Anyone who has ever been confined to bed with a high fever, a severe headache, difficulty moving, and body aches all over may find themselves wondering whether these symptoms could have been prevented by getting vaccinated earlier in the year.
Influenza is not simply a more severe version of the common cold. It is an illness that can force patients to take several days off to recover. In some cases, symptoms can become serious enough to require hospitalization, and for people in high-risk groups, the consequences may be far more severe than many realize.
This article will help readers understand influenza from the beginning, including its causes, how it differs from the common cold, how the vaccine works, and why receiving an influenza vaccination once a year remains important.
What Is Influenza, and How Is It Different from the Common Cold?
What Is Influenza, and What Causes It?
Influenza is a respiratory tract infection caused by the influenza virus. There are several types and strains of the virus, and they continually change from year to year.
The virus spreads easily through respiratory droplets released when an infected person coughs, sneezes, or talks. It can also be transmitted by touching contaminated surfaces and then touching the eyes, nose, or mouth. As a result, influenza can spread rapidly in crowded environments such as offices, schools, shopping malls, and public transportation.
In Thailand, influenza outbreaks commonly occur during the rainy season (approximately June to October) as well as in the early months of the year, reflecting the seasonal pattern of influenza transmission in tropical climates.
Differences Between Influenza and the Common Cold
Many people confuse influenza with the common cold because their names are similar. However, the two illnesses differ significantly in terms of severity, how quickly symptoms develop, and their overall impact on the body.
The common cold is usually caused by a variety of viruses, most commonly rhinoviruses. Symptoms typically develop gradually and include nasal congestion, a runny nose, sneezing, and a mild sore throat. Most people do not experience a high fever, and the illness generally resolves on its own within 7–10 days. In many cases, patients are still able to carry out their daily activities with only mild discomfort.
In contrast, influenza usually develops suddenly, with symptoms appearing within just a few hours. Patients often experience a high fever, severe headache, widespread muscle aches, and profound fatigue that can make it difficult to get out of bed. In some cases, influenza can lead to serious complications requiring hospitalization.
As a general rule, if a person is still able to get up, eat normally, and carry out their daily activities, they are more likely to have a common cold. However, if even changing position in bed feels exhausting and unusually difficult, the illness may be influenza rather than a common cold.
Comparison Table: Influenza vs. the Common Cold
| Feature | Influenza | Common Cold |
|---|---|---|
| Onset of Symptoms | Sudden onset within 1-2 hours | Gradual onset over 1-3 days |
| Fever | High fever, typically 38-40°C (100.4-104°F) | Low-grade fever or no fever |
| Muscle & Body Aches | Severe muscle and body aches | Mild or absent |
| Fatigue | Marked fatigue, sometimes making it difficult to get out of bed | Mild fatigue; most people can continue daily activities |
| Runny or Stuffy Nose | May occur, but not the main symptom | Very common and usually the primary symptom |
| Duration of Illness | 5-7 days, although fatigue may persist for several weeks | 7-10 days |
| Risk of Complications | Higher risk of complications, including pneumonia | Complications are uncommon |
| Vaccine Available | Yes (annual vaccination is recommended) | No vaccine available |
Symptoms of Influenza
The most common symptoms of influenza include:
- High fever, typically rising rapidly to 38–40°C (100.4–104°F)
- Severe headache, which in some cases may be intense enough to cause sensitivity to light
- Muscle and joint aches, particularly in the back, legs, and arms
- Extreme fatigue, with persistent tiredness even without physical activity
- Dry cough, which may become progressively more severe
- Sore throat and nasal congestion in some patients
- Nausea, vomiting, or diarrhea, which are more commonly seen in children
Most symptoms improve within 5–7 days. However, fatigue may persist for several weeks after the acute illness has resolved. In high-risk groups—such as young children, older adults, and individuals with underlying medical conditions—influenza can lead to serious complications, including pneumonia
Types of Influenza Viruses
Understanding the different types of influenza viruses helps explain why the influenza vaccine needs to be updated every year and why influenza continues to be an ongoing public health challenge.
Influenza A
Influenza A is the most commonly detected type of influenza virus and is the primary cause of widespread outbreaks at both national and global levels.
