as much as possible.

How to care for a patient with influenza A?

When caring for a family member with influenza A, the following measures can be taken:

• Both the caregiver and the patient should wear masks and wash hands frequently, preferably with running water or alcohol-based hand sanitizer. If possible, designate a separate space for the patient at home, or maintain a distance of at least one meter. Family members should monitor their own temperature daily and watch for flu-like symptoms.
• Monitor the patient’s symptoms, especially body temperature. If the temperature exceeds 38.5°C and the patient appears lethargic, administer fever-reducing medication.
• Ensure the patient’s clothing is clean and dry. Encourage the patient to rest, drink warm water, and conserve energy.
• Provide emotional support, communicate with the patient, and help alleviate anxiety or tension. Activities like listening to music or watching TV can help.
• Prepare easily digestible foods rich in protein and fiber, and avoid greasy dishes. Do not force the patient to eat if they have no appetite.
• Seek medical attention immediately if severe symptoms occur, such as intense coughing, coughing up blood, chest pain, or difficulty breathing.

Can people get infected with H1N1 influenza by eating pork?

No.

H1N1 influenza, also called human swine flu, is a reassortant virus combining genes from swine, human, and avian influenza viruses. However, it is not transmitted through food. Eating properly handled and cooked pork or pork products will not infect humans with the H1N1 virus.

When should a patient with influenza A seek medical attention?

• Persistent high fever (e.g., above 39°C) for several days without improvement.
• Signs of dehydration, such as no urination or absence of tears when crying.
• Severe coughing with excessive phlegm, coughing lasting more than three weeks, difficulty breathing, or chest pain.
• Ear pain or sudden hearing loss.
• Fluctuating condition with alternating improvement and worsening.

PREVENTION

Can Influenza A be prevented? How to prevent it?

Following these measures can help with prevention:

• Get vaccinated against the flu if possible.
• Avoid close contact with people who appear unwell or have symptoms like fever and cough.
• Wash hands frequently with soap and running water, use alcohol-based hand sanitizers, and maintain clean living environments.
• Maintain good hygiene habits, including adequate sleep, a balanced diet, sufficient fluid intake, and regular exercise.
• Wear masks outdoors. Avoid touching your eyes, nose, or mouth after contacting objects in crowded places to prevent indirect transmission. Wash hands after returning home.
• Ensure proper ventilation in living spaces.

If someone in the household has Influenza A, can children take oseltamivir for prevention?

It depends.

If the child is under 5 years old, at higher risk of flu complications, has not been vaccinated against the flu, or received the vaccine less than 2 weeks ago, oseltamivir is recommended for prevention.

If the child does not fall into the above categories, oseltamivir is not recommended as a preventive measure. Long-term use of medication carries risks of side effects and drug resistance.

Is it too late to vaccinate children and family members during the flu season?

No, and timely vaccination is recommended.

Seasonal flu outbreaks typically occur in winter and spring in both northern and southern regions of China. Vaccination during the peak flu season can still provide effective protection.

Coughing or a runny nose does not affect vaccination—children can receive the vaccine as long as they do not have a fever.

If a child has already contracted Influenza A but has not been vaccinated, vaccination is still recommended after recovery.

The flu vaccine covers 3–4 types of flu viruses and may even offer some cross-protection. Even after having the flu, vaccination can still provide protection against other flu viruses.

How to prevent flu in babies under 6 months who cannot be vaccinated?

First, it is recommended that all family members get vaccinated against the flu.

Vaccinating all family members creates a protective barrier, reducing the risk of transmitting the virus to the baby. Breastfeeding mothers who get vaccinated protect themselves while also passing antibodies to the baby through breast milk. However, it’s important to note that babies should still be vaccinated promptly after turning 6 months old.

Limit taking the baby to crowded places, though open parks are fine.

Parents should wash their hands thoroughly before handling the baby after returning home. If an older child in the household has the flu, try to assign a fixed caregiver. If this isn’t possible, the caregiver must wash their hands before attending to the baby.

Lastly, ensure the baby gets proper nutrition, spends time playing on their tummy, and gets enough sleep to boost immunity.