Influenza – Causes, Symptoms, Treatments

What is Influenza?

Influenza, also commonly known as the flu, is a respiratory illness caused by influenza viruses. These viruses infect the respiratory tract, which includes the nose, throat, and lungs. 

Influenza is most prevalent during the flu season, which typically occurs in the winter months in temperate regions. While influenza viruses can circulate year-round, colder weather creates conditions that favor its spread.

The exact timing and duration of flu seasons can vary. Flu activity often begins to increase in October and most of the time peaks between December and February. In some seasons, peak activity can occur in March or even as late as May. Influenza tends to peak in temperate regions during their winter months. So, for example:

• Northern Hemisphere: Flu season typically peaks between December and February.
• Southern Hemisphere: Flu season typically peaks between June and September.

Influenza infects between 5-15% of the global population annually during normal seasonal epidemics, and the World Health Organization (WHO) estimates that these infections result in 3 to 5 million cases of severe illness and about 290,000 to 650,000 respiratory deaths every year.

Types of Influenza Viruses

There are four main types of influenza viruses: A, B, C, and D. However, only two, influenza A and B, are responsible for causing seasonal flu epidemics in humans.

Influenza A viruses have the most variety. They can be further classified into subtypes based on two proteins on their surface: hemagglutinin (H) and neuraminidase (N). These subtypes are identified by an “H” and an “N” followed by a number, for instance H1N1 or H3N2. Due to this variation, influenza A viruses are also responsible for causing pandemics, which are global outbreaks of a new influenza virus strain.

Influenza B viruses are less varied than influenza A and are not classified into subtypes. However, there can be different lineages of influenza B viruses that circulate and cause illness.  In contrast to influenza A and B, influenza C typically causes only mild illness and is not thought to be a major contributor to seasonal flu epidemics. Finally, influenza D viruses primarily infect cattle and are not known to cause illness in humans.

Severity of Illness:

• Influenza A and B: These types are responsible for seasonal flu epidemics and can cause a range of illness severity, from mild to severe. In some cases, they can lead to complications like pneumonia, especially in vulnerable populations like young children, older adults, and those with chronic health conditions.
• Influenza C: This type generally causes milder illness compared to A and B, with symptoms that may resemble a common cold.
• Influenza D: This type doesn’t cause illness in humans.

Potential for Pandemics:

• Influenza A: Due to its ability to mutate and form new subtypes, influenza A has the potential to cause pandemics. Pandemics occur when a new influenza A virus strain emerges and spreads easily between people, often causing significant illness and death.
• Influenza B: This type doesn’t have the same pandemic potential as influenza A because of its lower variation.
• Influenza C and D: These types are not known to cause pandemics in humans.

Regular seasonal influenza epidemics are caused by influenza A viruses, but they’re usually strains that have circulated before, allowing some people to have some immunity. For a pandemic to occur, a completely new influenza A subtype needs to emerge. This often happens when influenza A viruses of different subtypes (e.g., avian and human) swap genetic material, creating a novel virus easily transmissible between humans.

The new influenza A strain not only needs to be novel but also highly contagious. This means it can be transmitted efficiently through respiratory droplets from person to person, causing widespread outbreaks.

How is the Influenza Virus Transmitted?

Influenza viruses primarily spread from person to person through respiratory droplets produced by an infected individual. Here’s a breakdown of the main pathway:

• Coughing and Sneezing: When an infected person coughs or sneezes, they expel respiratory droplets containing the virus. These droplets can travel short distances (up to six feet) through the air and can be inhaled by others nearby.

Secondary transmission pathway also exists:

• Contact with contaminated surfaces: Respiratory droplets can also land on surfaces like doorknobs, phones, or countertops. If a healthy person touches a contaminated surface and then touches their mouth, nose, or eyes, they can become infected. This is why frequent handwashing is crucial to prevent the spread of influenza.

Here’s a comparison of the two main pathways:

• Respiratory Droplets: This is the primary mode of transmission. The droplets are larger and heavier, so they travel a shorter distance and don’t stay airborne for long. However, they are highly contagious if inhaled directly.
• Contaminated Surfaces: This is a less common pathway, but it’s still important to consider. The virus can survive on surfaces for a limited time (usually a few hours), but proper hand hygiene can effectively prevent transmission through this route.

Additional factors to consider:

• Infectious period: The infectious period refers to the timeframe during which an infected person can transmit the virus to others. This can begin a day before symptoms even develop and last up to a week, sometimes longer in young children.
• Viral shedding: The amount of virus an infected person sheds can influence transmission. People with higher viral shedding are more likely to spread the virus to others.

Symptoms of Influenza

Influenza, despite sharing the common name “flu” with various respiratory illnesses, has its own characteristic set of symptoms. These symptoms typically appear rather abruptly, and while the exact experience can differ from person to person, there’s a recognizable pattern.