What makes Influenza A particularly concerning is that it does not infect only humans. It can also be found in several animal species, which increases the likelihood of mutation and genetic reassortment compared with other types of influenza viruses.
Well-known subtypes found in Thailand and worldwide include H1N1, which caused the 2009 influenza pandemic, and H3N2. Both subtypes are commonly included in annual influenza vaccines based on recommendations from the World Health Organization.
Influenza B
Influenza B also causes seasonal outbreaks, similar to Influenza A. However, it primarily infects humans and is rarely found in animals, which means it undergoes genetic changes more slowly than Influenza A.
The symptoms of Influenza B are similar to those of Influenza A and include fever, cough, sore throat, muscle and body aches, and fatigue. Influenza B is currently classified into two major lineages: B/Victoria and B/Yamagata. This is one of the reasons why quadrivalent influenza vaccines were developed—to provide protection against both B lineages in addition to the circulating Influenza A strains.
Influenza C
Influenza C is less common than other types of influenza and generally causes milder symptoms compared with Influenza A and B. It is more commonly found in children than in adults and does not typically cause widespread outbreaks. For this reason, Influenza C is not one of the primary strains included in influenza vaccines for the general population.
Influenza D
Influenza D primarily infects certain animals, particularly cattle and swine. To date, there is no clear evidence that this type causes widespread infection in humans. As a result, Influenza D is not included in seasonal influenza vaccines developed for human use.
What Does the Influenza Vaccine Protect Against?
The influenza vaccine is a biological preparation that stimulates the body’s immune system to develop protection against influenza viruses without causing the disease itself.
After vaccination, the immune system learns to recognize and respond to influenza viruses in advance. As a result, if a vaccinated person is exposed to the virus later, their immune system can respond more quickly and effectively than someone who has not been vaccinated.
The influenza vaccine offers several important benefits, including:
- Reducing the risk of influenza infection. The vaccine can significantly lower the chance of becoming infected with influenza. Its effectiveness depends on how closely the vaccine strains match the viruses circulating during a given influenza season.
- Reducing the severity of illness. People who become infected despite being vaccinated generally experience milder symptoms than those who are unvaccinated.
- Lowering the risk of hospitalization. Vaccination helps reduce the likelihood of severe illness requiring hospitalization, particularly among high-risk groups such as older adults and individuals with chronic medical conditions.
- Reducing transmission to others. Because vaccinated individuals are more likely to experience milder illness and shed less virus, the risk of transmitting influenza to others may also be reduced.
Although the influenza vaccine does not provide 100% protection, it remains one of the most well-established and scientifically supported measures for reducing the burden of influenza at both the individual and population levels.
What Types of Influenza Vaccines Are Available?
Comparison Table: Types of Influenza Vaccines
| Vaccine Type | Strains Covered | Administration | Suitable For |
|---|---|---|---|
| Trivalent Vaccine | 2 Influenza A strains (H1N1 and H3N2) + 1 Influenza B strain | Injection | All individuals aged 6 months and older |
| Quadrivalent Vaccine | 2 Influenza A strains (H1N1 and H3N2) + 2 Influenza B strains | Injection | All individuals aged 6 months and older |
| Inactivated Influenza Vaccine | Available in trivalent or quadrivalent formulations | Injection | Most individuals, including pregnant women |
| Live Attenuated Influenza Vaccine (LAIV) | 4 influenza strains | Nasal spray | Healthy individuals aged 2-49 years |
Trivalent Influenza Vaccine
The Trivalent Influenza Vaccine (IIV3) is designed to protect against three influenza virus strains: two Influenza A strains—A(H1N1) and A(H3N2)—and one Influenza B strain.
This vaccine has been used for many years and is supported by decades of established safety data. It provides effective protection against seasonal influenza and may be recommended by healthcare professionals based on individual risk factors and local vaccination guidelines.
Quadrivalent Influenza Vaccine
The Quadrivalent Influenza Vaccine (IIV4) provides protection against one additional Influenza B strain, covering both the B/Yamagata and B/Victoria lineages.
