Influenza: More Dangerous Than You Think – What Makes Each Strain Different?

Influenza may sound less threatening than COVID-19, but in reality, it can be more serious than expected. During the rainy season of 2023 in Thailand, flu cases increased significantly compared to the past three years. The previous drop in cases was largely due to the global COVID-19 pandemic, which led people to stay home and practice heightened hygiene, reducing opportunities for influenza transmission.

Additionally, fewer people received flu vaccinations during the pandemic, which led to lower immunity among the population—even though the virus continued circulating at a low level. As COVID-19 subsided and society returned to normal, flu cases began to rise again. According to the Department of Disease Control, more than 200,000 cases of influenza were reported in 2023 in Thailand, with no sign of decline due to the lack of widespread immunity.

How Is Influenza Different from the Common Cold?

Flu symptoms are typically more severe and longer-lasting than those of the common cold. A flu patient may develop a sudden high fever ranging from 39–40°C, along with intense fatigue and muscle aches. Influenza outbreaks are most common during the rainy and cold seasons.

In contrast, the common cold can occur year-round. Its symptoms are usually milder and can be managed with rest, fluids, and over-the-counter medications. However, those experiencing flu symptoms should see a doctor for antiviral treatment.

Characteristics of Influenza Type A

1. Mutation and Transmission

• Mutation: Influenza A has a high mutation rate, especially in the Hemagglutinin (HA) and Neuraminidase (NA) proteins. These mutations allow the virus to escape immunity developed from vaccines or previous infections and may lead to pandemics.
• Transmission: Type A can infect both humans and animals (e.g., birds and pigs). It spreads through respiratory droplets (coughs, sneezes), and via contaminated surfaces, entering the body through the nose or eyes.

2. Severity of Outbreaks

Influenza A is known for its high potential for widespread outbreaks. For example, the H1N1 "swine flu" pandemic in 2009 was caused by a strain of influenza A.

3. At-Risk Populations

Because it can infect multiple species and mutate rapidly, influenza A can lead to severe complications such as pneumonia, and poses a higher risk of cross-species transmission and new strain emergence.

Characteristics of Influenza Type B

• Mutation and Transmission: Influenza B mutates less frequently, resulting in more stable strains and vaccine effectiveness. It spreads only among humans and tends to cause localized outbreaks.
• Severity of Outbreaks: Infections can occur year-round, with peaks typically in winter. While outbreaks can affect regions or provinces, they tend to be less severe and have lower pandemic potential compared to Type A.
• At-Risk Populations: Type B infects only humans, with no known animal reservoir, and generally results in milder disease.

Summary: Which Strain Is More Dangerous?

Characteristics of Influenza

Influenza is an acute viral infection that affects the respiratory tract. Clinically, it is characterized by a sudden high fever, headache, muscle aches, and fatigue. It is one of the most significant emerging and re-emerging infectious diseases due to its history of causing global pandemics that have affected almost every continent, resulting in millions of cases and deaths.

Causes of Influenza

Influenza is caused by influenza viruses, which are divided into three types: A, B, and C.

• Type A causes widespread global outbreaks.
• Type B causes regional epidemics.
• Type C typically causes mild or asymptomatic infections and does not lead to epidemics.

Subtypes of Influenza A

Influenza A viruses are further classified by differences in two surface proteins: hemagglutinin (H) and neuraminidase (N). Common subtypes that infect humans include A(H1N1), A(H1N2), A(H3N2), A(H5N1), and A(H9N2). Influenza B viruses are not divided into subtypes.

Genetic Variation

Influenza viruses have segmented RNA genomes (7–8 segments), which makes genetic mutation relatively common. This variation leads to two main types of antigenic changes:

• Antigenic Drift: Small mutations in the RNA cause minor changes in the H or N proteins, resulting in local outbreaks.
• Antigenic Shift: A major genetic reassortment occurs when two different influenza A viruses infect the same cell, producing a new virus subtype that can lead to pandemics.

To date, 15 types of hemagglutinin and 9 types of neuraminidase have been identified, though only H1N1 and H3N2 commonly infect humans. Due to frequent antigenic changes, new flu strains can emerge across different regions and times.