The hallmark feature of influenza is often a fever or a feeling of being feverish, accompanied by chills. This fever often spikes within the first few days of catching the flu, and you might experience chills or sweats along with it. Another telltale sign is a cough, usually dry and hacking, that can be quite irritating and persistent. A sore throat, scratchy and uncomfortable, is another frequent complaint.

Just like the common cold, influenza can cause congestion and a runny nose. Additionally, muscle aches and general body aches are a classic feature, often described as feeling extremely run down or achy. Headaches are another frequent symptom, often dull and throbbing, along with fatigue that leaves you feeling exhausted and lacking in energy. Some people might also experience a loss of appetite.

It’s important to note that while these symptoms are common across most people, some groups may experience additional ones. Young children, particularly those under 5, might also experience vomiting and diarrhea alongside the usual flu symptoms. These digestive problems are less frequent in adults. On the other hand, older adults, especially those with underlying health conditions, may experience confusion, dizziness, and shortness of breath on top of the typical flu symptoms.

Complications of Influenza

Influenza infection can lead to a range of outcomes, from mild illness that resolves on its own to severe complications requiring hospitalization. 

For most healthy adults, influenza results in a relatively mild illness. People typically experience the core symptoms like fever, cough, fatigue, and body aches for a few days to a week. With proper rest, fluids, and over-the-counter medications, the body’s immune system usually fights off the virus, and a full recovery is expected.

However, in some cases, influenza infection can lead to more serious complications, especially for certain high-risk groups. These complications can affect the respiratory system, ears, sinuses, or even the heart.

• Pneumonia: This is a serious infection that inflames the lungs. It can be caused by the influenza virus itself or by bacteria that take advantage of the weakened immune system during influenza infection. Pneumonia can be life-threatening, especially in young children, older adults, and people with chronic health conditions.
• Bronchitis: Inflammation of the airways in the lungs can occur due to influenza infection. This can cause a persistent cough with mucus production.
• Sinusitis: Inflammation of the sinuses, the hollow spaces in the face around the nose, can be a complication of influenza. Symptoms include facial pain, congestion, and headache.
• Ear infections: Middle ear infections (otitis media) can sometimes occur, especially in young children, following influenza infection.
• Myocarditis and pericarditis: In rare cases, influenza can affect the heart muscle (myocarditis) or the sac surrounding the heart (pericarditis).

Groups at High Risk of Complications:

Certain groups of people are at higher risk of experiencing severe illness or complications from influenza infection. These include:

• Young children: Infants and young children (under 5 years old) have immature immune systems and are more vulnerable to severe influenza illness.
• Older adults: Adults aged 65 and older are more likely to have weakened immune systems and chronic health conditions that put them at higher risk of complications.
• Pregnant women: Pregnant women are at increased risk of severe illness from influenza and may also experience complications like premature birth.
• People with chronic health conditions: Individuals with chronic health conditions like heart disease, lung disease, diabetes, or weakened immune systems due to HIV/AIDS or medications are more susceptible to severe influenza complications.

How is Influenza Diagnosed?

Diagnosing influenza can sometimes be tricky because its symptoms mimic those of other respiratory illnesses like the common cold. However, healthcare professionals have a few tools at their disposal to determine if you have the flu:

1. Clinical Evaluation: Doctors will typically begin by asking you about your symptoms, their severity, and how long you’ve been feeling unwell. They will also inquire about your medical history and any risk factors you might have for influenza complications.
2. Rapid Influenza Diagnostic Tests (RIDTs): These are point-of-care tests that can provide a quick answer (often within 10-15 minutes) on whether you have influenza A or B. Rapid Influenza Diagnostic Tests (RIDTs) work by detecting influenza antigens, the proteins on the surface of the virus, in a respiratory sample (usually a nasal swab). While convenient and fast, RIDTs may not always be accurate, particularly early in the course of illness.
3. Molecular Assays: These tests are more accurate than RIDTs and can also differentiate between influenza A and B strains. Molecular assays detect the influenza virus’s genetic material (RNA) in a respiratory sample. However, they typically take longer to produce results (several hours) and may not be available in all healthcare settings.
4. Viral Culture: This is the most definitive test for diagnosing influenza but also the slowest. It involves collecting a respiratory sample and growing the virus in a laboratory culture. This process can take several days. While viral culture is rarely used for routine diagnosis due to its time constraints, it can be valuable for public health purposes and confirming unusual influenza strains.

The Importance of Vaccination

The most effective method for preventing influenza and its associated complications on an annual basis is vaccination with a flu shot. 

The influenza vaccine targets the influenza A and B strains that are predicted to be most prevalent in the upcoming flu season. Vaccination stimulates your immune system to develop antibodies against these specific strains. If you encounter these influenza viruses, your immune system is primed to fight them off, preventing you from getting sick or significantly reducing the severity of your illness.