The quadrivalent vaccine was developed to address the limitation that, in some influenza seasons, the circulating Influenza B lineage may differ from the single B strain included in the trivalent vaccine. By including both B lineages, the quadrivalent vaccine helps reduce the risk of this mismatch and provides broader protection against seasonal influenza.
Inactivated Influenza Vaccine
The Inactivated Influenza Vaccine (IIV) is the most widely used type of influenza vaccine in Thailand and around the world. It is typically administered as an intramuscular injection.
The term “inactivated” means that the influenza viruses contained in the vaccine have been killed or inactivated, making them unable to replicate or cause influenza infection. As a result, the vaccine cannot cause influenza. It is suitable for all individuals aged 6 months and older, including pregnant women and older adults.
Live Attenuated Influenza Vaccine
The Live Attenuated Influenza Vaccine (LAIV) contains influenza viruses that have been modified to weaken their ability to cause illness. It is usually administered as a nasal spray and is available in some countries.
This vaccine may stimulate a broader immune response because it more closely mimics natural influenza infection. However, it is not suitable for everyone, including people with weakened immune systems, pregnant women, and children under 2 years of age. Anyone considering this vaccine should always consult a doctor to determine whether it is appropriate for their individual health condition.
Why Is an Annual Influenza Vaccination Recommended?
This is one of the most common questions people ask, especially when comparing the influenza vaccine with other vaccines, such as the tetanus vaccine, which can provide protection for many years after a single series of doses. So why does the influenza vaccine need to be given every year?
Influenza Viruses Change Every Year
เInfluenza viruses have the ability to mutate rapidly. These changes occur through two main mechanisms: antigenic drift, which involves gradual changes in the virus’s surface proteins, and antigenic shift, which is a major genetic change that can result in the emergence of an entirely new influenza strain.
To ensure that influenza vaccines match the strains most likely to circulate during each influenza season, the World Health Organization (WHO) continuously monitors influenza activity worldwide and analyzes global surveillance data. Based on these findings, the WHO issues vaccine composition recommendations twice a year—one for the Northern Hemisphere and one for the Southern Hemisphere.
As a result, the influenza vaccine used in the current year may not provide sufficient protection against the strains expected to circulate in the following year, which is why the vaccine formulation is updated annually.
Immunity from the Vaccine Declines Over Time
Although the body develops immunity after influenza vaccination, the level of protection gradually decreases over time. This decline is particularly noticeable in older adults, whose immune systems tend to produce a weaker response to vaccination than those of younger individuals.
Receiving the influenza vaccine every year helps boost the immune response and maintain a level of protection that is sufficient to reduce the risk of influenza infection during each influenza season.
Helps Reduce Transmission to Close Contacts
Getting vaccinated is not only a way to protect your own health but also a way to help protect others.
People who are vaccinated and do not become ill are less likely to spread influenza to young children who are not yet old enough to be vaccinated, older family members, pregnant women, or colleagues with underlying medical conditions. What epidemiologists call herd immunity can develop when a sufficiently large number of people in the community are vaccinated.
The World Health Organization (WHO) and Thailand’s Department of Disease Control, Ministry of Public Health, recommend seasonal influenza vaccination for everyone aged 6 months and older, provided there are no medical contraindications. However, certain groups should be given particular priority.
Children Aged 6 Months and Older
Young children, particularly those under 5 years of age, are at a higher risk of developing complications from influenza, including pneumonia, middle ear infections (otitis media), and secondary bacterial infections.
Because their immune systems are still developing, the influenza vaccine plays an important role in strengthening their protection during this vulnerable stage of life.
Older Adults and People with Underlying Medical Conditions
Older adults (aged 65 years and above) and individuals with underlying medical conditions—such as cardiovascular disease, chronic lung disease, diabetes, kidney disease, liver disease, weakened immune systems, or HIV infection—are at a significantly higher risk of developing severe influenza and serious complications.
For these individuals, influenza is more than just an unpleasant seasonal illness. It can worsen existing medical conditions, increase the risk of hospitalization, and, in some cases, lead to life-threatening complications.