A standardized global naming system is used for influenza viruses, formatted as: Virus Type / Geographic Origin / Strain Number / Year of Isolation (Subtype). Example: A/Sydney/5/97(H3N2).

Avian Influenza Origins

Ecological studies show that mammalian influenza viruses likely originated from avian influenza viruses. Wild aquatic birds, such as ducks, act as reservoirs and can excrete large quantities of the virus without symptoms. The 1997 H5N1 outbreak in Hong Kong demonstrated transmission from shorebirds to ducks, then to chickens, and eventually to humans via fecal contamination. Reassortment of influenza viruses between species can lead to new subtypes capable of infecting humans.

Transmission of Influenza

Influenza spreads primarily through respiratory droplets released when infected individuals cough, sneeze, or talk. Crowded areas such as schools and factories facilitate transmission. It can also spread via contact with contaminated surfaces and touching the nose or mouth afterward.

Symptoms of Influenza

Symptoms typically appear 1–4 days after exposure and include:

• Sudden high fever
• Headache
• Chills
• Severe muscle aches and fatigue
• Nasal congestion and sore throat

Prolonged illness may result in post-viral bronchitis. Flu symptoms are more intense and last longer than those of the common cold. While most patients recover in 1–2 weeks, complications such as pneumonia can occur and may be fatal.

High-Risk Groups

• Pregnant women
• Children aged 6 months to 2 yearsI
• ndividuals aged 65 and older
  
• Patients with chronic conditions (e.g., COPD, asthma, heart disease, kidney failure, stroke, diabetes, thalassemia, cancer, neurological disabilities, HIV)
• Individuals with BMI > 35 or weight over 100 kg

Elderly individuals are particularly at risk of severe complications such as pneumonia, heart failure, acute myocardial infarction, and stroke.

Influenza vs. Common Cold

Duration of Illness

Symptoms appear 1–4 days post-exposure. Patients can transmit the virus starting 1 day before symptoms begin and remain contagious for 3–5 days after symptom onset. In children, this period may extend beyond 7 days. Asymptomatic individuals can still spread the virus.

Diagnosis

Clinical symptoms alone cannot reliably differentiate influenza from other respiratory infections. Lab tests may include:

• Detection of virus in nasal/throat swabs
• Antigen detection using fluorescent antibody techniques
• A fourfold increase in serum antibody levels using HI, CF, or ELISA tests

Treatment

Antiviral drugs such as Amantadine Hydrochloride or Rimantadine Hydrochloride are most effective when started within 48 hours of symptom onset and taken for 3–5 days.

• Children aged 1–9: 5 mg/kg/day in 2 doses
• Age 9 and up: 100 mg twice daily
• Adjust dose for elderly or patients with liver/kidney dysfunction

Patients with bacterial complications may require antibiotics. Avoid salicylates (e.g., aspirin) in children to prevent Reye’s syndrome.

Influenza Prevention

• Annual flu vaccination reduces disease severity and hospitalizations
• Flu and COVID-19 have similar symptoms, and dual vaccination is possible
• Those with egg allergies can safely receive the flu vaccine

Infection Control Measures

Since vaccines do not guarantee full protection, good hygiene remains essential:

• Wash hands frequently with soap or alcohol gel
• Avoid touching face (eyes, nose, mouth)
• Cover coughs/sneezes with tissue or elbow and wash hands afterward
• Disinfect frequently touched surfaces (phones, doorknobs)
• Avoid crowded places during flu season

Home Care Recommendations

• Drink plenty of fluids (water, juice, warm soup)
• Rest to support immune function
• Take over-the-counter pain relievers (avoid aspirin in children)

Why Get the Flu Vaccine Every Year?

Influenza viruses mutate frequently, and each year’s vaccine is updated to match circulating strains. Immunity from the vaccine typically lasts about one year, making annual vaccination necessary to ensure continued protection.

Flu vaccine effectiveness ranges from 70% to 90%, although response may be lower in elderly or immunocompromised individuals. Even in these cases, vaccination helps reduce the risk of severe complications, hospitalization, and death.

Flu shots can be administered year-round, but the best time is before the peak season. In Thailand, the flu season typically occurs:

• Rainy Season: May – October
• Cold Season: January – March