How the Vaccine is Adjusted Each Year:

The influenza virus is constantly evolving, and new strains emerge each year. To keep pace with this viral variation, the formulation of the flu vaccine is updated annually based on recommendations from the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC). Here’s a breakdown of the process:

1. Global Influenza Surveillance: A worldwide network of scientists continuously monitors influenza viruses circulating in different regions. They collect data on the viral strains, their characteristics, and their prevalence.
2. Strain Selection for the Vaccine: Twice a year, WHO convenes expert committees to analyze the global influenza surveillance data. Based on this analysis, they recommend the specific influenza A and B virus strains that should be included in the upcoming season’s flu vaccine.
3. Vaccine Development: Vaccine manufacturers then develop vaccines that target the recommended influenza virus strains. This process typically takes several months.

Importance of Annual Vaccination:

Since influenza viruses evolve, immunity from vaccination or prior infection wanes over time.  Additionally, the circulating influenza strains can change from year to year. That’s why getting a flu shot every year is essential for optimal protection.

Benefits of Vaccination:

• Reduces illness severity: Even if you do get the flu after vaccination, the illness is likely to be milder, with shorter duration and fewer complications.
• Protects high-risk groups: Vaccination is especially important for people at high risk of complications from influenza, such as young children, older adults, pregnant women, and those with chronic health conditions.
• Reduces hospitalization rates: Vaccination can significantly decrease the need for hospitalization due to influenza complications.
• Herd immunity: High vaccination rates in a community can help protect those who cannot be vaccinated due to medical reasons, creating herd immunity that indirectly protects vulnerable populations.

How is Influenza Treated?

Unlike some bacterial infections treated with antibiotics, influenza is a viral infection. Antibiotics won’t help against influenza. However, there are treatment options available to manage influenza symptoms and potentially shorten the illness. Here’s a breakdown of the treatment approaches for influenza:

1. Home Care and Supportive Measures:
   • Rest: Getting plenty of rest allows your body to focus its energy on fighting the virus.
   • Fluids: Staying hydrated is crucial to prevent dehydration, especially if you have a fever or experience vomiting and diarrhea. Drink plenty of fluids like water, broth, or clear juices.
   • Over-the-Counter Medications: Medications like acetaminophen or ibuprofen can help manage fever, aches, and pains.
   • Comfort Measures: A humidifier can help soothe a cough and congestion. Saline nasal sprays can also help clear congestion. Getting enough sleep and gargling with warm salt water can also provide some relief.
2. Antiviral Medications: These prescription medications are the only treatments that specifically target the influenza virus. They work by interfering with the virus’s ability to replicate and spread in your body. Antiviral medications are most effective when initiated within 48 hours of symptom onset. Here’s a breakdown of some antiviral medications used for influenza:
   • Neuraminidase inhibitors: These medications (e.g., oseltamivir [Tamiflu], zanamivir [Relenza]) block the enzyme neuraminidase, which the influenza virus needs to escape from infected cells and spread to new ones.
   • Cap-dependent endonuclease inhibitors: This newer class of antiviral medications (e.g., baloxavir marboxil [Xofluza]) disrupts the virus’s ability to copy its genetic material.

Benefits of Antiviral Medications:

• Reduced illness severity and duration: Antiviral medications can help lessen the severity of symptoms and shorten the duration of your illness by a day or two.
• Lower risk of complications: Early treatment with antivirals can potentially reduce your risk of developing complications like pneumonia, bronchitis, and sinus infections.
• Potentially reduced hospitalization: In some cases, antiviral medications can help prevent hospitalization, especially for high-risk individuals.

What Medications are Most Often Prescribed to Treat the Influenza Virus?

Unlike some bacterial infections treated with antibiotics, influenza is a viral infection, and antibiotics won’t help against it. However, there’s a specific class of medications designed to combat influenza viruses: antiviral medications. These are prescription drugs and the most often prescribed ones fall into two main categories:

1. Neuraminidase Inhibitors:
   1. This category represents the mainstay of influenza antiviral treatment and includes medications like oseltamivir (Tamiflu) and zanamivir (Relenza).
   2. Their mechanism of action targets an enzyme called neuraminidase. This enzyme is crucial for the influenza virus because it helps new virus particles exit infected cells and spread to healthy cells.
   3. By blocking neuraminidase, neuraminidase inhibitors essentially hinder the virus’s ability to replicate and spread within your body.
2. Cap-dependent Endonuclease Inhibitors:
   1. This is a newer class of antiviral medications with baloxavir marboxil (Xofluza) being a prominent example.
   2. Unlike neuraminidase inhibitors, these medications target a different part of the viral replication process. They disrupt the virus’s ability to copy its genetic material (RNA), hindering its ability to reproduce and spread.