Pregnant Women and Close Caregivers
Pregnant women undergo changes in their immune system that increase their risk of developing severe influenza if they become infected. The World Health Organization (WHO) and Thailand’s Department of Disease Control recommend influenza vaccination during pregnancy because the antibodies produced after vaccination can also be passed to the baby through the placenta, helping protect the infant during the first few months of life before they are eligible for vaccination.
Parents and caregivers of young children, caregivers of older adults, and healthcare workers are also strongly encouraged to receive the influenza vaccine. By reducing their own risk of infection, they help lower the chance of transmitting the virus to those who are most vulnerable to severe illness.
Who Can Benefit Most from the Influenza Vaccine?
People Who Regularly Interact with Large Numbers of Others
People who use public transportation every day, work in large offices, teach, provide services in retail businesses, or travel frequently are exposed to many different people in their daily lives. As a result, their risk of contracting influenza may be higher than that of people who spend most of their time at home.
Influenza vaccination can help reduce this risk and also lower the chance of bringing the virus home and spreading it to family members.
People Living with Young Children or Older Adults
In households with young children who are not yet eligible for vaccination, older adults, or family members with underlying medical conditions, ensuring that everyone else in the household is vaccinated is one of the most effective ways to protect those who are most vulnerable.
This approach is known as the Cocooning Strategy, which involves vaccinating close family members and caregivers to create a protective “cocoon” around individuals who are unable to receive the vaccine or are at a higher risk of developing severe influenza.
People Who Want to Maintain Their Productivity and Quality of Life
Influenza typically requires 3–5 days of recovery, and in some cases, even longer. Fatigue can also persist for several weeks after the acute illness has resolved, affecting work performance, productivity, income, and overall quality of life.
Receiving an annual influenza vaccination is therefore more than an investment in your health—it is also an investment in your time, productivity, and ability to stay active in your daily life.
Inform Your Healthcare Provider About Your Medical History
Before receiving the influenza vaccine, your healthcare provider will review your medical history to determine whether the vaccine is appropriate for you. Be sure to inform them about the following:
- Any history of allergic reactions to vaccines or vaccine ingredients, including egg allergy. Some influenza vaccines are manufactured using egg-based technology. However, egg-free influenza vaccines are now available for people with a history of severe egg allergy.
- Any medications you are currently taking, especially those that affect the immune system, such as immunosuppressive medications.
- Your current health status, including any underlying medical conditions or acute illnesses that may influence the timing or suitability of vaccination.
- Whether you are pregnant or breastfeeding, as this may help your healthcare provider determine the most appropriate type of influenza vaccine for you.
If You Have a High Fever or Are Acutely Ill, It Is Best to Postpone Vaccination
If you have a high fever, an acute illness, a severe cough, a severe sore throat, or are experiencing unusual fatigue on the day of your scheduled vaccination, it is generally recommended to postpone your appointment until you have recovered.
This recommendation is not because the influenza vaccine is unsafe during these situations. Rather, when your body is already fighting an active infection, your immune system may not respond to the vaccine as effectively. In addition, if symptoms develop after vaccination, it can be difficult to determine whether they are related to the vaccine or to the illness that was already present.
Get Enough Rest and Eat Normally Before Your Vaccination
On the day before and the day of your influenza vaccination, it is recommended to get adequate rest, eat your regular meals, and stay well hydrated. You should not skip meals before your vaccination, as doing so may increase the risk of dizziness, lightheadedness, or fatigue afterward.
Bring Your Vaccination History, If Available
If you have a vaccination record or remember when you last received an influenza vaccine, be sure to share this information with your healthcare provider before your appointment.
Because the influenza vaccine is recommended once a year, knowing your previous vaccination history helps your healthcare provider determine the most appropriate timing for your next dose and ensure that your vaccination schedule remains up to date.
Plan to Get Vaccinated Before the Influenza Season
The influenza vaccine does not provide immediate protection. After vaccination, your body typically needs about two weeks to develop an effective immune response. For this reason, it is best to receive the vaccine before influenza begins circulating in your community or before traveling to areas where influenza activity is high.
In Thailand, influenza vaccination is generally recommended before the rainy season (May–June) or at the beginning of the year (January–February), as these are the periods when seasonal influenza outbreaks occur most frequently.
